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Developing Policy Route Maps

I am new to this level and in search of guidance. Any insights would be greatly appreciated.

I am working on splitting a class C between 2 routers running BGP that are homed to the same AS. I believe that I want to advertise the class C out both for redundancy but want to direct half the outbound traffic to one site and half to the other. I know it could be done with static routes but that's just ugly. Policy route maps appear to be the way to go since they are based on source vs. destination. My neighbor in the AS will announce #.#.#.0 / 25 as

available via it's link to site 1 and #.#.#.128 / 25 available via the link to site 2 to avoid asymmetrical routing. This will probably be done with a metric from their end, again so that in the event of a link going down traffic will still find it's way.

The end result I hope to achieve is:

#.#.#.128 /25 is advertised from router1 as available via t1(a);

#.#.#.0 /25 is also advertised from router1 as available via t1(a) but with a higher weight to support failover?;

#.#.#.128 / 25 traffic enters and exits router1 to site1 via t1(a)

#.#.#.0 /25 is advertised from router2 as available via t1(b);

#.#.#.128 /25 is also advertised from router2 as available via t1(b) but with a higher weight to support failover?;

#.#.#.0 / 25 traffic enters and exits router2 to site2 via t1(b)

I hope this makes some sense. As a newbie the syntax of route-maps is another language. I believe this can be done with a combination of distribute list and ACL to restrict the advertisement or add weight to the appropriate subnet and a policy route map that directs traffic based on it's source subnet to the appropriate router. It should be noted that the class C has been further subnetted behind each router. I have consulted the Cisco web site for configuration recipes but haven't come across what appeared to be an applicable one.

Again thanks to any respondents for your insights.

1 REPLY
New Member

Re: Developing Policy Route Maps

I think you have 2 related concepts confused.

The first is Policy routing. You are correct that Policy routing can make forwarding decisions based on source and destination addresses. However, it is really a very sophisticated static route and affects only the forwarding engine on the router it is applied to. Policy routing is applied prior to any normal L3 forwarding. Packets that do not match the Policy are forwarded normal, using the routing table.. Policy routing is typically implemented using route maps. Be ware that packets generated by the router are not viewed by policy routing the same as packets transiting the router, so may not take the expected route.

BGP policy is the overall umbrella on which routes to advertise, using which BGP metrics. This is not part of the forwarding engine. In your case, I would advertise the /24 out of both interfaces, and a /25 with a better MED or shorter as path out the interface I wanted the traffic to use. Setting BGP parameters also uses route maps.

I think if you distinguish a difference between forwarding and routing, the issues will become clear.

Regards;

Doug

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