3com S7906E Manuel De Montage

 

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A router advertising BGP messages is called a BGP speaker. It establishes peer relationships with other 

BGP speakers to exchange routing information. When a BGP speaker receives a new route or a route 

better than the current one from another AS, it will advertise the route to all the other BGP peers in the 

local AS.  

To simplify configuration, multiple peers having an identical policy can be organized as a peer group. 

BGP runs on a router in either of the following two modes: 

z 

iBGP (internal BGP) 

z 

eBGP (external BGP) 

BGP is called iBGP when it runs within an AS and is called eBGP when it runs between ASs. 

BGP has five types of messages: 

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Open 

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Update 

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Notification 

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Keep-alive 

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Route-refresh 

They have the same header, as shown below: 

Figure 1-1 BGP message header 

 

 

z 

Marker: The 16-byte field is used to delimit BGP messages. The Marker must be all ones. 

z 

Length: The 2-byte unsigned integer indicates the total length of the message. 

z 

Type: This 1-byte unsigned integer indicates the type code of the message. The following type 

codes are defined: 1–Open, 2-Update, 3-Notification, 4–Keepalive, and 5–Route-refresh. The 

former four are defined in RFC1771, and the last one is defined in RFC2918. 

After a TCP connection is established, the first message sent by each side is an Open message for peer 

relationship establishment. An Open message contains the following fields: