Friday, December 10, 2010

CCNP - ENTERPRISE - Password recovery on a Catalyst switch

Part of the preparation work for my home lab is to clear off the old config on the recycled systems. This includes performing a password reset.

The steps on my catalyst switches are as follows:

  1. Access ROMMON - power off the device, press the Mode button on the front, connect the power. When the 'x1' LED goes out release the Mode button and ROMMON will be presented in your terminal
  2. Enable Flash - ROMMON: flash_int
  3. Enter the Following cmd - :loader_helper
  4. Check the directory structure - : dir flash:
  5. Rename the Startup-config file - :rename flash:config.text flash:config.bak
  6. Restart the system - :boot
  7. At the prompt enter 'no', you do not wish to run through the automation steps
  8. At this point you can either stop here and run the system as a fresh system or you can continue
  9. Rename the config.old back to config.text - switch#rename flash:config.bak flash:config.text
  10. Next, load the config file in the memory - switch#copy flash:config.text system:running-config
  11. At this point you are now running your original config. All you need to do now is reset all your access passwords
  12. Change the Enable Secret - switch(config)#enable secret [Secret]
  13. Change the Enable Password - switch(config)#enable password [Password]
  14. Change the Telnet password - switch(config)#line vty 0 4; switch(config-line)#password [Password] 
  15. Set the VTY line to prompt for the password - switch(config-line)#login
  16. Finally, save your running-config - switch#wr mem


In your working environment how often do you really need to access ROMMON? if everything is in order and you have up to date documentation then I suspect not very often.

So, I come to preparing my new home lab for my CCNP SWITCH course and I've had to remove the old configuration off the recycled kit.

For my access server I'm going to run an old 2610 router, not the most feature rich router but it has a single ethernet interface which I run inter-VLAN routing off. I have 2 catalyst 3550's for my distribution layer and I have 2 Catalyst 2950's for the access layer.

Accessing ROMMON.
Accessing ROMMON is actually very simple on the catalyst switches. Power off your device. Hold the mode button down on the front of the box, then connect the power lead.

Wait for the 'x1' interface indicator to go out and then release the mode button.

For the 2610 you power cycle the system and at the point the boot process is displayed on screen you hit ctrl+Break.

Thursday, December 9, 2010

CCNP - ENTERPRISE - To 'Delete' or to 'Erase'?

When do you use the commands Delete or Erase?

Well it depends on what you are wanting to do.

If you want to remove a file, you use the delete cmd, for example:
R1#delete flash:config.old
SW1#delete vlan.dat

If you want to remove system information you use the erase cmd, for example:
R1#erase startup-config

If in doubt use the ? to inspect your options:

Tuesday, October 26, 2010

CCNP - ENTERPRISE - Subnet Mask vs Wildcard Mask

Something I find I struggle with is when to use a subnet mask and when to use a wildcard mask. This article is to simply set out the instances when you will use one or the other (not the actual steps to apply them).

Subnet Mask:

  • When applying an IP to an interface
  • Routing protocol summary addresses
  • BGP
  • PIX security appliance ACL's
  • ASA security appliance ACL's
  • When creating DHCP pools on a Switch or Router

Wildcard Mask:
  • EIGRP network statements
  • OSPF network statements
  • VPN concentrator network lists (when setting the local and remote allowed networks)
  • Router ACL's
This list is not exhaustive and I will add to it as I come across new instances where you use either a Subnet mask or Wildcard mask. 

Monday, September 27, 2010

CCNP - TSHOOT - TSHOOT in the real world...Part2

The second trouble ticket that I was able to demonstrate my TSHOOT knowledge was the following:

Trouble Ticket #2 - the customer reported that hosts at SiteC were not getting a DHCP allocation from the server at SiteD.

The customer reported that when they performed a traceroute from the LAN at SiteC to on the LAN at SiteD the trace stopped. The network looked like this:

SiteC --> R1 at SiteC --> 'the core network' at R2 --> R3 at SiteD --> DHCP Server on SiteD LAN

For this one I took a look at our Nagios Monitoring and could see that there were no issues reported on R1, R2, or R3. As such I jumped on to R1 at SiteC and performed a traceroute to The trace was successful! Yet from the LAN at SiteC the trace failed?

Based on this information I formed my hypothesis. I asked the customer whether there could be an access-list/firewall between R3 and the DHCP server that could be blocking traffic from the LAN at SiteC? The customer investigated their equipment and found that an earlier change to an access-list on their equipment at SiteD was now stopping DHCP requests from SiteC. They corrected their configuration and the issue was resolved.

Trouble Ticket #2 - Closed

With this ticket because our network monitoring was reporting no issues with R1, R2, R3, I took the 'Shoot from hip' approach and looked to eliminate all L3 issues in one go. The trace route worked indicating that the server was responding to ping and that all L3 connectivity between SiteC and SiteD was fine. I could have tested the L1/L2 physical connectivity on each router in turn but our monitoring would have pick this up  and therefore I was confident the physical connectivity was fine. Once L3 connectivity had been proven on the equipment I have admin access on I could formulate my hypothesis and put it to the customer to test.

CCNP - TSHOOT - TSHOOT in the real world...

When studying for any certification exam it's very easy to end up concentrating on the goal of passing the exam instead of the actual goal which should be gaining knowledge and achieving a certain standard of expertise that can be practically utilised in the real world. The exam simply confirms that you have a certain level of knowledge. The point is to enable you to use it in the real world.

So here I am, studying for my TSHOOT exam, which will complete my CCNP, and in the week before the exam comes 2 real world examples of how studying for the TSHOOT exam has helped me investigate and resolve the issue in hand. The next 2 posts will outline the problem, methodology used and how the TSHOOT exam materials directly related to resolving the problems.

Trouble Ticket #1 - The customer contacted us to state that SiteA no longer had access to the internet via it's default route. 

 Following the TSHOOT troubleshooting methodology I gathered as much relevant information as I could from the client. I then reviewed the information, eliminated the possible causes until I was down to a small   number of possible issues, I then formed my hypothesis, tested it and reviewed the outcome.

For this particular problem the customer stated that following the closure of one office and the migration to a new site access to the internet via it's default route was no longer possible. The customer supplied me their default route - and that it originally routed via the now closed office. I also had the new network diagram outlining the new office and the appropriate routes. The diagram was essentially as follows:

SiteA Firewall ---> R1at SiteA ---> 'The core network' at R2 ---> R3 at SiteB - (trunk link)--> SiteB L3 switch ---> SiteB's ASA firewall --> INTERNET

First job was to see where the routing began to fail. So on to R1 and check the interfaces were up (yes) and then I moved on to the routing table to check for a route for This was a static route pointing to our core network (for this article we'll call all of it R2). I performed a traceoute on R1 to and it failed at R2.

