Path Attributes Field

Path Attributes Field
The Path Attributes field is of variable length and contains a sequence of attributes about a
path. The Path Attributes field is present in every UPDATE message. Of course, it may be
empty, which will be indicated by a TPAL of 0.
The information contained in the Path Attributes field is used to track specific route information
and is also used for routing decisions and filtering. Each path attribute is broken down
into an
<
attribute type
,
attribute length
,
attribute value
>
triple.
The Attribute Type field is 2 bytes in length and consists of the Attribute Flags byte followed
by the Attribute Type Code byte.
Attribute Flags
The attribute flags state whether path attributes are one of the following:

Well-known mandatory—This attribute must be recognized by all implementations of
BGP and be present in the UPDATE message. A BGP session will be terminated if a wellknown
attribute is not present in the UPDATE message.

Well-known discretionary—This attribute must be recognized by all implementations of
BGP but doesn’t need to be present in the UPDATE message.
Length
Prefix
244
Chapter 8 
Border Gateway Protocol

Optional transitive—This attribute allows for optional attributes that are not recognized
by an implementation of BGP to be passed along to a BGP speaker’s peers.

Optional non-transitive—If an optional attribute is not recognized by an implementation
of BGP and the transitive flag is not set, the optional attribute will not be passed on to the
BGP speaker’s peers.
The attribute’s flag sets which category a path attribute belongs to through the use of its bits:

The first high order bit (bit 0) is the optional bit. If the bit is set to 1, the path attribute
is optional. If the bit is set to 0, the path attribute is well-known.

The second high order bit (bit 1) is the transitive bit. This bit defines whether an optional
attribute is transitive or not. An optional transitive attribute will have the bit set to 1 and
an optional non-transitive attribute will have the bit set to 0. If an attribute is well-known,
the transitive bit will always be set to 1.
 The third high order bit (bit 2) is the partial bit. The partial bit will state whether the
optional transitive attribute is partial or complete. A complete optional transitive
attribute will have the bit set to 0, and a partial will have the bit set to 1. All wellknown
and optional non-transitive attributes will have the partial bit set to 0.
 The fourth high order bit (bit 3) is the extended length bit. The extended length bit is
used to specify whether the attribute length is 1 or 2 bytes. An attribute length of 1 byte
will have the bit set to 0, and for an attribute length of 2 bytes, the bit will be set to 1.
 The four lower order bits (bits 4–7) are unused. They will be set to 0 and ignored.