Loss of information inside aggregates

When an Aggregate is sent we lose the finer granularity of the actual AS Paths to the Aggregated prefixes.

Using AS_SET we can advertise the AS path information that would normally not be sent in an aggregate advertisement.

We setup our network as shown in Figure 11-13 of Internet Routing architectures in chapter 11.

The only new command we encounter is an option appended to the aggregate-address command under Router BGP configuration.

RTA(config)#aggregate-address 192.68.0.0 255.255.0.0 as-set

Cleaning up we run the following commands on routers RTA & RTF:

RTA(config)#no route-map SUPPRESS/ALLOW
RTA(config)#no access-list 1
RTA(config)#no aggregate-address 172.16.0.0 255.255.0.0

Before we use the as_set option, just add the aggregate-address command and view the BGP table on router RTK.

RTA(config)#aggregate-address 192.68.0.0 255.255.0.0

RTK>sh ip bgp
BGP table version is 18, local router ID is 172.16.220.2
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
*> 172.16.1.0/24 172.16.220.1 0 0 3 i
*> 172.16.20.0/24 172.16.220.1 0 0 3 i
*> 172.16.65.0/26 172.16.220.1 0 3 i
* 172.16.220.0/24 172.16.220.1 0 0 3 i
*> 0.0.0.0 0 32768 i
*> 192.68.0.0/16 172.16.220.1 0 0 3 i
*> 192.68.5.0 172.16.220.1 0 3 i
*> 192.68.10.0 172.16.220.1 0 3 2 i
*> 192.68.11.0 172.16.220.1 0 3 1 i

Notice, the 192.68.0.0 aggregate address only shows AS 3 in the path, when in fact the more specific 192.68.0.0 networks reach into AS 1 & AS 2.

Apply the as_set option and review the changes.

RTA(config)#aggregate-address 192.68.0.0 255.255.0.0 as-set

RTK>sh ip bgp
BGP table version is 19, local router ID is 172.16.220.2
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
*> 172.16.1.0/24 172.16.220.1 0 0 3 i
*> 172.16.20.0/24 172.16.220.1 0 0 3 i
*> 172.16.65.0/26 172.16.220.1 0 3 i
* 172.16.220.0/24 172.16.220.1 0 0 3 i
*> 0.0.0.0 0 32768 i
*> 192.68.0.0/16 172.16.220.1 0 0 3 {2,1} i
*> 192.68.5.0 172.16.220.1 0 3 i
*> 192.68.10.0 172.16.220.1 0 3 2 i
*> 192.68.11.0 172.16.220.1 0 3 1 i
RTK>

You will notice how the aggregate 192.68.0.0/16 changes to include a SET{2,1} within its path information.This indicates that the aggregate actually summarizes routes that have passed via ASs 1 or 2. The AS_SET information becomes important in avoiding routing loops because it keeps an indication of where the route has been.

Given that the aggregate with the AS_SET contains information about each individual route, that is summarized, changes in the individual route will cause the aggregate to be updated. In the example if 192.68.11.0/24 goes down, the path information of the aggregate will change from 3{3,1} to 3 2, and the aggregate will be updated. It the aggregate summarizes 100's or 1000's of routes , the aggregate will constantly be oscillating if the rotes forming the aggregate are unstable.