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Chapter 7
BGP's Basic Components
Optional Parameters Length
This field is used to indicate the length of the Optional Parameters field in the
OPEN message. If there are no optional parameters in the field, then this
length is set to zero.
Optional Parameters
This field contains any optional parameters inserted into the OPEN message.
Each optional parameter includes a one-octet parameter type, a one-octet
parameter length, and a variable-length parameter value.
UPDATE Message
This type of message is the actual topology information sent between two
BGP speakers. An UPDATE messages can contain a new route, routes to be
withdrawn, or both. However, only one new route can be advertised by an
UPDATE message.
The UPDATE message adds additional fields to the BGP common header,
as shown in Figure 7.9.
F I G U R E 7 . 9
The additional fields added to the BGP common header when using the
UPDATE message type
Let's look at the additional fields added to the BGP common header when
the UPDATE message type is used.
Withdrawn Routes Length
This field is used to indicate the length of the Withdrawn Routes field and
specifies this information in the number of octets. The BGP specification
itself officially calls this field the Unfeasible Routes Length field.
Withdrawn Routes
The Withdrawn Routes field can contain a list of IP prefixes for which the
BGP speaker sending the UPDATE message wants to notify its BGP peer that
a route path either no longer exists or cannot be accessed due to the addition
of a policy. We'll discuss this topic in more detail in Chapter 9. Each IP prefix
Common
Header
Total Path
Attributes
Length
(2 octets)
Total Path
Attributes
(length
varies)
Withdrawn
Routes
(length
varies)
Network layer
Reachability
Information
(length varies)
Withdrawn
Routes
Length
(2 octets)
+
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