Information Technology Reference
In-Depth Information
Special Treatment of AS_PATH
The loop-prevention mechanism inside a confederation is based on the AS_PATH attribute.
Confederation introduces two new AS_PATH segment types:
•
AS_CONFED_SEQUENCE
—An ordered set of member AS numbers in the local
confederation that the route has traversed.
•
AS_CONFED_SET
—An unordered set of member AS numbers in the local
confederation that the route has traversed.
In Cisco IOS software, member AS numbers are enclosed in parentheses in the output of
show ip bgp
.
NOTE
How AS_PATH is updated within a confederation depends on the type of sessions.
Figure 7-34 shows AS_PATH changes for three types of peerings (to simplify the figure,
only one-way advertisement is shown):
Intraconfederation eBGP session
—The member AS number is prepended to the AS_
CONFED_SEQUENCE of the AS_PATH, such as from R4 to R2.
Internal BGP session
—The AS_PATH is not modified, as in the path from R2 to R1.
External BGP session
—The member AS numbers are removed from the AS_PATH, and
the confederation number is prepended to the AS_PATH, as shown in the path from R3 to R6.
Figure 7-34
AS_PATH with BGP Confederation
AS 65000
AS 200
{(65002) 200}
R1
R2
{(65002) 200}
R5
{(65000 65002) 200}
AS 65001
AS 65002
172.16.0.0/16
{200}
X
{(65001 65000 65002) 200}
R3
R4
Confederation
ID 100
AS 300
172.16.0.0/16
{100 200}
R6
