Orhan Ergun 3 Comments

You need route redistribution for many reasons.

In this post,the drivers for the route redistribution but more importantly the best practices for applying route redistribution will be explained in great detail.

Redistribution allows one domain information to be leaked to another one.

This can be between the companies, same company but different routing protocol but in IS-IS case same routing protocol but different level.

Redistribution comes with its cost, in this article I will explain the drivers for route redistribution.

How really it works and best practices from the network design point of view will be covered.

  • You may have partner network which uses different routing protocol than your network, although it doesn’t have to be different protocol. Better practice is to create a BGP neighborship between the two companies and provide the reachability to each other.

By doing this, you continue to use separate IGP protocols in each site and start to use BGP for partner network routes. Biggest advantage of doing this; link flaps, route flaps, intentional or unintentional operator mistakes will not cause routing protocol convergence in your network.

Best Practice : If you have multiple points where you redistribute routes, be aware of routing loops. You should use route-map, distribute-list, route-tag, some sort of filtering mechanisms.

Best Practice : If you are running link state protocol as your IGP and you have broadcast segment, it is better to use Designated router and redistribution point ( ASBR ) as same router. In this way, you will reduce the amount of flooding.

  • You may want to redistribute default route at the Internet edge into your IGP protocol. You need to be really careful while doing this.Use filtering mechanisms to allow only default route.
  • If company is using IS-IS as an IGP and multi level of IS-IS is deployed , you may need to leak some routes from level 2 domain into level 1 domain.This operation is done by redistributing selective addresses from level 2 domain into level 1 domain.

Why would I want to leak prefixes into level 1 ?

You have to leak loopback addresses of all PE devices from level 2 into level 1 domain if MPLS is enabled in the network. ( Seamless or Unified MPLS is exception ).In that case you learn remote PE addresses and label binding through BGP.

  • Enterprise might be receiving MPLS VPN service from Service Provider and they maybe using different PE-CE routing protocol than their IGP. In this case, redistribution is necessary. All above best practices applies in this case.
  • VPN service providers need to redistribute customers routing protocol into Multi Protocol BGP if the customers is not using BGP as their PE-CE routing protocol.In this case Service Provider takes the routes from VRF, make them VPN route by adding RD and RT values and send over the SP network.

Conclusion : Avoid redistribution if you can. It is easy to create routing loops and to manage it to avoid loop, might be too complex to configure.

It’s Your Turn

Do you have any bad experience with route redistribution? Share your experience and suggestions in the comments box below.

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