Information Technology Reference
In-Depth Information
Weight
We i g h t i s a s s i g n e d l o c a l l y o n a r o u t e r t o s p e c i f y a p r e fe r r e d p a t h i f m u l t i p l e p a t h s e x i s t
out of a router for a destination. Weights can be applied to individual routes or to all
routes received from a peer. Weight is specific to Cisco routers and is not propagated to
other routers. The weight value ranges from 0 to 65,535. Routes with a higher weight are
preferred when multiple routes exist to a destination. Routes that are originated by the lo-
cal router have a default weight of 32,768.
Yo u c a n u s e w e i g h t r a t h e r t h a n l o c a l p r e fe r e n c e t o i n f l u e n c e t h e s e l e c t e d p a t h t o e x t e r n a l
BGP peers. The difference is that weight is configured locally and is not exchanged in BGP
updates. On the other hand, the local preference attribute is exchanged between iBGP
peers and is configured at the gateway router.
When the same destinations are advertised from both Router B and Router C, as shown in
Figure 11-15, Router A prefers the routes from Router C over Router B because the routes
received from Router C have a larger weight (600) locally assigned.
AS 100
Internet
AS 200
Router C
5.1.1.1
Router B
4.1.1.1
AS 500
Weight = 600
Weight = 400
Router A
Figure 11-15
BGP Weight
BGP Decision Process
By default, BGP selects only a single path to reach a specific destination (unless you spec-
ify maximum paths). The Cisco implementation of BGP uses a simple decision process.
When the path is selected, BGP puts the selected path in its routing table and propagates
the path to its neighbors.
To s e l e c t t h e b e s t p a t h t o a d e s t i n a t i o n , C i s c o r o u t e r s r u n n i n g B G P u s e t h e fo l l o w i n g a l g o -
rithm in the following order:
1.
If the specified next hop is inaccessible, drop the path.
2.
drop the path.
