3com S7906E Manuel De Montage

 

1-3 

Figure 1-2 BGP Open message format 

 

 

z 

Version: This 1-byte unsigned integer indicates the protocol version number. The current BGP 

version is 4. 

z 

My autonomous system: This 2-byte unsigned integer indicates the Autonomous System number 

of the sender. 

z 

Hold time: When establishing a peer relationship, two parties negotiate an identical hold time. If no 

Keepalive or Update is received from a peer within the hold time, the BGP connection is considered 

down. 

z 

BGP identifier: An IP address that identifies the BGP router 

z 

Opt Parm Len (Optional Parameters Length): Length of optional parameters, which is set to 0 if no 

optional parameter is available. 

z 

Optional parameters: Used for multiprotocol extensions, and other functions. 

The Update messages are used to exchange routing information between peers. It can advertise a 

feasible route or remove multiple unfeasible routes. Its format is shown below: 

Figure 1-3 BGP Update message format 

Total path attribute length

Withdrawn routes

Path attributes

NLRI

Unfeasible routes length

2 Octets

N Octets

2 Octets

N Octets

N Octets

 

 

Each Update message can advertise a group of feasible routes with identical attributes, and the routes 

are contained in the network layer reachable information (NLRI) field. The Path Attributes field carries 

attributes of these routes. Each Update message can also carry multiple withdrawn routes in the 

Withdrawn Routes field. 

z 

Unfeasible routes length: The total length of the Withdrawn Routes field in bytes. A value of 0 

indicates no route is withdrawn from service, and the Withdrawn Routes field is not present in this 

Update message. 

z 

Withdrawn routes: This is a variable length field that contains a list of withdrawn IP prefixes. 

z 

Total path attribute length: Total length of the Path Attributes field in bytes. A value of 0 indicates 

that no Network Layer Reachability Information field is present in this Update message. 

z 

Path attributes: List of path attributes related to NLRI. Each path attribute is a triple <attribute type, 

attribute length, attribute value> of variable length. BGP uses these attributes to avoid routing loops, 

and perform routing and protocol extensions.