The output of RTC's BGP table shows that the only prefix that was learned from RTA is the aggregate 172.16.0.0/16. Because RTF is also performing the same aggregation, RTC will also learn an aggregate that is originated from RTF (next hop RTD via AS2).

    RTC#sh ip bgp
    BGP table version is 22, local router ID is 192.68.11.1
    Status codes: s suppressed, d damped, h history, * valid, > best,
    i - internal Origin codes: i - IGP, e - EGP, ? - incomplete
       Network          Next Hop          Metric LocPrf Weight Path
    *> 172.16.0.0       172.16.20.2                          0 3 i
    *                   192.68.6.1                           0 2 3 i
    *> 192.68.10.0      192.68.6.1             0             0 2 i
    *> 192.68.11.0      0.0.0.0                0         32768 i

Looking at the specific 172.16.0.0/16 aggregate entry, the following display provides more information about the aggregate itself. Note the presence of the "atomic-aggregate" attribute, which indicates that the prefix 172.16.0.0/16 is an aggregate. Also note the presence of the "aggregated by 3 192.68.5.1" and "aggregated by 3 172.16.2.254" statements, which represent the "aggregator" attribute. The "aggregator" attribute (Chapter 5) indicates the AS number and Router ID of the router who originated the aggregate; in this case, AS3 and the Router IDs of RTA and RTF.

     RTC#sh ip bgp 172.16.0.0
     BGP routing table entry for 172.16.0.0/16, version 22
     Paths: (2 available, best #1, advertised over EBGP)
       3, (aggregated by 3 172.16.2.254)
         172.16.20.2 from 172.16.20.2 (172.16.2.254)
           Origin IGP, valid, external, atomic-aggregate, best
       23, (aggregated by 3 192.68.5.1)
         192.68.6.1 from 192.68.6.1 (192.68.10.1)
           Origin IGP, valid, external, atomic-aggregate

Aggregates can also be generated by using static routes. This is illustrated in the following RTA and RTF configuration.

RTA configuration:

    router bgp 3
     no synchronization
     network 172.16.0.0
     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 route 172.16.0.0 255.255.0.0 null0
    ip as-path access-list 10 permit ^$

RTF configuration:

    router bgp 3
     no synchronization
     network 172.16.0.0
     neighbor 172.16.2.254  remote-as 3
     neighbor 172.16.2.254 next-hop-self
     neighbor 192.68.5.2 remote-as 2
     neighbor 192.68.5.2 filter-list 10 out
     no auto-summary

    ip route 172.16.0.0 255.255.0.0 null0
    ip as-path access-list 10 permit ^$

The preceding configuration places a static instance of 172.16.0.0/16 in the routing table. Note that the static entry is pointing to null0 (pit bucket). If RTA or RTF have no knowledge of the more specific routes of 172.16.0.0, then traffic will be dropped. This is to prevent loops in case RTA or RTF are themselves following defaults to their providers (see "Aggregation and Loops," in Chapter 3, "Handling IP Address Depletion").

Aggregate Plus More Specific Routes

In some cases, more specific routes, in addition to the aggregate, need to be passed (leaked) to a neighboring AS. This is usually done in ASs multihomed to a single provider. An AS (the provider) that gets the more specific routes would be able to make a better decision about which way to take to reach the route. (You have already seen how an AS receiving different metrics can direct the traffic accordingly.)

In figure 10-11, AS3 is multihomed to a single provider AS1. RTA and RTF in AS3 can send the aggregate 172.16.0.0/16 and the more specific routes toward AS1. The following configuration of RTA and RTF will do so:

Figure 10-11  More BGP aggregation scenarios.

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
     network 172.16.220.0 mask 255.255.255.0
     aggregate-address 172.16.0.0 255.255.0.0
     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 ^$

RTF 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
     network 172.16.220.0 mask 255.255.255.0
     aggregate-address 172.16.0.0 255.255.0.0
     neighbor 172.16.2.254  remote-as 3
     neighbor 172.16.2.254 next-hop-self
     neighbor 192.68.5.2 remote-as 1
     neighbor 192.68.5.2 filter-list 10 out
     no auto-summary

    ip as-path access-list 10 permit ^$