Forming the Aggregate Based on a Subset of Specific Routes

Having control over which individual prefixes form the aggregate is very useful in deciding which attributes the aggregate is going to carry. In the previous example, if we could exclude prefix 192.68.11.0/24 from being part of the prefixes that form the aggregate, the aggregate would not inherit the "no-export" community attribute.

The advertise map is yet another form of a route map that enables you to form the aggregated route based on a limited selection of the more specific routes.

In figure 10-14, RTA and RTF are getting routes 192.68.11.0/24 and 192.68.10.0/24 from ASs 1 and 2, respectively. If RTA and RTF are to aggregate these routes into 192.68.0.0/16 using the "as-set" option, the aggregate cannot be sent back to either AS1 or AS2 because it contains {1 2} in the AS_path information. This is due to the normal BGP behavior in detecting loops.

Figure 10-14  Aggregation with advertise-map.

Assume that the required behavior is to have the aggregate 192.68.0.0/16 be sent back to AS1 and not to AS2. The solution is to have AS1 not be part of the AS_path of the aggregate; then, AS1 will not drop the aggregate. This could be achieved by having RTA and RTF form the aggregate based on the 192.68.10.0/24 prefix only, using the advertise-map option.

RTA's configuration would be set as in the following example. RTF would have the same relative configuration.

RTA configuration:

    router bgp 3
     no synchronization
     network 172.16.1.0 mask 255.255.255.0
     network 172.16.10.0 mask 255.255.255.0
     network 172.16.65.0 mask 255.255.255.192
     aggregate-address 192.68.0.0 255.255.0.0 as-set advertise-map
     SELECT_MORE_SPECIF_ROUTES
     neighbor 172.16.1.2 remote-as 3
     neighbor 172.16.1.2 update-source Loopback0
     neighbor 172.16.20.1 remote-as 1
     neighbor 172.16.20.1 filter-list 10 out
     no auto-summary

    ip as-path access-list 10 permit ^$
    
    access-list 1 permit 192.68.10.0 0.0.0.255

    route-map SELECT_MORE_SPECIF_ROUTES permit 10
     match ip address 1

By permitting prefix 192.68.10.0/24, the above advertise map causes RTA to base its aggregate calculation on 192.68.10.0/24 only. Thus, 192.68.11.0/24 is not included in the formation of the aggregate.

The following show ip bgp command output illustrates that the path information of the aggregate is now 2 and not {1 2}. This means that the aggregate can now be advertised to AS1 because the AS_path does not include AS1. AS2 will not be able to receive the aggregate.

    RTA#show ip bgp 192.68.0.0
    BGP routing table entry for 192.68.0.0 255.255.0.0, version 31
    Paths: (2 available, best #2, advertised over IBGP)
      2 (aggregated by 3 192.68.5.1)
        172.16.1.2 from 172.16.1.2 (192.68.5.1)
          Origin IGP, localpref 100, valid, internal, atomic-aggregate
      2 (aggregated by 3 172.16.2.254)
        0.0.0.0
          Origin IGP, localpref 100, weight 32768, valid, aggregated,
           local,
          atomic-aggregate, best

Looking Ahead

The BGP attributes are the basic elements in interdomain network design. Combining and manipulating different attributes will result in a unique routing policy for your autonomous system. The next chapter takes what you have learned so far and goes further in showing implementations for major design problems facing every network. The chapter also shows examples of controlling Internet stabilty by using route dampening.