On R2 I did the same, checked that interfaces were up and then checked the routing, This is where the first issue occurred. The static route on R2 was still pointing to the closed office. So the change was made. Static route was now pointing to R3 at SiteB.

I performed a traceroute to the default route from R1 again and it still didn't work. This time the trace stopped at R3.

On to R3 the routing table revealed that there was no route for at all. After referring to the customer they advised me that the traffic had to go via a specific VLAN on the L3 switch so on R3 we needed to create VLAN10 and allowed VLAN10 on the trunk link to the L3 switch at SiteB. Finally the route for was set to go via the SVI for VLAN10.

With this created I tested the route to from R3. It still didn't work? With the information at hand I was able to formulate my hypothesis.

Was VLAN10 a) created on the L3 switch? and b) allowed on the trunk link?

Next I checked the state of the VLANs. VLAN10 was in place on R3, and was allowed on the trunk link to the L3 switch. The trunk link was up and protocol was up. Everything that I had admin access to was now properly configured and had been tested however the traceroute was still not returning. I put this to the customer who inspected their L3 Switch. They reported that VLAN10 had not been created after all on their equipment.

Once the customer applied their configuration on their equipment the route from SiteA to the ASA firewall on the LAN of SiteB worked. It was only after the issue was investigated that it became apparent the customer had neglected to account for the routing required for SiteA when they planned the closure of their old office. As a result none of the required configuration had been prepared before hand and SiteA had been cut off once the old office closed.

Trouble Ticket #1 - Closed.

As you can see on this occasion I was able to use a 'Divide and Conquer' trouble shooting approach. Starting L3 and then checking L1 and L2 when the traceroute failed. On R3 I was able to narrow the issue to L2 and formulate my hypothesis which was then tested.

Friday, September 24, 2010

CCNP - TSHOOT - Troubleshooting the Routers...

Continuing my TSHOOT methodology, in this post I'm going to look at troubleshooting the network between R4, R3, R2,R1 and out to the web server at

Having not sat the TSHOOT exam the impression I'm getting is the Trouble Ticket (TT) will state something like Client1 cannot access [insert resource]. As such you need to identify the system the issue is on and the resolution.

Again I'll be referring to the actual  TSHOOT topology - here

Let's say Client 1 cannot access the web server at Based on my previous article, also assume Client 1 can ping at R4. In this post I will outline how I see myself troubleshooting the connectivity issue to the web server.

I'll want to prove connectivity to R1. So in turn I'll ping each interface on the way to R1 at If at any point the ping is not returned then log in to the router that you were trying to ping and begin troubleshooting.

1) Are the interface on the link UP/UP? check that one of them is not admin'd down:
#sh ip interfaces brief

2) With interfaces up, is the L2 Frame-Relay topology correct? the diagram shows that the routers are connected via point-to-point links therefore I'm expecting to see a config similar to:
R4(config)#int s0/0/0
R4(config-if)#encapsulation frame-relay point-to-point
R4(config-if)#no shutdown
R4(config)#int s0/0/0.34 point-to-point
R4(config-if)#ip address
R4(config-if)#frame-relay interface dlci 403

3) Assuming L2 is fine move on the routing protocols. Things to check here are:
i - is route redistribution functioning correctly on R4? check the routing table, check config
ii - Are the adjacencies up? Check the neighbor tables/topology tables.
iii -Are the correct networks being advertised to the neighbor? check the ospf network statements
iv -  is there any authentication in place? if so it must match at each end
v - is the routing table including the correct route as per the network diagram? If the route is not in the routing table it won't be advertised out to the neighbor.
vi - Is the routing config command 'passive-interface default' set? if it is then updates will be suppressed by default. You need to either remove the command using #no passive-interface default
Or you specifically list the interfaces you want to participate in dynamic routing using:
#passive-interface default
#no passive-interface f0/0
#no passive-interface f0/1
and so on...

4) Up to R1 I think that would cover off all the major connectivity issues you'd be likely to get. If you still have problems look closer at R1.

On R1 check you can get to the next-hop outside of the network. If you can the config on R1 should be sound. If you can't it could indicate any of the following.
i - Check over the interfaces as per previous points. Ensure they are UP/UP. if they aren't then troubleshoot accordingly.
ii - Check routing/BGP - is the BGP peering up? if not check the config, if the update source being used is a Loopback address then you need the update-source command set in the BGP config. Check the IP's of the Peers, check the remote-as command is correct
iii - Is there a default route? I'd expect there to be one. Simple question. Is it pointing to the correct outbound next-hop? not in towards the LAN?
iv - Are there any access-lists? if so, what are they permitting and what are they blocking? Could the implicit deny  be causing an issue here?
v- Is the default route being correctly redistributed? I'm expecting to see something like:
R1(config)#router ospf 1
R1(config)#default-information originate
vi - Is NAT in operation? check which interface is inside, outside, check the ip nat source statements. Check any NAT pools that my be configured? if a pool is not configured then I'm looking for the 'overload' option in the command :
#ip nat source list 10 interface s0/0/1 overload
With out Overload then port address translation is not working and you'd only be able to have 1 host using NAT at any one time.

Based on the exam topology I'd hope that by running through my structured check list from L1, L2 to L3 to L4 and above that the issue in hand would be revealed.

When I'm in the exam I'll be taking my cue from the information in the TT. As such I might not need to bother with L1/L2 checks if the remote peer is responding as expected.

My next article is going to look at the IPv6 topology and look at how I might be troubleshooting problems there.

Monday, September 20, 2010

CCNP - ENTERPRISE - Considering the Client...

In the TSHOOT exam the bulk of the trouble tickets will pretty much be Client1 cannot access [insert resource here], identify the problem device and present a solution.

The TSHOOT topology is freely available from the Cisco website - here - and so we can take a look and start to suggest possible problems that can be presented. On it we have BGP, OSPF, EIGRP and redistribution, NAT, DHCP, IPv6 Tunnels, OSPFv3 and RIPng redistribution, we've got first hop redundancy and etherchannel, plus OSPF over Frame-Relay. Some pretty tasty stuff.

So, where to start? Well, lets say that Client1 can't access the web server at As I'm looking at client issues assume that the config between the web server all the way in to the network to R4 is fine.

