2.3.1 BGP message type

Before establishing a peering connection for BGP, the two neighbors must perform a standard TCP three-way handshake to open the TCP connection on port 179. BGP messages are unicast delivered to neighbors over a TCP connection

BGP message type

* Open messages

* Keeplive

* Update

* Notification

1.Open Open message

After the TCP session is established, both neighbors send Open messages with the following Open message information:

* BGP version number

* Autonomous system number

The AS number of the session initiating router, which can be identified as EBGP or IBGP

* hold time

Represents the maximum amount of time that a router can wait before receiving an Keepalive or Update message.

The default holding time of cisco is 180 seconds. If the holding time of both neighbors is not the same, then a shorter holding time will be taken as the mutually acceptable holding time.

* BGP identifier

Use the maximum loop return address

If the loop does not have an address, use the largest physical interface address

* optional parameters

Support for authentication, multiprotocol support, routing refresh

2.Keepalive message

If the router accepts the parameters specified in the Open message sent by its neighbor, it responds with a Keepalive message.

Cisco sends a Keepalive message every 60s by default, or sends a Keepalive message with a period of 1 to 3 of the agreed hold time

3.update message

Update messages are used to declare feasible routes, routes that have been revoked, or both, including the following information:

* NLRI network layer reachable information

This field is one or more tuples that declare ID addresses and prefixes and their length

* path attribute

* revoked rout

Describe destinations that have become unreachable and are out of service

4.Notification message

As soon as the router detects an error, it sends a Notification message and closes the BGP connection