Information Technology Reference
In-Depth Information
Figure 4-5
AS_PATH Updates with Local AS
AS_PATH: 2 100
Local-AS
AS 100
AS 200
AS 2
AS_PATH: 2 200
Local-AS
AS 2
Local AS can be used together with peer groups, but it cannot be customized for individual
peers in a peer group. Local AS cannot have the local BGP AS number or the remote peer's
AS number. The
local-as
command is valid only if the peer is a true eBGP peer. It does not
work for two peers in different member autonomous systems in a confederation.
NOTE
Example 4-29 shows a sample BGP configuration in AS 200 border routers for Figure 4-5.
192.168.1.1 is the IP address of a BGP speaker in AS 100. On 192.168.1.1, 2 instead of 200
is configured as the remote AS (not shown).
Example 4-29
Sample Local AS Configuration in AS 200
router bgp 200
neighbor 192.168.1.1 remote-as 100
neighbor 192.168.1.1 local-as 2
Figure 4-6 shows another example of Local AS. In this case, AS 200 is configured with
Local AS with two remote autonomous systems, AS 100 and AS 300. When AS 200 border
routers advertise prefix 172.16.0.0/16 to AS 300, the AS_PATH is 2 200 2 100. Because
loop detection is done only for incoming updates from an eBGP peer, this AS_PATH is not
considered a condition of a loop. AS 300 accepts the prefix because it does not detect any
loop of AS 300. Similarly, AS 100 accepts prefix 10.0.0.0/8. Multiple occurrences of the
Local AS number in the eBGP updates indicate more than one point of Local AS sessions.
Figure 4-6
Local AS in Two Connections
172.16.0.0/16
10.0.0.0/8
Local-AS 2
Local-AS 2
AS 100
AS 200
AS 300
10.0.0.0/8
A
S_PATH: 2 200 2 30
0
172.16.0.0/16
A
S_PATH: 2 200 2 100