When I'm in the exam I'm going to be looking to ensure connectivity to Client1's default gateway and the DHCP server at R4 is sound. When I find that Client1 is unable to ping the default gateway or obtain a DHCP allocation here is what I'll be looking for:
1) Client1 IP Assignment - The L2/L3 topology diagram states that the address is obtained via DHCP (R4 at On Client1 do >ipconfig - if the address is move to ASW1, if the address is 169.X.X.X then DHCP is not working properly for some reason. In both cases move to ASW1

2) Interface VLAN membership - Client1 should be connected to int f1/0/1 - is it? if not then correct as appropriate. Interface f1/0/1 should be a member of VLAN 10 - does the VLAN exist? :
#sh vlan

Is int f1/0/1 a member of VLAN 10?:
#sh run int f1/0/1 OR #sh vlan

Does ASW1 have any port security?
#sh run int f1/0/1
if so what is configured here? Does any static MAC address match that of Client1? does the port limit number of MAC addresses per interface? Problem is here is if port security is incorrect. Otherwise move on

3) Is VLAN 10 allowed on the trunks?
Firstly prove int Po13 is an active Etherchannel and that interfaces f1/0/19-20 are members
#sh etherchannel summary - Trouble shoot accordingly if this is not the case.

Next is int Po13 a trunk link?
#sh interfaces trunk - if not then problem is here.

Next which VLAN's are allowed on the trunk links?
#sh interfaces po13 trunk - if VLAN10 is not here then you won't get to DSW1

Next, can you see any VLAN filtering in place? if so check that any permited ACL has the correct ip range otherwise traffic from will not be allowed to use VLAN10. Also check the vlan access-map config ensure that the traffic allowed in the ACL is forwarded not dropped.

Do step 3 on both ASW1 and DSW1. If one side is not configured correctly then connectivity will fail.
If everything looks good you should be able to at least ping VLAN10's dfg at on DSW1, move on.

4) Check routing on DSW1:
- DSW1 is an L3 switch that needs to send your DHCPDISCOVER to the DHCP server on a different network. As such ensure ip routing is enabled:
DSW1#sh run | begin ip routing
- Check L3 connectivity between DSW1 and R4 - Ping, check interfaces are UP/UP
DSW1#sh ip interface brief
- Check DSW1 int f1/0/1 is not admin'd down:

- If all looks good here check that the ip-helper command is applied to interface VLAN 10:
DSW1(config)#int vlan10
DSW1(config-if)#ip helper-address
(this will tell DSW1 to forward UDP broadcasts received on this interface)

5) You should now be at R4. If all of the above looks good then you should look above L2/L3 to DHCP or other factors.

6) DHCP configuration:
- Is DHCP running on R4? look for service dhcp in #sh run
- Are there any leases active? check - #sh ip dhcp bindings
Check the Hardware Address with that of Client1 to ensure the correct Client has an assignment. If you've discounted steps 1-4 I'd be surprised if this was the case.
- Check the DHCP address pool is correctly configured:
DSW1#sh run | begin ip dhcp
look for any kind of misconfig here be it incorrect subnet defined for the pool or an exclude address incorrect

7) At this point I'd expect to have uncovered an issue. If that's not case there's still some tactics I can try.
i) Kevin Wallace suggested Abort! - if you think you've narrowed the issue to a point but are unsure of the final solution, hit Abort, dive in to a different ticket and cross check the config. If the config is different the here is your fault. If it's the same then you can discount that area as the source of the problem and move on
ii) Let the question guide you - you've narrowed the issue to, lets say, DSW1 but are still unsure. Review the options available to you in part 2 and part 3 of the trouble ticket and use the information there to gleen if you are looking in the right place.

I think I've covered off the major issues that could be presented. I guess HSRP on DSW1 could be an issue but if DSW2 is the active router (#sh standby) then you should still be able to reach the DHCP server at

Please let me know if I've missed a point or am miss guided in any way. Thanks

Wednesday, September 15, 2010

CCNP - ENTERPRISE - OSPF - Adjacency requirements

For an OSPF adjacency to form the 2 neighbors must agree on several parameters within the Hello Packet before the adjacency can form. These are:

i) Each must have a unique Route-ID
ii) Each must be in the same Area
iii) Authentication setting must match
iv) Timers must match.

One other parameter that must be agreed upon is the router priority for DR/DBR elections.

CCNP - ENTERPRISE - Manipulating Routing Updates - Route-map permissions

When compiling a Route-map(RM), you set an access control list (ACL) then use the Route-map to match addresses set out in that ACL to apply your chosen criteria in the Set field of the Route-map.

Now the question is given the combination of permit or deny statements in the ACL and the permit or deny statement of the Route-map what is the out come for a packet.

The following is what happens to a given packet when the permit or deny statements are considered:

ACL = Permit
RM= Permit
Result = Packet Permitted to proceed via the route-map. That's to say the packet is permitted to be permitted.

ACL = Deny
RM = Permit
Result = Packet Denied. The packet is denied from being permitted.

ACL = Permit
RM = Deny
Result = Packet Denied. The packet is permitted to be denied.

ACL = Deny
RM = Deny
Result = Packet PERMITTED. The packet is denied from being denied. If it isn't allowed to be denied, it must, therefore, be permitted.

Bit of a weird one to get your head round but it's an obvious trick to chuck in there when you're under pressure so keep an eye out.

Tuesday, September 14, 2010

CCNP - ENTERPRISE - IPv6 - IP address types

The following is a list of IPv6 address types. The high order bits are displayed and their function:

i) 001 = Global - 200
ii) 1111 1111 = Multicast - FF
iii) 1111 1110 11 = Site Local - FEC0
iv) 1111 1110 10 = Link Local - FE80
v) ::X:X:X:X = IPv4 compatible address, where the first 96 bits are set to 0 (hence the ::) and the remaining 32 bits are converted to hex from the IPv4 address

Other addresses include:
::1 or 0:0:0:0:0:0:0:1 = Loopback
::/128 = unspecified address which is essentially the DFG for IPv6, all the bits are set to 0.
IPv6 private addresses start - 1111 1110 1 - therefore both site local and link local are private

CCNP - ENTERPRISE - EIGRP - Adjacency Requirements

The following criteria need to be meet before an EIGRP adjacency will form:
i) Authentication (if in place)
ii) AS Number
iii) Source IP MUST be the primary address for the interface - secondary IP's will not result in the adjacency forming
iv) K values must match

N.B. - Timers do not have to match but they must be equal.
- Adjacency will flap if timers are mismatched.
- Therefore ensure you have a reliable time source.

