The TCP/IP Guide - BGP Connectivity Maintenance: <i>Keepalive</i> Messages

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The TCP/IP Guide
9 TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9 TCP/IP Internet Layer (OSI Network Layer) Protocols
           9 TCP/IP Routing Protocols (Gateway Protocols)
                9 TCP/IP Exterior Gateway/Routing Protocols (BGP and EGP)
                     9 TCP/IP Border Gateway Protocol (BGP/BGP-4)
                          9 BGP Detailed Messaging, Operation and Message Formats

BGP Route Information Exchange: Update Messages

BGP Error Reporting: Notification Messages

BGP Connectivity Maintenance: Keepalive Messages
(Page 2 of 2)

BGP Keepalive Message Format

The point of a Keepalive message is the message itself; there's no data to be communicated. In fact, we want to keep the message short and sweet. Thus, it is really a “dummy” message containing only a BGP header—a nice change after that incredibly long Update message format! The format of the Keepalive message can be found in Table 140 and Figure 195.

**Table 140: BGP *Keepalive* Message Format**
Field Name	Size (bytes)	Description
*Marker*	16	*Marker:* This large field at the start of each BGP message is used for synchronization and authentication. See the discussion of the BGP general message format for details.
*Length*	2	*Length:* The total length of the message in bytes, including the fields of the header. Keepalive messages are fixed in length at 19 bytes.
*Type*	1	*Type:* BGP message type, value is 4 for Keepalive messages.

**Figure 195: BGP *Keepalive* Message Format**

There is also one other, special use of Keepalive messages: as acknowledgment of receipt of a valid Open message during initial BGP session setup.

Key Concept: BGP Keepalive messages are sent periodically during idle periods when no real information needs to be sent between connected BGP speakers. They serve only to keep the session alive, and thus contain only a BGP header and no data.

BGP Route Information Exchange: Update Messages

BGP Error Reporting: Notification Messages

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