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BPG Common Header

269

WELL-KNOWN DISCRETIONARY

A well-known discretionary attribute might be included in a route descrip-
tion but does not have to be included. These attributes are as follows:

Local Preference The LOCAL_PREF (Type Code 5) attribute is a well-
known discretionary attribute that can contain only a single AS and can
be used only with iBGP.

Atomic Aggregate The ATOMIC_AGGREGATE (Type Code 6)
attribute is a well-known, discretionary attribute that is used to inform
BGP speakers of policy routing decisions that have been made when there
is more than one route, also known as overlapping routes. This attribute
is basically used to indicate that a prefix is or is not to be used. Therefore,
the ATOMIC_AGGREGATE has a path length of 0.

OPTIONAL TRANSITIVE

An optional transitive attribute may not be recognized by some implemen-
tations of BGP and is not expected to be. These attributes are used in many
private BGP-enabled networks. If an implementation of BGP does not rec-
ognize the optional transitive attribute of a message, it will mark the message
as a partial message but still propagate the message to its neighbors. The
optional transitive attributes are as follows:

Aggregator The AGGREGATOR (Type Code 7) attribute is an
optional transitive attribute of six octets in length: two octets identifying
the ASN and four octets identifying the IP address. This attribute can be
attached to a message that is performing aggregation to identify the AS
and the router that performed the aggregation.

T A B L E 7 . 4

ORIGIN Type Codes

Code
Value

Type

IGP (the originating AS, which has learned about this NLRI
from its own IGP)

EGP (the AS sending the NLRI, which was first learned from
an eBGP speaker)

INCOMPLETE (the NLRI obtained this route statically, such as
from a configured static route) This code may also be used
when any redistributed route from an IGP to BGP has an
incomplete flag.

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