Bgp Attributes; Local Preference Attribute; Metric (Multi-Exit Discriminator) Attribute - Nortel Alteon OS 42C4911 Application Manual

Alteon OS Application Guide

BGP Attributes

The following two BGP attributes are discussed in this section: Local preference and metric
(Multi-Exit Discriminator).

Local Preference Attribute

When there are multiple paths to the same destination, the local preference attribute indicates
the preferred path. The path with the higher preference is preferred (the default value of the
local preference attribute is 100). Unlike the weight attribute, which is only relevant to the
local router, the local preference attribute is part of the routing update and is exchanged among
routers in the same AS.
The local preference attribute can be set in one of two ways:
/cfg/l3/bgp/pref
This command uses the BGP default local preference method, affecting the outbound
direction only.
/cfg/l3/rmap/lp
This command uses the route map local preference method, which affects both inbound
and outbound directions.

Metric (Multi-Exit Discriminator) Attribute

This attribute is a hint to external neighbors about the preferred path into an AS when there are
multiple entry points. A lower metric value is preferred over a higher metric value. The default
value of the metric attribute is 0.
Unlike local preference, the metric attribute is exchanged between ASs; however, a metric
attribute that comes into an AS does not leave the AS.
When an update enters the AS with a certain metric value, that value is used for decision mak-
ing within the AS. When BGP sends that update to another AS, the metric is reset to 0.
Unless otherwise specified, the router compares metric attributes for paths from external
neighbors that are in the same AS.
180 Chapter 11: Border Gateway Protocol 42C4911, January 2007