Riverstone Networks WICT1-12 - Manual (Page 265)

Riverstone Networks RS Switch Router User Guide Release 8.0 16-7
BGP Configuration Guide Basic BGP Tasks
3. Exit Configure mode.
4. Re-enter Configure mode.
5. Add the peer-host back to the peer-group.
If the
as-count
option is part of the startup configuration, the above steps are unnecessary.
16.2.8 Creating BGP Confederations
In a BGP autonomous system, each iBGP router has to peer with all other iBGP routers over a direct link. This is
known as a "routing mesh." In a large AS, the number of peers and the number links between peers can be significant.
Creating a BGP confederation reduces the number of links between BGP peers by consolidating small autonomous
systems into a larger AS. To BGP routers that are not part of the confederation, sub-AS's appear as a single AS with a
single, externally-visible AS number. Each BGP confederation router uses its internal, sub-AS number to
communicate with peers that are members of its confederation. Therefore, a router in a BGP confederation must be
configured with both the AS number of the confederation (the externally-visible AS number) as well as the number of
its sub-AS.
In Figure 16-1 , a BGP confederation with the AS number 64801 consists of sub-AS’s 100, 101, 102, and 103. BGP
routers outside the confederation see only AS 64801; they cannot see the sub-AS’s. For example, R2 resides in AS
1000. When R2 communicates with R1, it sees R1 as part of AS 64801; R2 does not know that R1 is a member of AS
102.
Figure 16-1 BGP confederation
BGP confederations assume that a single IGP runs across the entire confederation. BGP sessions between routers that
are in the same confederation but in different sub-AS’s are also known as EIBGP sessions. EIBGP sessions are similar
to EBGP sessions with the following differences:
• Local preference attributes in learned routes are allowed to traverse sub-AS’s instead of being
ignored. For example, in
Figure 16-1 , R1 establishes a local preference value for a route advertised
by R2. AS 102 must be allowed to advertise the local preference value to the other sub-AS’s in the
confederation.
• The next hop attribute is set by the first-hop router in the confederation and is then allowed to
traverse sub-AS’s without being changed.
R1 R2
AS 100 AS 102
AS 101 AS 103
AS 1000 AS 64801
BGP Confederation