Sunday, September 12, 2010

CCNP - TSHOOT - Maintenance Models

The main models used in assisting with the maintenance of a network are as follows:

  1. FCAPS - which is - Fault mgmt, Configuration mgmt, Accounting mgmt, Performance mgmt and Security mgmt. Defined by ISO (International Organisation for Standardisation).
  2. ITIL - the IT Infrastructure Library, which is a collection of best practices. When implementing this PLEASE PLEASE PLEASE remember it is Best Practice not THE Practice. I've worked at companies who have idiots Service Delivery Mangers who think that to be 'compliant' they need to implement all of the guidelines regardless of how big the firm is or how suitable the process is to their enterprise </rant>
  3. TMN - Telecommunications Management Network, the ITU-T's variant to FCAPS tailored to the telecommunications field.
  4. Cisco Lifecycle Services - You should be familiar with this from your ROUTE and SWITCH studies Chpt1. Covering the PPDIOO - Prepare, Plan, Design, Implement, Operate, and Optimize life cycle.

CCNP - ENTERPRISE - Creating/Converting addresses

One easy way to pick up marks in the BSCI exam is to practise creating or converting addresses in one format to another, quickly.

So far I've spotted 4 situations where you would be asked to identify suitable converted addresses for a given address.

These are:
1) Identify the correct Multicast MAC address for a given Multicast IP addresses
- Multicast MAC addresses always start 01-00-5e. You need to find the last 23 bits to add to these first 25 bits to create a 48bit MAC.
- Take you Multicast IP and convert to binary
- section off the last 23 bits starting from the RIGHT in to 4 bit sections. the last section will contain only 3 bits so to get a Hex figure for this just tack a 0 on to the start of it. 
- Next convert your binary to Hex
- Finally add these to the 01-00-5e to get your Multicast MAC
e.g -
Binary = 11100000.0 101 1010. 0001 0001 0010 1011
Hex -                      | 5 |  A |  1   |  1  |   2   |   B
MAC = 01-00-5e-5a-11-2b

2) When regarding IPv6 6to4 tunnels, identify a suitable IPv6 address for a given IPv4 address that is assigned to a physical interface.
 Using a Global address you know 2 things A) the high order bits always start 001 = 2000::/3 and generally end with 0001 in the first 16bits and B) a Global prefix is 48 bits long ( or /48). With this in mind  you know your first 16 bits, 0010 0000 0000 0001: X:X:X:X:X:X:X, so you are looking for the remaining 32 bits to form the address
Copy out into binary the IPv4 address of the physical interface the tunnel will be associated with then convert it to hex, e.g.)
    1100 0000. 1010 1000. 0110 0011. 0000 0001
      c     0        a      8       6     3      0     1
- Combine this with your first 16 bits and you have your Global IPv6 to apply to your tunnel interface

3) Given the ASN 5662 what would a suitable GLOP address be.
- For this type of question you know that the 1st octet is always 233 and you can choose what the last octet can be (1-255) so you need to calculate the 2nd and 3rd octet values.
- Take the ASN 5662
-Convert it to binary and pad the left of the binary figure with zero's until you have 16 bits (octets 2 and 3 combined) -0001011000011110
- Divide the 16 bits in 2 and you are left with 2 octets (8bits each)
- convert these to decimal - 00010110 = 22, 00011110 = 30 and add to you GLOP address starting 233.X.X.X
- GLOP addresses always have /24 subnet mask because the implementer can select 255 addresses to be assigned locally. As a result in this example the GLOP address will be 233.22.30.XXX/24

4) Finally there is the question that asked you about subnetting. Be it a host is not communicating (it has the wrong subnet mask), Which path would the router select (you need to work out the correct Network in a routing table for a given IP), will a packet be permitted or denied in the ACL (again you need to work out if your IP is with the range permitted or denied with in the subnet mask stated) 
- For all these types of questions you are looking at basic subnetting. Aim to get these calculations down to less the 20 SECONDS. 
- Imagine a slide ruler in your head. Slide it to the where the subnet mask stops and bang, you have your subnet increments. From here you know the Network address, 1st host in the subnet, last host in the subnet, broadcast address for the subnet, and the next network address.
- Practice this over and over until you can see in your head the octet with the bit values and where you stop for each subnet.

If you can get each of the situations above nailed, and nailed quickly you can buy time on the harder questions. Practice, practice, and practice again.

CCNP - ENTERPRISE - Multicast - Address scopes

Multicast uses a reserved Class D with the first 4 high order bits in the 1st octet assigned 1110.

Therefore you can work out that the address range assigned for Multicast is to

IANA then broke the address scope down further:
1) Locally Scoped, Reserved Link Local, addresses: to
This range is the IANA 'well known' multicast range which includes your addresses for EIGRP (, OSPF ( and RIPv2 ( PIMv2 (

2) Globally Scoped addresses: - to
-These can be allocated dynamically across the internet
-GLOP addresses fall into this scope (
-224.2.X.X was allocated to the 'MBone' or Multicast Backbone which is now a defunct technology due to little uptake by large institutions and the resources required by the equipment to manage the multicast traffic.

3) Limited (administratively) scoped addresses: - to
- Reserved for inside corporate networks, similar to private IP's
- Organisations can use limited scoped addresses for local multicast apps

This range was further subdivided in to: to
- Organisation wide scoped addresses - site local address.

CCNP - ENTERPRISE - Multicast - GLOP addresses

The GLOP address range, (pronounced GLOP not G-L-O-P), was originally specified in RFC2770 and was an experimental, public, statically assigned multicast address for publishers and ISP's to source content on the internet.

The method of assigning one of these experimental address was called GLOP. Implementers were assigned 255 addresses from the subnet. The actual address assigned was determined from the ASN the implementer already used.

The address assigned set out the values of the 2nd and 3rd octet of the GLOP address. That is to say, all GLOP addresses start 233 (octet #1), the you had octet #2 and #3 to allocate, and finally you knew you had 255 addresses to choose from so the 4th octet was always a value of your choice 1-255.

Octet 2 and Octet 3 were determined by a calculation involving the ASN already assigned to the implementer and therefore, in theory, the GLOP address that resulted was unique (i.e. - not allocated to another organisation).

