Information Technology Reference
In-Depth Information
effective BGP routing architecture. This section highlights some of the common BGP
attributes, with more-detailed discussion in later chapters.
The following attributes currently are supported in Cisco IOS software:
•
ORIGIN
•
AS_PATH
•
NEXT_HOP
•
MULTI_EXIT_DISC
•
LOCAL_PREF
•
AT OMIC_AGGREGATE
•
AGGREGATOR
•
COMMUNITY
•
ORIGINATOR_ID
•
CLUSTER_LIST
•
Multiprotocol Reachable NLRI (MP_REACH_NLRI)
•
Multiprotocol Unreachable NLRI (MP_UNREACH_NLRI)
The following is a brief overview of some of the more common BGP attributes. Additional
attributes are discussed in later chapters.
ORIGIN
This attribute indicates a prefix's source. There are three possible origins:
•
IGP
—ORIGIN of 0
•
EGP
—ORIGIN of 1
•
INCOMPLETE
—ORIGIN of 3
A prefix with a lower ORIGIN value is preferred during a path selection. A prefix's
ORIGIN attribute is automatically defined when a prefix is injected into BGP but can be
modified through the use of a route map. For example, if a prefix is redistributed into BGP
using the
redistribute
command, its ORIGIN is set to 3; if a prefix is injected into BGP via
the
network
command, its origin is set to 0. In effect, routes originated by the
network
command are preferred over those that are redistributed.
AS_PATH
AS_PATH lists in reverse order the autonomous systems traversed by a prefix, with the
last AS placed at the beginning of the list. The primary purpose of the AS_PATH is to
provide loop prevention for inter-AS routing. The accepted number of autonomous systems
