Information Technology Reference
In-Depth Information
Routers A and G are configured to peer with each other and with Routers B, C, and D. The
configuration of Routers C and D is different from the rest; they are configured to peer
with Routers A and G only. All route reflectors in the same cluster must have the same
cluster ID number.
Router B is the route reflector for the second cluster. Router B peers with Routers A and G
and with Routers E and F in its cluster. Routers E and F are route-reflector clients and peer
only with Router B. If Router B goes down, the cluster on the right goes down because no
second route reflector is configured.
Confederations
Another method to reduce the iBGP mesh within an autonomous system is BGP confeder-
ations. With confederations, the autonomous system is divided into smaller, sub au-
tonomous systems, and the whole group is assigned a confederation ID. The sub-ASNs or
identifiers are not advertised to the Internet but are contained within the iBGP networks.
The routers within each private autonomous system are configured with the full iBGP
mesh. Each sub-autonomous system is configured with eBGP to communicate with other
sub-autonomous systems in the confederation. External autonomous systems see only the
ASN of the confederation, and this number is configured with the BGP confederation
identifier.
In Figure 11-12, a confederation divides the autonomous system into two.
Routers A, B, and G are configured for eBGP between the sub autonomous systems. You
configure the
bgp confederation identifier
command on all routers The confederation
identifier number is the same for all routers in the network. You use the
bgp confedera-
tion peers
command to identify the ASN of other sub-autonomous systems in the confed-
eration. Because Routers A and G are in autonomous system 10, the peer confederation to
Router B is autonomous system 20. Router B is in autonomous system 20, and its peer con-
federat ion to Router s A and G is autonomou s s ystem 10. Router s C and D are par t of au-
tonomous system 10 and peer with each other and with Routers A and G. Routers E and F
are part of autonomous system 20 and peer with each other and with Router B.
BGP Administrative Distance
The Cisco IOS software assigns an administrative distance to eBGP and iBGP routes, as it
does with other routing protocols. For the same prefix, the route with the lowest adminis-
trative distance is selected for inclusion in the IP forwarding table. For BGP, the adminis-
trative distances are
eBGP routes:
20
■
Key
To p i c
iBGP routes:
200
■
BGP Attributes, Weight, and the BGP Decision Process
The BGP protocol uses path attributes to select the best path to a destination. This sub-
BGP decision process.