To determine the value of octet #2 and octet #3 do the following: (example taken from RFC2770)
i)Take the ASN 5662
ii)Convert it to binary and pad the left of the binary figure with zero's until you have 16 bits (octets 2 and 3 combined) - 0001011000011110
iii)Divide the 16 bits in 2 and you are left with 2 octets (8bits each)
iv) convert these to decimal - 00010110 = 22, 00011110 = 30 and add to you GLOP address starting 233.X.X.X
v) GLOP addresses always have /24 subnet mask because the implementer can select 255 addresses to be assigned locally. As a result in this example the GLOP address will be 233.22.30.XXX/24

Further reading:

Friday, September 10, 2010

CCNP - ENTERPRISE - OSPF - Default Path Cost values

The OSPF metric is cost, which is calculated using the equation 100Mbps/Bandwidth of interface.

The 100Mbps is a reference bandwidth which is applied in order to calculate the Cost of an interface. The Cost is an indication of the overhead to send packets across that link. Lower Costs are better.

OSPF uses the following default metric costs for different types of interface:
56K dial-up - 1785
T1 (1.544Mbps serial link) - 64
E1 (2.048Mbps serial link) - 48
Ethernet - 10
100Mb Fast Ethernet - 1
1000Mb Gigabit Ethernet - 1

The default OSPF Cost is used to calculate the best path. The best path is then entered in to the routing table (assuming there isn't another protocol with a better AD with the same path).

In order to refine your traffic shaping you can change the reference bandwidth so that you can determine the best path when considering FE and GE links, as you'll note that the default cost is the same and there fore determining the best path between the two could result in sub-optimal routing.

To change the reference bandwidth do:
R1(config)#router ospf 1
R1(config-router)#auto-cost reference-bandwidth [ref-bw]
!where ref-bw can be a value of 1-4294967

To override the default cost value that would result from the values stated above, you can manually set a cost value on an interface:
R1(config)#int s0/0
R1(config)#ip ospf cost [int-cost]
!where int-cost is a value of 1 -65535

CCNP - ENTERPRISE - EIGRP - Metric Weights

When redistributing another routing protocol in to EIGRP you need to specify the metric weights in order to redistribute the routes correctly and efficiently.

The command you need to perform redistribution is:
R1(config)#router eigrp 1
R1(config-router)#redistribute ospf 1 metric 1500 10 255 10 1500

Alternatively you can define a 'Seed' metric which is applied to all redistributed routes and so you don't need to specify the individual metrics each time.

R1(config)#router eigrp 1
R1(config-router)#default-metric 1000 100 250 100 1500
R1(config-router)#redistribute ospf 1

The values stated above represent each of the values for the EIGRP metric:
R1(config-router)#default-metric bandwidth delay reliability loading mtu 

In both of the examples above you are manually setting out the metric for the routes once they are redistributed in to EIGRP.

If you fail to set either a default metric or a specific metric in your redistribute command then EIGRP will assign a metric of 'infinity' and the routes will fail. So if you end up scratching you head wondering why your desired routes don't appear in the routing table take a look at your metrics.

The metric set out above represent the K-values for each of the criteria that make up the EIGRP Metric. The K values are:
i) Bandwidth (K1) - Minimum bandwidth along the path in Kbps. This is a value of 1 - 4294967295
ii) Load (K2) - Used as a way of managing traffic off heavily used links. Value is 0 to 255 where 255 equals is 100% utilisation of the available bandwidth
iii) Delay (K3) - Latency of the path in 10's of Microseconds. This is a value of 1 - 4294967295
iv) Reliability (K4) - A value representing how likely the path is to be available or fail. Value is between 0 and 255, with 255 equalling 100% reliable.
v) MTU (K5)- Used to set a path MTU for a given route. Value is 1 - 65535

CCNP - ENTERPRISE - OSPF - NBMA network types review

One of the things I have trouble with is remembering the different characteristic's of each of the OSPF network types.

Below is a chart simply highlighting each point.

For configuring each network type please refer to my earlier posts.


Wednesday, September 8, 2010

CCNP - ENTERPRISE - BGP - Neighbor States Notes

If a router stays in an IDLE condition check:
i) If the neighbor announces the route in it's local IGP
ii) Verify you have not entered an incorrect IP in your neighbor statements

If a router enters or remains in ACTIVE state it could be because:
i) Neighbor doesn't have a route to the source IP of the BGP Open packet generated by the router - check the routing table on the neighbor and add a suitable route via a static entry or an IGP if one is missing.
ii) The neighbor is peering with the wrong address - check neighbor statements via #sh run
iii) The neighbor doesn't have a neighbor statement for this router - add one!
iv)The AS number in the neighbor statement is misconfigured on one or both peers. - check the neighbor statements for a mis-typed Remote-AS entry.

CCNP - ENTERPRISE - BGP - Neighbor States

When establishing neighbor sessions, BGP transitions through a number of states. These are:

  1. IDLE - router is searching routing table to see whether a route exists to the neighbor*
  2. CONNECT - Router found a route to the neighbor and 3 way TCP handshake is complete
  3. Open Sent - Open msg with parameters for BGP session is sent
  4. Open Confirm  - Router received an agreement on the parameters to establish a session. Alternatively, the router enters ACTIVE state is not responds is received to the Open Sent msg
  5. ESTABLISHED - peering established, routing begins.
To view this activity you can use the debug options to see the process in action.

R1#debug ip bgp all
R1#debug ip bgp events

Remember, a debug is processor intensive so remove the debug once you are finished:
R1#undebug all 
R1#u all 

*I can't remember if I've already stated this but in the UK the word is spelt NEIGHBOUR. For consistency with Cisco IOS commands I'm spelling it NEIGHBOR when I need to use the word. Thought I'd just clarify that as I'm not some dumb ass that can't spel. ...erm...

CCNP - ENTERPRISE - BGP - Message Types

As with all routing protocols there are a number of different messages types with differing duties/purposes.

For BGP we have:

  1. OPEN - Includes BGP version number, AS number (ASN), Hold Time, BGP router-id, other optional parameters such as Authentication criteria
  2. Keepalives - Exchanged to prevent the hold time expiring, where hold time is 0 keepalives are not sent. Keepalives are sent every 60seconds
  3. UPDATE - information on 1 path only. Multiple paths require multiple update messages. All attributes in an update refer to the path. This includes - Withdrawn routes, Path attributes, Network Layer Reachability (list of ip prefixes reachable via the path)
  4. Notification Messages - sent due to error condition being met. BGP connection is closed immediately after one of these is sent.

Tuesday, September 7, 2010

CCNP - ENTERPRISE - BGP - Synchronisation

Synchronisation rule states that a BGP router can not advertise an external neighbor destination from iBGP peers unless that route is also known via an IGP (such as EIGRP, OSPF, RIPv2 etc.)

The thinking behind this is that in the event that a router along the path to the destination is not running BGP then you don't end up with a black hole with packets getting dropped. The IGP has a route the the destination so a path will still exist.

