Information Technology Reference
In-Depth Information
When the link between R2 and R4 is down, the aggregate from R4 is removed. Under this
condition, R2 stops the injection of the prefix 172.16.1.0/24. This is shown in the BGP RIB
on R1 in Example 4-28. When the link between R3 and R5 is down as well, both 172.16.0.0
and 172.16.2.0 are also removed from AS 100 (not shown).
Example 4-28
BGP RIB on R1 When the Link Between R2 and R4 Is Down
R1#show ip bgp
BGP table version is 56, local router ID is 192.168.14.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i172.16.0.0 192.168.35.5 100 0 200 400 i
*>i172.16.2.0/24 192.168.35.5 100 0 200 400 i
Local A
When two ISPs merge their networks, many challenges related to BGP design arise. When
one AS is being replaced by another AS, its former peering autonomous systems might not
honor the new AS and might continue to insist on the previous peering agreements. For
example, if ISP A has a private peering agreement with ISP B, and if ISP A is acquired by
ISP C, ISP B might not want to peer with ISP C but might honor the previous peering
agreement with ISP A.
An ISP generally has various peering agreements with other ISPs. Changing the AS number
on a large scale might be too disruptive to its peering sessions with other ISPs. Also, chang-
ing the AS number on all the routers in one large AS during one maintenance window might
not be feasible or recommended. During the migration, both autonomous systems must
coexist and continue to communicate. The BGP Local AS feature helps reduce these
challenges.
With the Local AS feature, a BGP speaker can be physically in one AS and acts as such to
some neighbors while it appears to be another AS to other neighbors. When sending and
receiving AS_PATH to and from neighbors with Local AS configured, BGP prepends the
Local AS to the real AS. For these neighbors, BGP uses the Local AS as the remote AS in
the configuration. Thus, the Local AS number appears as if it were another AS inserted
between the two real autonomous systems.
Figure 4-5 shows an example. When AS 2 is configured on AS 200 as a Local AS, the
AS_PATH is prepended with AS 2 for updates from AS 100. When AS 100 receives updates
from AS 200, the AS_PATH is prepended with AS 2.
