8.3 BGP Neighbor Negotiation
8.3.5 UPDATE message and routing updates
Routing updates contain all the information BGP uses to construct a loop-free picture of the internetwork. The following are the basic blocks of an UPDATE message:
  • Network-layer reachability information (NLRI)
  • Path attributes
  • Unreachable routes

Figure illustrates these components in the context of an UPDATE message format. Each NLRI is an IP prefix indicating (or advertising) a reachable destination. The path attribute list provides BGP with the capabilities of detecting routing loops and the flexibility to enforce local and global routing policies. An example of the BGP path attributes is the AS_path attribute, which is a sequence of AS numbers a route has traversed before reaching this BGP router.

AS3 in Figure , for example, is receiving BGP updates from AS2 indicating that 10.10.1.0/24 is reachable via two hops; the first hop is AS2, and the second is AS1. If an NLRI with an AS_path containing AS3 is received on a router within AS3, it will be discarded as a looped path.

The third part of the UPDATE message is a list of routes that have become unreachable -- or in BGP terminology, WITHDRAWN. With the example illustrated in Figure , if 10.10.1.0/24 is no longer reachable or experiences a change in its attribute information, BGP can withdraw the route that it advertised by sending an UPDATE message that lists the new network information.