TCP/IP Tutorial and Technical Overview - IBM Redbooks

RFC 4271 recommends a 90 second hold timer and a 30 second keepalive
timer.
Sending reachability information
Reachability information is exchanged between peers in UPDATE messages.
BGP does not require a periodic refresh of the entire BGP routing table.
Therefore, each BGP speaker must retain a copy of the current BGP routing
table used by each peer. This information is maintained for the duration of the
connection. After neighbors have performed the initial exchange of complete
routing information, only incremental updates to that information are exchanged.
An UPDATE message is used to advertise feasible routes or withdraw infeasible
routes. The message can simultaneously advertise a feasible route and withdraw
multiple infeasible routes from service. Figure 5-24 depicts the format of an
UPDATE message:
► Network layer reachability information (NLRI).
► Path attributes (we discuss path attributes in “Path attributes” on page 223.
► Withdrawn routes.
Number of Octets
Figure 5-24 BGP UPDATE message
Several path attributes can be used to describe a route.
Withdrawn routes
The unfeasible routes length field indicates the total length of the withdrawn
routes field.
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2
Variable
2
Variable
Variable
Common Header
Unfeasible Routes Length
Withdrawn Routes
Total Path Attribute
Length
Path Attributes
Network Layer
Reachability Information
Type = 2