By default, Synchronisation is switched off in Cisco IOS an and there fore BGP can advertise a route without it first being advertised by an IGP.

There are 2 situations when you can safely switch off synchronisation:

  • When you have a fully meshed iBGP topology - resulting in the destination being reached with the need of an IGP
  • When the AS is NOT an transit AS - where all destination networks are within the AS and accessible due to you having a full mesh iBGP topology.
R1(config)#router bgp 123
!This turns on synchronisation (which is disabled by default)

R1(config)#router bgp 123
R1(config-router)#no synchronization
!This turns off synchronisation

Tuesday, August 31, 2010

CCNP - ENTERPRISE - Manipulating Routing updates

So, this evening I've been battling Distribution lists and the appropriate application of them and for some reason I just couldn't get that external EIGRP route to be filtered out when redistributing in to OSPF.

After a bit of reading around I've figured out I was approaching it from the wrong end. Assume you have R1, R2, and R3. R1 is running EIGRP, R2 is performing redistribution in to EIGRP and OSPF, and OSPF is running on R3.

My error was that I was working on R2 and wondering why it was that I had configured my access-list to deny my chosen route, applied the cmd - #distribution-list 1 out ospf 1 within the #router ospf 1 process, and nothing had happened.

The problem I was encountering was that in order for OSPF to properly calculate the shortest path, all the Link-State Databases through out your area must be synchronised. As such you can't simply deny your chosen network on the redistributing router as the network would not then be in synch.

The solution was to log on to R3, the router I wanted to have the route filtered from. Create the access list to deny the chosen route (then permit any - remember the implicite deny that would otherwise take affect). I then entered #distribution-list 1 in, from within the ospf routing process and job done! My desired route if filtered out and the rest remain.

R3(config)#access-list 4 deny
R3(config)#access-list 4 permit any
R3(config)#router ospf 1
R3(config-router)#network area 0
R3(config-router)#network area 0
R3(config-router)# distribute-list 4 in

CCNP - ENTERPRISE - Routing Protocol Metrics

Continuing the theme of grouping together common attributes/characteristics (see my posts on Summarisation and Authentication), in this article I'm going to set out the Metric for each routing protocol tested on the BSCI.

RIPv2 :  Distance Vector protocol
Metric = Hop count, 1-15, with 16 being 'infinity' or unreachable

EIGRP: Advanced Distance Vector protocol
Metric = Calculation based on Bandwidth (K1), Load(K2), Delay(K3), Reliability(K4), MTU (K5) although MTU is tracked through the path to find the smallest MTU - it is NOT used in the metric calculation.

Calculation is:  Metric = 256*([K1*Bw + K2*Bw/(256-Load) + K3*Delay]*[K5/(Reliability + K4)])

 [K5/(Reliability + K4)] is disregarded if K5 = 0

Default K-values in use are K1 and K3 therefore is you use the default settings the default metric is based on Bandwidth and Delay.

OSPF: Link-State routing protocol
Metric = Cost where cost is calculated by - cost= 10000 0000/bandwith in bps

  • The Cost is an indication of the overhead required to send a packet over a specified interface. 
  • Cost is inversely affected by the bandwidth, the greater the bandwidth the lower the cost
  • The Cost of the outbound interface is used
  • To change the Cost of a given interface and therefore influence path selection you apply the command #ip ospf cost [cost value] to the outbound interface concerned
IS-IS: Link-State routing protocol
Metric = Arbitrary value between 0 -63, you decide what it means. Default value is 10

  • To fine tune IS-IS you manually assign a metric value to each interface configured for IS-IS
  • Similar to the OSPF bandwidth
  • use the cmd #isis metric [metric] [level1| level2] to change the metric and assign it the appropriate routing level.
BGP: Distance Vector exterior routing protocol
Metric - is the Multi-Exit Discriminator value.
  • the Lower the MED the better
  • Used to decided how to enter an AS
  • Default is 0
  • Optional, Non-transitive
  • Usually only shared between 2 AS's that have multiple eBGP connections with each other

Monday, August 30, 2010

CCNP - ENTERPRISE - Manipulating Routing Updates - Routing Table Codes

I got caught out during some practice questions on routing table codes. So I thought it might be an idea to list the out put from #sh ip route and just set down what they are.

Simple question to nail - given the codes [X, Y, Z] which is the least trust worthy. I should have had this down from my CCNA but there you are. You need to first understand what the code is, then cross reference it with a suitable Administrative Distance. (Check my Charts and Table page above if you're unsure of the AD for a given protocol)

Protocol code:

C - connected,
S - static,
R - RIP,
M - mobile
EX - EIGRP external,
IA - OSPF inter area
N1 - OSPF NSSA external type 1,
N2 - OSPF NSSA external type 2
E1 - OSPF external type 1,
E2 - OSPF external type 2
i - IS-IS,
su - IS-IS summary,
L1 - IS-IS level-1,
L2 - IS-IS level-2
ia - IS-IS inter area,
* - candidate default,
U - per-user static route
o - ODR,
P - periodic downloaded static route

CCNP - ENTERPRISE- BGP - Route Reflectors

A route reflector acts like a DR (for OSPF) or DIS (in IS-IS) where it acts as a central location where multiple BGP routers can peer with it. The Route Reflector is the Server and all other BGP peers are clients.

The benefit of this is that you can by pass the Full Mesh requirement the iBGP requires. Instead a Route Reflector server propagates routes to the peers. A Route Reflector advertises a route learned from one iBGP peer to another iBGP peer. (remember by it's very nature iBGP peers to not forward updates received as it is assumed the network is a Full Mesh. The use of a Route Reflector helps cuts down the admin required for a Full Mesh configuration.

If you had 10 routers, in a full mesh you'd be looking at 100 specific statements across the network. Introducing a Route Reflector cuts this down to 20 statements (11 on the RR and 9 across the remaining 9 routers).

There are rules on how a Route Reflector will propagate routes within the iBGP AS.
1) If a route is received from a non-client peer ( a BGP peer not using a route reflector), reflect to clients only
2) If a route is received from a client peer, refelct to all non-client peers AND client peers, except the originator of the route
3) If a route is received from an eBGP peer, reflect to all client and non-client peers.

Clients require no configuration at all. Process is transparent. Just configure your neighbor statements as usual.

On the Route Reflector:
R1(config)#router bgp 100
R1(config-router)#neighbor route-reflector-client
R1(config-router)#neighbor route-reflector-client
Note that when configuring the neighbor as a route reflector client the adjacency will go down and back up.

