Aggregate with a Subset of the More Specific Routes

In figure 10-12, we will show how AS3 can utilize a combination of aggregation and more specific routes to influence what link AS1 uses to reach AS3's networks. RTA will send over its direct link to AS1 the aggregate 172.16.0.0/16 plus the more specific routes 172.16.1.0/24, 172.16.10.0/24, and 172.16.65.0/26. RTF will send over its direct link to AS3, the aggregate 172.16.0.0/16, plus the more specific route 172.16.220.0/24 only. As a result, AS1 is forced to reach 172.16.220.0/24 via RTF and all the other routes in AS3 via RTA.

Figure 10-12  BGP aggregates with subset of specific routes.

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 suppress-map SUPPRESS
     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 172.16.220.0 0.0.0.255
    access-list 1 deny any

    route-map SUPPRESS permit 10
     match ip address 1

The suppress-map is another form of route-map that can be used to indicate the more specific routes to be suppressed or the more specific routes to be allowed. When a route is permitted through the suppress map, the route is suppressed. If the route is not permitted (denied), the route is not suppressed—that is, allowed. Note that the deny logic here does not prevent the route from being advertised; rather, it prevents it from being suppressed.

In RTA's configuration, we have used a suppress map called SUPPRESS that will prevent 172.16.220.0/24 from being advertised and enable all other routes. As a result, RTA will announce the aggregate 172.16.0.0/16, plus the more specific routes 172.16.1.0/24, 172.16.10.0/24, and 172.16.65.0/26. The following is RTA's BGP table; note how the suppressed entries have the "s" at the far left.

    RTA#sh ip bgp
    BGP table version is 17, local router ID is 172.16.2.254
    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
    * i172.16.0.0       172.16.1.2                  100      0 i
    *>                  0.0.0.0                          32768 i
    *> 172.16.1.0/24    0.0.0.0                0         32768 i
    *> 172.16.10.0/24   172.16.1.2            20         32768 i
    *> 172.16.65.0/26   172.16.1.2            20         32768 i
    s> 172.16.220.0/24  0.0.0.0                0         32768 i
    * i192.68.10.0      172.16.1.2             0    100      0 1 i
    *>                  172.16.20.1                          0 1 i
    * i192.68.11.0      172.16.1.2             0    100      0 1 i
    *>                  172.16.20.1            0             0 1 i

On the other hand, RTF will use a similar logic to advertise the aggregate, plus the more specific route 172.16.220.0/24. RTF's configuration will include a suppress map called ALLOW that allows the prefix 172.16.220.0/24 and suppresses everything else. As a result, AS1 will be forced to use RTF to reach 172.16.220.0/24. The naming of the suppress maps SUPPRESS and ALLOW reflects the main function of the route map. In RTA's configuration, it made more sense to suppress a specific entry and allow the rest because the number of routes to be allowed is large. In RTF's configuration, it made sense to allow a specific entry and suppress the rest because the number of routes to be suppressed is large.

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 suppress-map ALLOW
     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 ^$
    access-list 1 deny 172.16.220.0 0.0.0.255
    access-list 1 permit any

    route-map ALLOW permit 10
     match ip address 1

The preceding configuration of RTF will allow the aggregate 172.16.0.0/16 and the more specific route 172.16.220.0/24 to be advertised; all other more specific routes will be suppressed. The following is RTF's BGP table:

        RTF#sh ip bgp
        BGP table version is 17, local router ID is 192.68.5.1
        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       0.0.0.0                          32768 i
        * i                 172.16.2.254                100      0 i
        s> 172.16.1.0/24    0.0.0.0                0         32768 i
        s i                 172.16.2.254           0    100      0 i
        s> 172.16.10.0/24   0.0.0.0                0         32768 i
        s i                 172.16.2.254          20    100      0 i
        s> 172.16.65.0/26   0.0.0.0                0         32768 i
        s i                 172.16.2.254          20    100      0 i
        *> 172.16.220.0/24  172.16.1.1            20         32768 i
        *> 192.68.10.0      192.68.5.2             0             0 1 i
        * i                 172.16.20.1                 100      0 1 i
        *> 192.68.11.0      192.68.5.2                           0 1 i
        * i                 172.16.20.1                 100      0 1 i