BGP route aggregation

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By Jeremy Stretch

Route aggregation (or summarization) with BGP is extremely flexible. Unfortunately, this flexibility leads to difficulty remembering all the different tweaks which can be used to manipulate aggregate routes. In this article, we'll look at all of the aggregation configuration parameters and how each can be used to influence the advertisement of an aggregate route.


Contents

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First let's have a look at our sample lab:

Image:BGP_route_aggregation_lab.png

Routers one and two each advertise four /24 routes to their respective networks. These routes are advertised to R3, which in turn advertises them to R4. Inspecting the BGP table on R4, we can see four routes each from AS 10 and 20. 

R4# show ip bgp
BGP table version is 9, local router ID is 10.0.0.10
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 172.16.0.0/24    10.0.0.9                               0 30 10 ?
*> 172.16.1.0/24    10.0.0.9                               0 30 10 ?
*> 172.16.2.0/24    10.0.0.9                               0 30 10 ?
*> 172.16.3.0/24    10.0.0.9                               0 30 10 ?
*> 172.16.4.0/24    10.0.0.9                               0 30 20 ?
*> 172.16.5.0/24    10.0.0.9                               0 30 20 ?
*> 172.16.6.0/24    10.0.0.9                               0 30 20 ?
*> 172.16.7.0/24    10.0.0.9                               0 30 20 ?

Since R4 has only a single path to all of these subnets, maintaining eight independent routes is unnecessary. We can implement aggregation at R3 to optimize the BGP table on R4.

Creating an Aggregate Route

The aggregate-address command can be used to generate a summary route on R3. We'll create a 172.16.0.0/21 summary to include all more-specific routes while not leaving any overlap. 

R3(config-router)# aggregate-address 172.16.0.0 255.255.248.0

Let's verify that R4 sees the new aggregate route: 

R4# show ip bgp

   Network          Next Hop            Metric LocPrf Weight Path
*> 172.16.0.0/24    10.0.0.9                               0 30 10 ?
*> 172.16.0.0/21    10.0.0.9                 0             0 30 i
*> 172.16.1.0/24    10.0.0.9                               0 30 10 ?
*> 172.16.2.0/24    10.0.0.9                               0 30 10 ?
*> 172.16.3.0/24    10.0.0.9                               0 30 10 ?
*> 172.16.4.0/24    10.0.0.9                               0 30 20 ?
*> 172.16.5.0/24    10.0.0.9                               0 30 20 ?
*> 172.16.6.0/24    10.0.0.9                               0 30 20 ?
*> 172.16.7.0/24    10.0.0.9                               0 30 20 ?

Suppressing More-Specific Routes

BGP route aggregation differs from IGP summarization in that the default behavior is to continue advertising all the more-specific routes of summarized by an aggregate. As we can see here, the aggregate was injected without affecting any of the more-specific routes. We can suppress all the summarized routes by recreating the aggregate route, this time appending the summary-only keyword. 

R3(config-router)# aggregate-address 172.16.0.0 255.255.248.0 summary-only

Checking on R4 again, we can see all the more-specific routes have been withdrawn, leaving only the aggregate. Note that the aggregate appears to originate in AS 30 (to which the aggregating router belongs). 

R4# show ip bgp

   Network          Next Hop            Metric LocPrf Weight Path
*> 172.16.0.0/21    10.0.0.9                 0             0 30 i
Retrieved from "http://wiki.nil.com/BGP_route_aggregation"