Sunday, August 29, 2010

BSCI - 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.

Configuration:
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 192.168.1.0 0.0.0.255 area 0
R1(config-router)#neighbor 192.168.1.2*
R1(config-router)#neighbor 192.168.1.3
R1(config-router)#neighbor 192.168.1.4
!
R1(config)#int s0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#encapsulation frame-relay
R1(config-if)#frame-relay map ip 192.168.1.2 102^
R1(config-if)#frame-relay map ip 192.168.1.3 103
R1(config-if)#frame-relay map ip 192.168.1.4 104
!

*when you use this cmd with out setting a neighbor priority the default is 1
^ maps 192.168.1.2 to DLCI 102 - this is locally significant.

On R2:
R2(config)#router ospf 1
R2(config-router)#network 192.168.1.0 0.0.0.255 area 0
R2(config-router)#neighbor 192.168.1.1 priority 10* 
!
R2(config)#int s0/0
R2(config-if)#ip address 192.168.1.2 255.255.255.0
R2(config-if)#encapsulation frame-relay
R2(config-if)#frame-relay map ip 192.168.1.1 25
R2(config-if)#frame-relay map ip 192.168.1.3 25
R2(config-if)#frame-relay map ip 192.168.1.4 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 192.168.1.0 0.0.0.255 area 0
R3(config-router)#neighbor 192.168.1.1 priority 10
!
R3(config)#int s0/0
R3(config-if)#ip address 192.168.1.3 255.255.255.0
R3(config-if)#encapsulation frame-relay
R3(config-if)#frame-relay map ip 192.168.1.1 35
R3(config-if)#frame-relay map ip 192.168.1.2 35
R3(config-if)#frame-relay map ip 192.168.1.4 35
!

On R4:
R4(config)#router ospf 1
R4(config-router)#network 192.168.1.0 0.0.0.255 area 0
R4(config-router)#neighbor 192.168.1.1 priority 10
!
R4(config)#int s0/0
R4(config-if)#ip address 192.168.1.4 255.255.255.0
R4(config-if)#encapsulation frame-relay
R4(config-if)#frame-relay map ip 192.168.1.1 45
R4(config-if)#frame-relay map ip 192.168.1.2 45
R4(config-if)#frame-relay map ip 192.168.1.3 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?).

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