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terminology, WITHDRAWN.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.
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