GNS3-Topology: EvilRouters-Weighting To Influence BGP Routing
Written by LBSources from http://www.gns3-labs.com on March 14th, 2009 | 4 CommentsThis is an older lab that is part of the BGP series labs that Jeremy over at EvilRouters.net is building. The labs are great and they will surely serve someone some great lab experience by the time hes done with them all.
This lab picks up from:
Don’t get too confused – yes the topology has dramatically changed 
In Jeremy’s next lab he will use the local preference attribute to influence INBOUND BGP Routing
Weight
Weight is assigned locally on a router to specify a preferred path if multiple paths exist 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 local router have a default weight of 32,768.
You can use weight instead of local preference to influence the selected path to external 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.
Routers Used: 3640
IOS: c3640-jk9s-mz.124-16a
Feature of Topology: BGP, Weight Attribute
The Goal of this lab?
We can see that we have two routes to each of: 192.168.2.0/24, 192.168.3.0/24, and 192.168.4.0/24. All things being equal, BGP will choose the shortest path to each network. We can see this evidenced by the fact that BGP has chosen to send traffic for 192.168.2.0/24 to AS 65002 (R2) and traffic for 192.168.4.0/24 to AS 65004 (R4). Notice, however, that while there are two routes to 192.168.3.0/24 (AS 65003), BGP has chosen to send traffic for that network through AS 65002 (R2). The path through R2 was chosen because it is the “more stable” route (R1’s adjacency with R2 was formed before R1’s adjacency with R4).
Let’s assume, however, that (for whatever reason) we want to route traffic for 192.168.3.0/24 through AS 65004 (R4). The easiest (but not always best) way to do this is by using BGP’s weight attribute.
Image: 
Download: GNS3-Labs:: EvilRouters-Weighting To Influence BGP Routing
Enjoy .. LBS
13,035 viewsTags: BGP, BGP Weight Influence, EvilRouters.net, GNS3 Lab and Exercise, GNS3 Topology
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