Information Technology Reference
In-Depth Information
When Local AS is used, the AS_PATH length becomes longer. If AS_PATH length is used
as a deciding factor in selecting preference, AS_PATH prepending might be needed on
other paths so that path selection is not affected.
NOTE
During AS migration, it is possible that some routers are in the original AS and others are
in the new AS. When a border router is migrated to the new AS and is configured with Local
AS to remote peers, the updates from this border router to other routers that are still in the
old AS are denied, because the other routers detect an AS_PATH loop.
Figure 4-7 shows what happens. Before the migration, both R1 and R3 are in AS 2. When
R1 is migrated to AS 200 (the new AS), the Local AS is configured with R2 in AS 100.
When R3 receives the prefix 172.16.0.0/16, it detects its own AS in the AS_PATH, and the
update is denied. Example 4-30 shows the output of
debug ip bgp update
in on R3.
Figure 4-7
Updates Denied on R3 with Local AS on R1
AS 200
AS 2
AS 100
Local-AS 2
X
R1
R3
R2
172.16.0.0/16
AS_PATH: 200 2 100
172.16.0.0/16
Example 4-30
Loop Detection on R3 as Captured by
debug ip bgp update in
*Apr 22 04:59:32.563 UTC: BGP(0): 192.168.13.1 rcv UPDATE w/ attr: nexthop
192.168.13.1, origin i, originator 0.0.0.0, path 200 2 100, community , extended
community
*Apr 22 04:59:32.563 UTC: BGP(0): 192.168.13.1 rcv UPDATE about 172.16.0.0/16 --
DENIED due to: AS-PATH contains our own AS;
path 200 2 100
As mentioned previously, loop detection is performed on the inbound of an eBGP session.
Because the session between R1 and R3 is now eBGP, this detection is enforced.
