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Community Member

Core/Distribution Routing

We currently have the setup that is attached to this post. Currently due to certain bandwidth limits by our providers we are statically using routemaps routing traffic between the distribution and core routers to each provider for each of our network segments.

Now we want to change this and run a protocol between the core and distribution layer so all clients on the access layer will have the best path outwards to the internet.

Is it possible or even advisable to run BGP on the distribution layer as well so that it will know which core router A or B to send the traffic to? My thinking is that way it will have the same routing table as the core routers and be able to plot the best path.

Also can someone point me to ANY whitepapers/configuration guides, etc that can give me detailed commands/listing of how to implement a network design like this, core/distribution/access.

Thank you.

Community Member

Re: Core/Distribution Routing

Unless you have some specific reason why the choice of next outbound hop (i.e. which core switch) from the distribution switches actually matters, I'd stick to using an IGP at the distribution layer and only feed it summary routing information (maybe only default routes) from BGP at the core.

I ignorantly make this suggestion with little knowledge of what you're really trying to achieve, or what your entire environment looks like, but while you could extend iBGP into the distribution layer, does the associated resource consumption and complexity make it worthwhile?

In the interest of simplicity you should be able to keep an IGP on the inside (behind the core) and let each core switch make a decision on which way to route outbound traffic - either via its own Internet connection or via the other core switch (A or B).

Cisco have plenty of documentation on campus design, edge routing and EGP-IGP redistribution - the search engine on will give you more pointers than I can.

Community Member

Re: Core/Distribution Routing

Yes, your idea is possible.

The shortest AS_PATH in normal.

You can see How the Best Path Algorithm Works:

Community Member

Re: Core/Distribution Routing

With that diagram.

I would run BGP on all 4 routers.

Since RouterA and RouterB would have providers hooked to it, and full routing tables. They would pass that to Router C and D as well so they can decide which route to go out directly without first going to A then to B, this way they can skip ahead directly to the prope router.

Question: Would I need to setup any filter lists on C and D in regards to bgp for making sure C or D is not readvertising known routes back to A/B? I'm thinking I just need to advertise the directly connected routes on C/D back to A/B. correct?

Would their be a filter list between C/D? Or should it be the full routing table so that if A or B goes down, then Router C/D would have an alternate path back to the Core?



Re: Core/Distribution Routing

I agreed to enable routing protocol at all switches due to multiple paths to remote. What I suggest not to use filter but to use local preference at BGP to fine tune the preferred path for different subnet & users manually. However, you have to determine the priority of each paths to reach the remote.

You can also make the routes from providers to make it as preferred or less preferred path for particular destination.

Hope this helps.

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