Lenovo Flex System Fabric CN4093, Application Manual

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CN4093 Application Guide for N/OS 8.4
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:
 Using the BGP default local preference method, affecting the outbound
direction only.
 Using 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 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.
CN 4093(config)# router bgp
CN 4093(config_router_bgp)# local-preference <0-4294967294>
CN 4093(config_router_bgp)# exit
CN 4093(config)# route-map 1
CN 4093(config_route_map)# local-preference <0-4294967294>
CN 4093(config_route_map)# enabled
CN 4093(config_router_map)# exit
CN 4093(config)# router bgp
CN 4093(config_router_bgp)# neighbor {
<number>/group <number>
} route-map
{
<in/out> <1 ‐ 255>
}