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BPG Common Header
being withdrawn adds two fields to the Withdrawn Routes field. An IP Prefix
Length, which is one octet, identifies the length of the second field, called the
IP Prefix field. This field is of variable length and identifies the IP prefix for
the route that needs to be withdrawn. If the prefix does not equal at least
eight bits, then the rest of the field is padded with additional bits to make
each integer a multiple of eight bits.
Total Path Attributes Length
This field is used to indicate how large the Total Path Attributes field is.
Total Path Attributes
There are many path attributes that can be placed in the Total Path
Attributes field. Each attribute has a type code and several bits that describe
each attribute's usage. These attributes are associated with prefixes found in
the Network Layer Reachability Information (NLRI) field. Each bit indi-
cates a different attribute type. Table 7.2 describes the attribute types (A 0
bit equals OFF and a 1 bit equals ON).
T A B L E 7 . 2
Attribute Types
Attribute Type
ON=partial optional attribute; must be passed on.
OFF=well-known non-transitive; does not need to be
passed on.
ON=extended length bit; the total length of the attribute is
more than one octet. (By setting the extended length bit,
attributes can be longer than 255 bytes.)
OFF=the length of the attribute is one octet.
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