Internet Routing Architectures (CISCO):Configuring Basic BGP Functions and Attributes

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Given the preceding configuration of RTA and RTF, AS1 will only be able to reach 172.16.220.0/24 via the RTD-RTF link and 172.16.1.0/24, 172.16.65.0/26, 172.16.10.0/24 via the RTC-RTA link. This is illustrated in the BGP table of RTD.

    RTD#sh ip bgp
    BGP table version is 19, local router ID is 192.68.10.1
    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
    * i172.16.0.0       192.68.6.2                  100      0 3 i
    *>                  192.68.5.1                           0 3 i
    *>i172.16.1.0/24    192.68.6.2             0    100      0 3 i
    *>i172.16.10.0/24   192.68.6.2            20    100      0 3 i
    *>i172.16.65.0/26   192.68.6.2            20    100      0 3 i
    *> 172.16.220.0/24  192.68.5.1            20             0 3 i
    *> 192.68.10.0      0.0.0.0                0         32768 i
    *>i192.68.11.0      192.68.6.2             0    100      0 i

RTD has only one choice to reach 172.16.220.0/24, and that is via the RTD-RTF link. In case of link failure, the aggregate is still advertised via both links, and the route will follow the aggregate.

In some situations, administrators require that some neighbors receive some of the specific routes already suppressed. Suppression could have been done via the summary-only parameter or the neighbor command. In this case, Cisco provides a different form of route map called the unsuppress map that is applied on a per neighbor bases. The unsuppress map allows previously suppressed routes to be advertised. If for example, RTA wanted to prevent 172.16.220.0/24 from being suppressed toward 172.16.1.2 (RTF), the following RTA router configuration would be used:

    neighbor 172.16.1.2 unsuppress-map AllowSpecifics

    route-map AllowSpecifics permit 10
    match ip address 1

    access-list 1 permit 172.16.220.0 0.0.0.255

The preceding configuration will allow advertisement of prefix 172.16.220.0/24 toward RTF.

Loss of Information Inside Aggregates (AS-SET)

Aggregation causes loss of granularity. The detailed information that exists in the specific prefixes will be lost when summarized in the form of aggregates. The purpose of AS-SET is to try to save the attributes carried in the specific routes in a mathematical SET that gives a better idea of the elements of the aggregate.

In figure 10-13, RTA is aggregating prefixes 192.68.10.0/24 and 192.68.11.0/24 coming from AS2 and AS1, respectively. Without AS-SET, the aggregate 192.68.0.0/16 will be considered as having originated from AS3 and will lose all the specific attribute information that the individual prefixes 192.68.10.0/24 and 192.68.11.0/24 have. We will present two configuration possibilities for RTA—first without AS-SET and second with AS-SET. We will see how the aggregate 192.68.0.0/16 will look in both scenarios.

Figure 10-13 Dealing with loss information with aggregates.

RTA configuration without AS-SET:

    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 192.68.0.0 255.255.0.0
     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
     neighbor 172.16.220.2 remote-as 5
     no auto-summary

     ip as-path access-list 10 permit ^$

The following shows how the aggregate 192.68.0.0/16 will look when it is received by RTK. Note that the aggregate has lost the individual path information because the AS_path consists of AS number 3 only.

    RTK#sh ip bgp
    BGP table version is 8, local router ID is 172.16.220.2
    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
    *> 172.16.1.0/24    172.16.220.1           0             0 3 i
    *> 172.16.10.0/24   172.16.220.1          20             0 3 i
    *> 172.16.65.0/26   172.16.220.1          20             0 3 i
    *> 172.16.220.0/24  172.16.220.1           0             0 3 i
    *> 192.68.0.0/16    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

Using the AS-SET concept, the aggregate will be sent out from RTA with a SET of the path information.

RTA configuration with AS-SET:

    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 192.68.0.0 255.255.0.0 as-set
     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
     neighbor 172.16.220.2 remote-as 5
     no auto-summary

    ip as-path access-list 10 permit ^$

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