That's it. R1 will 'reflect' routes to the clients set out in the neighbor statements.

Verify your route reflector clients using:
R1#sh ip bgp neighbor
The out put will be quite lengthy but there will be a statement in the output highlighting the 'Route Reflector Client'

Sunday, August 29, 2010

CCNP - ENTERPRISE - OSPF - Network Types - Point-to-Multipoint Non-Broadcast

Point-to-Multipoint Non-Broadcast:

  • Cisco proprietary
  • Single subnet required
  • No DR/BDR elected
  • Neighbors specifically configured - this is the difference between standard and Cisco Point-to-Multipoint
R1(config)#interface Serial0
R1(config-if)#ip address
R1(config-if)#ip ospf network point-to-multipoint non-broadcast
R1(config-if)#encapsulation frame-relay
R1(config-if)#frame-relay local-dlci 100
R1(config-if)#frame-relay map ip 102
R1(config-if)#frame-relay map ip 103
R1(config-if)#frame-relay map ip 104
R1(config-if)#no shut
R1(config)#router ospf 1
R1(config-router)#network area 0

On R2:
R2(config)#interface S0/0
R2(config-if)#ip address
R2(config-if)#encapsulation frame-relay
R2(config-if)#ip ospf network point-to-multipoint non-broadcast
R2(config-if)#frame-relay local-dlci 201
R2(config-if)#frame-relay map ip 201
R2(config-if)#no shut
R2(config)#router ospf 1
R2(config-routrer)#network area 0

CCNP - ENTERPRISE - OSPF - Network Types - Broadcast


  • Cisco proprietary
  • Full mesh required
  • Single subnet required
  • DR/BDR elected
  • Timers = 10 seconds
Lacking a diagram, assume we have 4 routers in a hub and spoke design. R1 is the Hub and R2,R3, and R4 are spoke routers. Frame-relay is in use.

On R1:
R1(config)#router ospf 1
R1(config-router)#network area 0
R1(config)#int s0/0
R1(config-if)#ip address
R1(config-if)#encapsulation frame-relay
R1(config-if)#ip ospf network broadcast
R1(config-if)#ip ospf priority 10
R1(config-if)#frame-relay map ip 102 broadcast*
R1(config-if)#frame-relay map ip 103 broadcast
R1(config-if)#frame-relay map ip 104 broadcast
*broadcasts and multi-casts are now forwarded

On R2:
R2(config)#router ospf 1
R2(config-router)#network area 0
R2(config)#int s0/0
R2(config-if)#ip address
R2(config-if)#encapsulation frame-relay
R2(config-if)#ip ospf network broadcast
R2(config-if)#ip ospf priority 0
R2(config-if)#frame-relay map ip 25 broadcast
R2(config-if)#frame-relay map ip 25 broadcast
R2(config-if)#frame-relay map ip 25 broadcast

On R3:
R3(config)#router ospf 1
R3(config-router)#network area 0
R3(config)#int s0/0
R3(config-if)#ip address
R3(config-if)#encapsulation frame-relay
R3(config-if)#ip ospf network broadcast
R3(config-if)#ip ospf priority 0
R3(config-if)#frame-relay map ip 35 broadcast
R3(config-if)#frame-relay map ip 35 broadcast
R3(config-if)#frame-relay map ip 35 broadcast

On R4:
R4(config)#router ospf 1
R4(config-router)#network area 0
R4(config)#int s0/0
R4(config-if)#ip address
R4(config-if)#encapsulation frame-relay
R4(config-if)#ip ospf network broadcast
R4(config-if)#ip ospf priority 0
R4(config-if)#frame-relay map ip 45 broadcast
R4(config-if)#frame-relay map ip 45 broadcast
R4(config-if)#frame-relay map ip 45 broadcast

In the example above you should note that R1 has a priority of 10 manually configured whilst R2, R3, and R4 each have their ospf priority set to 0. This will ensure that regardless of any future configuration R1 will always be the OSPF DR.

CCNP - ENTERPRISE - OSPF - Network Types - Point-to-Point


Uses Sub-Interfaces
Sub-interfaces used to address Split-Horizon issues that can result in using a single physical interface
Separate sub-interfaces require separate subnets
No DR/BDR elections
Neighbors automatically form.

Lacking a diagram, assume we have 4 routers in a hub and spoke design. R1 is the Hub and R2,R3, and R4 are spoke routers. Frame-relay is in use.

On R1:
R1(Config)#router ospf 1
R1(config-router)#network area 0
R1(config)#int s0/0
R1(config-if)#encapsulation frame-relay
R1(config-if)#no shutdown
R1(config)#int s0/0.102 point-to-point
R1(config-if)#description Link_To_R2
R1(config-if)#ip addr
R1(config-if)#frame-relay interface-dlci 102
R1(config)#int s0/0.103 point-to-point
R1(config-if)#description Link_To_R3
R1(config-if)#ip addr
R1(config-if)#frame-relay interface-dlci 103
R1(config)#int s0/0.104 point-to-point
R1(config-if)#description Link_To_R4
R1(config-if)#ip addr
R1(config-if)#frame-relay interface-dlci 104

On R2:
R2(Config)#router ospf 1
R2(config-router)#network area 0
R2(config)#int s0/0
R2(config-if)#encapsulation frame-relay
R2(config-if)#no shutdown
R2(config)#int s0/0.25
R2(config-if)#description Link_To_R1
R2(config-if)#ip addr
R2(config-if)#frame-relay interface-dlci 25

On R3:
R3(Config)#router ospf 1
R3(config-router)#network area 0
R3(config)#int s0/0
R3(config-if)#encapsulation frame-relay
R3(config-if)#no shutdown
R3(config)#int s0/0.35
R3(config-if)#description Link_To_R1
R3(config-if)#ip addr
R3(config-if)#frame-relay interface-dlci 35

On R4:
R4(Config)#router ospf 1
R4(config-router)#network area 0
R4(config)#int s0/0
R4(config-if)#encapsulation frame-relay
R4(config-if)#no shutdown
R4(config)#int s0/0.45
R4(config-if)#description Link_To_R1
R4(config-if)#ip addr
R4(config-if)#frame-relay interface-dlci 45

CCNP - ENTERPRISE- OSPF - Network Types - Point-to-Multipoint


  • Full mesh not required
  • Allows for NBMA networking
  • No DR/BDR elections
  • Timers = 30 seconds /120 seconds
  • Do not need to manually configure neighbor statements
  • Specific networks advertised through out.
    Lacking a diagram, assume we have 4 routers in a hub and spoke design. R1 is the Hub and R2,R3, and R4 are spoke routers. Frame-relay is in use.

