| Rather than advertise reachable
destinations as a network and a subnet mask, BGP advertises them
using NLRI, which consists of prefixes and prefix lengths. The
prefix represents the reachable destination, and the prefix length
represents the number of bits set in the subnet mask. For example,
10.1.1.0 255.255.255.0 has a prefix of 10.1.1.0 and a prefix length
of 24 (there are 24 bits set in the subnet mask), and thus would be
advertised by BGP as 10.1.1.0/24.
The NLRI consists of multiple
instances of the 2-tuple <length, prefix>. A tuple is a
mathematical term for a set of elements (in this case, the 2 refers
to the fact that there are only two elements in the set). Thus, the
NLRI <19, 192.24.160.0> represents the prefix of 192.24.160.0,
and the length is a 19-bit mask. In decimal terms, this NLRI refers
to a supernet: 192.24.160.0 255.255.224.0.
Withdrawn Routes
Withdrawn routes provide a list of routing updates that are no
longer reachable and that need to be withdrawn (removed)
from the BGP routing table. Withdrawn routes have the same format as
NLRI.
An update message that has no NLRI or
path attribute information is used to advertise only routes to be
withdrawn from service.
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