Route Redistribution


Route Redistribution

Configs : Route Redistribution

Net_file : route_redistribution_net_file

I must confess I have been doing a lot of route redistribution recently and the more I do it the more I realise how tricky it can be. Above is a simple topology where we are going to have multiple points of redistribution on routers R3 and R4

The first issue is caused by Administrative Distance (AD), which determines which routing protocol is going to be trusted. OSPF AD 110, RIP AD 120 where in this case OSPF will be preferred as it has lowest AD.

So let’s perform mutual redistribution on R3 and R4. The following networks will be injected from RIP into OSPF: 10.1.1.0/24,10.1.2.0/24, 10.1.3.0/24, 10.1.4.0/24, and 10.1.5.0/24.

R3(config-router)#do sh run  | se router
router ospf 1
log-adjacency-changes
redistribute rip subnets
network 192.168.1.3 0.0.0.0 area 0
router rip
version 2
redistribute ospf 1 metric 5
network 10.0.0.0
no auto-summary

R4(config-router)#do sh run | se router
router ospf 1
log-adjacency-changes
redistribute rip subnets
network 192.168.5.4 0.0.0.0 area 0
router rip
version 2
redistribute ospf 1 metric 5
network 10.0.0.0
no auto-summary

R3 and R4 will eventually start exchanging redistributed RIP networks .Because OSPF’s AD is lower than RIP’s, R3 and R4 will prefer the sub-optimal path through OSPF to reach the redistributed RIP routes what essentially creates a routing loop. R3 prefers OSPF path via R1 to reach the 10.x.x.x networks (except for 10.1.1.0/24 as it is directly connected to it) and vice versa.

To fix this problem we will use 2 different methods and distribute-list will be the first one :
R3 config:

R3(config)# access-list 10 deny 10.1.2.0 0.0.0.255
R3(config)# access-list 10 deny 10.1.3.0 0.0.0.255
R3(config)# access-list 10 deny 10.1.4.0 0.0.0.255
R3(config)# access-list 10 deny 10.1.5.0 0.0.0.255
R3(config)# access-list 10 permit any
R3(config)# router ospf 1
R3(config-router)# distribute-list 10 in fastethernet0/0

R4 config would be similar.

Another method is the distance command to adjust the AD of specific routes where in this case can accomplished two ways:
Lowering the AD of the local RIP-learned routes
• Raising the AD of the external OSPF-learned routes

To force the RIP routes to be preferred, R3 config :

R3(config)# access-list 10 permit 10.1.2.0 0.0.0.255
R3(config)# access-list 10 permit 10.1.3.0 0.0.0.255
R3(config)# access-list 10 permit 10.1.4.0 0.0.0.255
R3(config)# access-list 10 permit 10.1.5.0 0.0.0.255
R3(config)# access-list 10 deny any
R3(config)# router rip
R3(config-router)# distance 70 10.1.1.0 0.0.0.255 10

Under RIP process an administrative distance of 70 is applied to updates from routers on the 10.1.1.0 network, for the specific networks matching access-list 10.

R4 configuration would be similar.

Another solution is to raise AD of the external OSPF routes.
R3(config)# router ospf 1
R3(config-router)# distance ospf external 240

Routing loop is only one of the issue with this topology , route feedback is another problem. OSPF routes redistributed into RIP on R3 will eventually reach R4, and then be redistributed again back into OSPF. This is a perfect example of route feedback and suboptimal routing.

Route tags is a great solution.
R3(config)# route-map OSPF_RIP deny 5
R3(config-route-map)# match tag 120
R3(config-route-map)# route-map OSPF_RIP permit 10
R3(config-route-map)# set tag 110
R3(config)# router rip
R3(config)# redistribute ospf 1 route-map OSPF_RIP
R3(config)# route-map RIP_OSPF deny 5
R3(config-route-map)# match tag 110
R3(config-route-map)# route-map RIP_OSPF permit 10
R3(config-route-map)# set tag 120
R3(config)# router ospf 1
RouterC(config)# redistribute rip route-map RIP_OSPF

 

Enjoy !

Tom

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About ccie4all
Hello, and welcome to the first post of my CCIE blog This blog has got one simple goal and that is to improve our skills in Cisco Networking field so we can become best engineers on a job market. Wordpress Blog https://ccie4all.wordpress.com/ information about the changes made to Gns3 BGP , MPLS and R&S CCIE labs. In order to access and download all provided materials and receive important updates from Gns3 BGP , MPLS and R&S CCIE labs under GNS3 tab in the main header please go ahead and subscribe to https://ccie4all.wordpress.com/ ! All other posts have not been affected and can be accessed at any given time. Enjoy ! Tom

One Response to Route Redistribution

  1. Pingback: REDISTRIBUTION CCNP INE | Tom G CCIE Blog

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