IBM RackSwitch G8000, Application Manual

Page: 254 / 362

252
RackSwitch G8000: Application Guide
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.
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.
Selecting Route Paths in BGP
BGP selects only one path as the best path. It does not rely on metric attributes to
determine the best path. When the same network is learned via more than one BGP
peer, BGP uses its policy for selecting the best route to that network. The BGP
implementation on the G8000 uses the following criteria to select a path when the
same route is received from multiple peers.
1. Local fixed and static routes are preferred over learned routes.
2. With iBGP peers, routes with higher local preference values are selected.
3. In the case of multiple routes of equal preference, the route with lower AS path
weight is selected.
AS path weight = 128 x AS path length (number of autonomous systems
traversed).
4. In the case of equal weight and routes learned from peers that reside in the
same AS, the lower metric is selected.
Note: A route with a metric is preferred over a route without a metric.
5. The lower cost to the next hop of routes is selected.
6. In the case of equal cost, the eBGP route is preferred over iBGP.
7. If all routes are from eBGP, the route with the lower router ID is selected.
When the path is selected, BGP puts the selected path in its routing table and
propagates the path to its neighbors.
>> # router bgp
>> (config_router_bgp)# local-preference
>> (config_router_bgp)# exit
>> # route-map 1
>> (config_route_map)# local-preference
>> (config_router_map)# exit