    On R1:
    R1(config)#router ospf 1
    R1(config-router)#network area 0
    R1(config)#int s0/0
    R1(config-if)#ip address
    R1(config-if)#encapsulation frame-relay
    R1(config-if)#ip ospf network point-to-multipoint
    R1(config-if)#frame-relay map ip 102^
    R1(config-if)#frame-relay map ip 103
    R1(config-if)#frame-relay map ip 104
    ^ maps to DLCI 102 - this is locally significant.

    On R2:
    R2(config)#router ospf 1
    R2(config-router)#network area 0
    R2(config)#int s0/0
    R2(config-if)#ip address
    R2(config-if)#encapsulation frame-relay
    R2(config-if)#ip ospf network point-to-multipoint
    R2(config-if)#frame-relay map ip 25
    R2(config-if)#frame-relay map ip 25
    R2(config-if)#frame-relay map ip 25

    On R3:
    R3(config)#router ospf 1
    R3(config-router)#network area 0
    R3(config)#int s0/0
    R3(config-if)#ip address
    R3(config-if)#encapsulation frame-relay
    R3(config-if)#ip ospf network point-to-multipoint
    R3(config-if)#frame-relay map ip 35
    R3(config-if)#frame-relay map ip 35
    R3(config-if)#frame-relay map ip 35

    On R4:
    R4(config)#router ospf 1
    R4(config-router)#network area 0
    R4(config)#int s0/0
    R4(config-if)#ip address
    R4(config-if)#encapsulation frame-relay
    R4(config-if)#ip ospf network point-to-multipoint
    R4(config-if)#frame-relay map ip 45
    R4(config-if)#frame-relay map ip 45
    R4(config-if)#frame-relay map ip 45

    Note - the lack of manually assigned neighbor statements and the fact you still need to configure your frame-relay maps.
    -Also do not forget the ip ospf network point-to-multipoint cmd.

    CCNP - ENTERPRISE - OSPF - Network Types - NBMA

    Non-Broadcast Multi-Access :
    • Typically a full mesh topology (each router has a link to every one router). 
    • Is the default network configuration for Frame Relay/ATM
    • Neighbors are statically configured (see config below)
    • Must use a single subnet to link all the interfaces in the mesh
    • a DR/BDR is elected
    • Hello Timers = 30 seconds Hello/120 seconds hold down
    Note: With Frame Relay, when adding a new network behind one of the branch routers (not a DR router) it will be advertised out via OSPF but not actually pingable.
    - to resolve this issue you need to manually add a Frame-Relay map to the outgoing interface on the router the new network is sat behind and on the DR.

    Lacking a diagram, assume we have 4 routers in a hub and spoke design. R1 is the Hub and R2,R3, and R4 are spoke routers. Frame-relay is in use.

    On R1:
    R1(config)#router ospf 1
    R1(config-router)#network area 0
    R1(config)#int s0/0
    R1(config-if)#ip address
    R1(config-if)#encapsulation frame-relay
    R1(config-if)#frame-relay map ip 102^
    R1(config-if)#frame-relay map ip 103
    R1(config-if)#frame-relay map ip 104

    *when you use this cmd with out setting a neighbor priority the default is 1
    ^ maps to DLCI 102 - this is locally significant.

    On R2:
    R2(config)#router ospf 1
    R2(config-router)#network area 0
    R2(config-router)#neighbor priority 10* 
    R2(config)#int s0/0
    R2(config-if)#ip address
    R2(config-if)#encapsulation frame-relay
    R2(config-if)#frame-relay map ip 25
    R2(config-if)#frame-relay map ip 25
    R2(config-if)#frame-relay map ip 25

    *when you use this cmd with the priority cmd the highest will be elected DR.

    On R3:
    R3(config)#router ospf 1
    R3(config-router)#network area 0
    R3(config-router)#neighbor priority 10
    R3(config)#int s0/0
    R3(config-if)#ip address
    R3(config-if)#encapsulation frame-relay
    R3(config-if)#frame-relay map ip 35
    R3(config-if)#frame-relay map ip 35
    R3(config-if)#frame-relay map ip 35

    On R4:
    R4(config)#router ospf 1
    R4(config-router)#network area 0
    R4(config-router)#neighbor priority 10
    R4(config)#int s0/0
    R4(config-if)#ip address
    R4(config-if)#encapsulation frame-relay
    R4(config-if)#frame-relay map ip 45
    R4(config-if)#frame-relay map ip 45
    R4(config-if)#frame-relay map ip 45

    You'll note that on the hub router, R1, each DLCI is different. This is because it is sending packets to 3 different routers. On the spoke routers, R2, R3, R4, the DLCI is the same. This is because they are directing all traffic to R1 for all destinations and therefore use the same outbound link. Resulting in the same DLCI being allocated. (if you follow me?).

    Saturday, August 28, 2010

    CCNP - ENTERPRISE - OSPF - Network types

    OSPF Network types. This is not 'Area types where we are looking at making the OSPF process more efficient'. This is the 'nature of the network that OSPF is running on'.

    There are generally 3 broad types of network that OSPF can be used within:
    1) Broadcast Access - this is a multi-access network. Ethernet.
    2) Point-To-Point - Such as lease lines, T1/T3 connections, ISDN.
    3) Non-Broadcast Multi-Access (NBMA) - Any WAN config represented by a Cloud, Frame-Relay, ATM etc

    In your lab work you'll generally configure either option 1, fastethernet links in to a switch and all your routers connected to the switch resulting in DR/BDR elections. Alternatively you'll have configured option 2, directly connecting your routers using serial cables resulting in there being only one device at either end, thereby eliminating the need for DR/BDR operations.

    It is here that we arrive at our third option and the one I struggle with. Just keep telling yourself 'if it was easy it wouldn't be worth achieving'

    There are 5 modes of NMBA network:
    1) NBMA - RFC 2328 standard
    2) Point-To-Multipoint - RFC 2328 standard
    3) Point-To-Point - Cisco propriortary
    4) Broadcast - Cisco propriortary
    5) Point-To-Multipoint Non-Broadcast - Cisco proprietary

    The next articles cover these network modes.

    CCNP - ENTERPRISE - OSPF - Path Selection

    When OSPF considers which path to take to a destination if there are multiple paths available it will first consider the destination advertised from the lowest LSA (where type1 LSA = Low and type5 LSA =high)

    OSPF will the select the path with the lowest cost.