Describing Bgp Messages; Open - Enterasys Security Router X-PeditionTM User Manual

Overview
BGP can be categorized as a path vector routing protocol which defines a route as a pairing
between a destination and the qualities of the path to that destination. The main role of a BGP-
speaking node is to trade network reachability data with adjacent BGP nodes known as neighbors
or peers. This reachability data includes a list of AS's that have been traversed along the way. By
using this list, an AS connectivity grid can be built from which routing loops may be pruned and
some policy decisions at the AS level may be enforced.
Like other protocols, BGP uses the Transmission Control Protocol (TCP) as its transport protocol.
Running over a reliable transport protocol eliminates the need for BGP to use fragmentation,
retransmission, acknowledgment, and sequencing BGP uses TCP port 179 to build its links.
BGP neighbors exchange full routing data when the TCP link between neighbors is first
established. When routing table changes are detected, BGP routers send only changed routes to
their neighbors. BGP routers do not transmit periodic routing updates, and BGP routing updates
advertise only the best path to a destination network.
BGP permits policy-based routing which choose among multiple paths to a destination and
control the redistribution of routing data.
BGP-4 supports Classless Inter-Domain Routing (CIDR), which dispenses with network classes, as
wells as route aggregation, including the aggregation of AS paths and supernetting.

Describing BGP Messages

BGP nodes exchange four types of messages, including Open, Update, Keepalive, and Notification.
They are described as follows.

Open

After two BGP nodes are connected via TCP, they exchange BGP open messages to build a BGP
link between them. Once the connection is set up, the nodes trade BGP messages and data traffic.
Open messages consist of the BGP header as well as the following fields:
• Version: The current protocol version number of the message. The current BGP version
number is 4.
• Local AS number: Autonomous System number of the sender.
6-2 Configuring the Border Gateway Protocol
Figure 6-1 Differentiating EBGP from IBGP