Another method to reduce the iBGP mesh within an autonomous system is to use BGP confederations. With confederations, the autonomous system is divided into smaller, sub-autonomous 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 4-13, a confederation divides the autonomous system into two.

Figure 4-13 BGP Confederations

Routers A, B, and G are configured for eBGP between the sub-autonomous systems. This involves using the bgp confederation identifier command on all routers, where confederation identifier is the same for all routers in the network. You use the bgp confederation peers command to identify the ASN of other sub-autonomous systems in the confederation. Because Routers A and G are in AS 10, the peer confederation to Router B is AS 20. Router B is in AS 20, and its peer confederation to Routers A and G is AS 10. Routers C and D are part of AS 10 and peer with each other and with Routers A and G. Routers E and F are part of AS 20 and peer with each other and with Router B.

BGP Administrative Distance

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 administrative distance is selected for inclusion in the IP forwarding table. For BGP, the administrative distances are

• eBGP routes: 20
• iBGP routes: 200

BGP Attributes, Weight, and the BGP Decision Process

BGP uses path attributes to select the best path to a destination. This section describes BGP attributes, the use of weight to influence path selection, and the BGP decision process.

BGP Path Attributes

BGP uses several attributes, or metrics, for the path-selection process. BGP uses path attributes to communicate routing policies. BGP path attributes include next hop, local preference, autonomous system path, origin, multi-exit discriminator (MED), community, atomic aggregate, and aggregator. Of these, the autonomous system path is one of the most important attributes: It lists the number of autonomous system paths to reach a destination network.

BGP attributes can be categorized as well known or optional. Well-known attributes are recognized by all BGP implementations. Optional attributes do not have to be supported by the BGP process.

Well-known attributes can be further subcategorized as mandatory or discretionary. Mandatory attributes are always included in BGP update messages. Discretionary attributes might or might not be included in BGP update messages.

Optional attributes can be further subcategorized as transitive or nontransitive. Routers must advertise a route with transitive attributes to their peers even if they do not support the attribute locally. If the path attribute is nontransitive, the router does not have to advertise the route to its peers.

The following sections cover each attribute category.