Optional Parameters field format

Parameter Type
The Parameter Type field is 1 byte long and identifies the individual
parameter.
Parameter Length
The Parameter Length field is 1 byte long and contains the length of the
Parameter Value field.
Parameter Value
The Parameter Value field is of variable length and is interpreted based on
the value of the Parameter Type field.
FIGURE 8 . 4
Optional Parameters field format
UPDATE Message
After BGP speakers have become peers, they will exchange incremental UPDATE messages.
UPDATE messages contain the routing information for BGP. The information contained in
the UPDATE message is used to construct a loop-free routing environment.
Not only does the UPDATE message contain a feasible route to use, it also contains any
unfeasible routes to withdraw. A single UPDATE message can contain at most one feasible
route to use but may contain multiple unfeasible routes to withdraw.
It’s important to understand the distinction between a route and a prefix. Each
UPDATE message may advertise only a single feasible route, but it may advertise
multiple prefixes in its NLRI field. Said a different way, each UPDATE message
can advertise a single way to get to multiple networks. It makes sense,
then, that any network that is not accessible at the end of the advertised route
would not be in the Network Layer Reachability Information (NLRI) field of the
same UPDATE message.
The UPDATE message contains the fixed-size BGP header and the following fields, as
depicted in Figure 8.5.