Monday, March 28, 2011

CCNP - SWITCH - Create an Implementation Plan for Inter-VLAN Routing

Inter-VLAN routing can be implemented in several ways. You might wish to configure an external router, use an SVI on a Layer3 (L3) switch, or use routed ports on an L3 switch.

Which ever way you choose to implement your configuration you'll need an implementation plan.

The following steps outline the considerations you need to make with regards to using an external router:
1) Know how many VLANs need routing, their ID number, name, and which ports will connect to the router
2) On the router, work out your sub-interface requirements. You'll have a single link running from the router to the switch configured as a trunk. On the router side you'll then need a sub interface for each VLAN you're wishing to provide inter-VLAN routing for. Number each sub-interface logically i.e. - the same as the VLAN it represents. Ensure the encapsulation method is the same across each sub-interface and across the link to the switch. (usually Dot1q all round).
3) Decide on the Native VLAN and ensure it is the same on either end of the link.
   use the command - #int f0/0.[id]
                                #encapsulation dot1q [vlan-id] native*
4) Finally use the interface command - #encapsulation dot1q [vlan-id] - to associate a VLAN to a sub-interface

*Remember that traffic on the Native VLAN is not tagged

The following steps outline the considerations you need to make with regards to using an SVI:
1) On your L3 switch identify the VLANs that require a default gateway.
2) For any SVI's not already present on your L3 switch you will need to create then. As such you will need to decide on suitable numbering for the SVI (should be the VLAN ID number) plus an IP address to associate with it. Don't forget to No Shutdown the interface.
3) To perform L3 routing functions you need to set the L3 switch to be able to perform the routing. To achieve this use the global command - #ip routing - this will enable to switch to route between your VLANs
4) Define any appropriate dynamic routing protocols. Typically required if you are configuring a larger enterprise network that may be subject to change. You can deploy RIP, EIGRP, OSPF which ever you feel is appropriate.
5) Finally with the information above gathered consider if you require any given SVI to be excluded from contributing to the SVI state Up-Down calculation. Do this using the 'Autostate' feature

The following steps outline the considerations you need to make with regards to using an Routed port:
1) Plan your addressing for each VLAN and which port is going to be a routed port.
2) The remaining steps are much like the SVI. Configure your routed port using the command #no switchport - which removes L2 capability. Apply an IP address and enable the port via the #no shutdown command.

 REMEMBER - when you are under pressure and your links are not up use the command - #sh ip int brief - to make sure you have enable the port concerned. If you see 'Administratively Down' against your problem port go back and do a #'no shutdown - Obvious I know but you wouldn't believe how many times I've seen students scratching their heads in a lab because of this.

3) To allow inter-VLAN routing use the global command - #ip routing - then apply any dynamic routing protocols as required.

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