How to Configure BGP Weight Attribute

Weight is a Cisco proprietary BGP attribute that can be used to select a certain path. Here’s what you need to know about weight:

• Weight is the first BGP attribute in the list.
• Cisco proprietary so you won’t find it on other vendor routers.
• Weight is not exchanged between BGP routers.
• Weight is only local on the router.
• The path with the highest weight is preferred.

Let me give you an example of BGP weight:

R1 in AS 1 can reach AS 3 through AS 2 or AS 4. If we want to ensure AS 2 is always used as the best path, you can change the weight. In my example, the weight for the path to AS 2 is set to 500 and higher than the weight of 400 for AS 4. Let’s see what this looks like on real Cisco routers. This is the topology that I will use:

Above, we have a simple scenario with two autonomous systems. R2 and R3 both have network 2.2.2.0/24 configured on their loopback0 interface, and I’ll advertise that in BGP.

R1(config)#router bgp 1
R1(config-router)#bgp router-id 1.1.1.1
R1(config-router)#neighbor 192.168.12.2 remote-as 2
R1(config-router)#neighbor 192.168.13.3 remote-as 2

R2(config)#router bgp 2
R2(config-router)#bgp router-id 2.2.2.2
R2(config-router)#neighbor 192.168.12.1 remote-as 1
R2(config-router)#neighbor 192.168.23.3 remote-as 2
R2(config-router)#network 2.2.2.0 mask 255.255.255.0

R3(config)#router bgp 2
R3(config-router)#bgp router-id 3.3.3.3
R3(config-router)#neighbor 192.168.13.1 remote-as 1
R3(config-router)#neighbor 192.168.23.2 remote-as 2
R3(config-router)#network 2.2.2.0 mask 255.255.255.0

Above you’ll find the configuration for BGP. I configured the manual router ID for a reason. R2 and R3 have the same IP address on the loopback interface, which means they would get the same router ID, and they would be unable to form a BGP neighbor adjacency. Let’s take a detailed look at R1:

R1#show ip bgp 
BGP table version is 2, local router ID is 192.168.13.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 2.2.2.0/24       192.168.12.2             0             0 2 i
*                   192.168.13.3             0             0 2 i

Router R1 decided to use 192.168.12.2 as the next hop. All the BGP attributes are the same, so it came down to the router ID to select a winner.

Now let’s change this behavior using the weight attribute…

R1(config)#router bgp 1
R1(config-router)#neighbor 192.168.13.3 weight 500

You can configure weight per neighbor using the weight command. All prefixes from this neighbor will have a weight of 500.