NETWORK NOTES : BGP - Basics Message Type

BGP Message

BGP messages are unicast to the one neighbor over the TCP connection.

BGP uses four message types:

· Open

· Keepalive

· Update

· Notification

Open Message

After the TCP session is established, both neighbors send Open messages,

1. BGP version number— This specifies the version (2, 3, or 4) of BGP that the originator is running.it defaults to BGP-4. If a neighbor is running an earlier version of BGP, it rejects the Open message specifying version 4; the BGP-4 router then changes to BGP-3 and sends another Open message specifying this version. This negotiation continues until both neighbors agree on the same version.

2. Autonomous system number— This is the AS number of the originating router. It determines whether the BGP session is EBGP (if the AS numbers of the neighbors differ) or IBGP (if the AS numbers are the same).

3. Hold time— This is the maximum number of seconds that can elapse before the router must receive either a Keepalive or an Update message. The hold time must be either 0 seconds (in which case, Keepalives must not be sent) or at least 3 seconds; the default Cisco hold time is 180 seconds. If the neighbors' hold times differ, the smaller of the two times becomes the accepted hold time.

4. BGP identifier— This is an IP address that identifies the neighbor.

The numerically highest loopback address is used.

5. Optional parameters—optional capabilities, as authentication, multiprotocol support, and route refresh.

Keepalive Message

If a router accepts the parameters specified in its neighbor's Open message, it responds with a

Keepalive. Subsequent Keepalives are sent every 60 seconds by Cisco default, or a period equal to

one-third the agreed-upon hold time.

The Keepalive message consists of only the 19-octet BGP message header, with no additional data.

Update Message

The Update message advertises feasible routes, withdrawn routes, or both.

Network Layer Reachability Information (NLRI)— This is one or more (Length, Prefix)

tuples that advertise IP address prefixes and their lengths

Path Attributes. The attributes provide the information that allows BGP to choose a shortest path, detect routing loops, and determine routing policy

Withdrawn Routes— These are (Length, Prefix) tuples describing destinations that have

become unreachable and are being withdrawn from service.

**each update message describes only a single BGP route (because the path attributes describe only a single path, but that path might lead to multiple destinations