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Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes | Page 5
BGP: Concepts and Terminology
Update attributes
As mentioned above, each BGP update message contains a set of attributes. These attributes
describe some of the properties of the routes, and can be used in making decisions about
which routes to accept and which to reject. Some of the attributes are:
Origin
How a prefix came to be routed by BGP at the origin Autonomous System (AS). Prefixes are
learned from various sources such as directly connected interfaces, manually configured static
routes, or dynamic internal or external routing protocols, and then put into BGP.
AS-path
The list of Autonomous Systems (ASs) through which the announcement for the prefix has
passed. As prefixes pass between Autonomous Systems each one adds its Autonomous
System Number (ASN).
Next-hop
The address of the next node that the router should send packets destined for the specified
prefixes to, in order to get the packets closer to the destination.
Multi-Exit-
Discriminator
(MED)
A metric expressing the optimal path to reach a particular prefix in or behind a particular AS.
Local-preference
A metric used in IBGP so each host knows which path inside the AS it should use to reach
the advertised prefix. EBGP peers do not send this value, and ignore it on receipt.
Atomic-aggregate
A non-transitive attribute that allows BGP peers to inform each other about decisions they
have made regarding overlapping routes. Non-transitive means that if the attribute is
received by a device that does not recognise the attribute, it is dropped and not passed on
to the next router.
Aggregator
Can be attached to an aggregated prefix to specify the AS and router that performed the
aggregation.
Community
Indicates where a prefix is relevant to—for example, if it is relevant to the whole Internet, or
just within an AS.