Blade ICE G8124 Manuale Utente

266  

  Chapter 19: Border Gateway Protocol

BMD00220, October 2010

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

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:

The following commands use the BGP default local preference method, affecting the outbound 
direction only. 

The following commands use the route map local preference method, which affects both 
inbound and outbound directions. 

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 making 
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.

>> #

>> (config_router_bgp)#

>> (config_router_bgp)#

>> #

>> (config_route_map)#

>> (config_router_map)#