AS_Path Manipulation

Considering RTF's BGP table, RTF has picked the direct path via AS1 to reach 192.68.11.0/24 because it is shorter. The following configuration shows how the AS_path information can be manipulated to make the AS_path longer by prepending AS path numbers. Considering figure 10-6, we will prepend two extra AS path numbers to the AS_path information sent from RTC to RTA to change RTF's decision about reaching 192.68.11.0/24.

RTC configuration:

    router bgp 1
     network 192.68.11.0
     neighbor 172.16.20.2 remote-as 3
     neighbor 172.16.20.2 route-map AddASnumbers out
     neighbor 192.68.6.1 remote-as 2
     no auto-summary

    route-map AddASnumbers permit 10
     set as-path prepend 1 1

The preceding configuration prepends two additional AS_path numbers 1 and 1 to the AS_path information sent from RTC to RTA. If you look at RTF's BGP table, you will see that RTF can now reach 192.68.11.0/24 via next hop 192.68.5.2—that is, via path 2 1. RTF will prefer this path because it is shorter than the direct path via AS1, which has a path information of 1 1 1.

    RTF#sh ip bgp
    BGP table version is 18, 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
    * i172.16.1.0/24    172.16.2.254           0    100      0 i
    *>                  0.0.0.0                0         32768 i
    * i172.16.10.0/24   172.16.2.254          20    100      0 i
    *>                  0.0.0.0                0         32768 i
    * i172.16.65.0/26   172.16.2.254          20    100      0 i
    *>                  0.0.0.0                0         32768 i
    * i172.16.220.0/24  172.16.2.254           0    100      0 i
    *>                  172.16.1.1            20         32768 i
    *> 192.68.10.0      192.68.5.2             0             0 2 i
    *> 192.68.11.0      192.68.5.2                           0 2 1 i
    * i                 172.16.20.1            0    100      0 1 1 1 i

Using Private ASs

This example demonstrates how BGP can be configured to prevent the leakage of private AS numbers into the Internet. Consider figure 10-7; AS1 will prevent private AS number 65001 from being leaked to the Internet when BGP routes are propagated.

Figure 10-7  Stripping private AS numbers.

RTA configuration:

    router bgp 65001
     network 172.16.220.0 mask 255.255.255.0
     neighbor 172.16.20.1 remote-as 1
     no auto-summary

RTC configuration:

    router bgp 1
     network 192.68.11.0 mask 255.255.255.0
     neighbor 172.16.20.2 remote-as 65001
     neighbor 192.68.6.3 remote-as 7
     neighbor 192.68.6.3 remove-private-AS
     no auto-summary

Note how RTC is using the remove-private-AS keyword in its neighbor connection to AS7. The following output shows the BGP tables of RTC and RTE.

    RTC#show ip bgp
    BGP table version is 72, 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
    *> 172.16.220.0/24  172.16.20.2            0             0 65001 i
    *> 192.68.11.0      0.0.0.0                0         32768 i
    
    RTE#sh ip bgp
    BGP table version is 245, local router ID is 192.68.30.1
    Status codes: s suppressed, * valid, > best, i - internal
    Origin codes: i - IGP, e - EGP, ? - incomplete
    
       Network             Next Hop          Metric LocPrf Weight Path
    *> 172.16.220.0/24  192.68.6.2                      0 1 i
    *> 192.68.11.0      192.68.6.2           0          0 1 i

Note that prefix 172.16.220.0/24 has an AS_path of 65001 in RTC's BGP table and an AS_path of 1 in RTE's BGP table. RTC has stripped the private AS path information when propagating the update to AS7.