Understanding the significance of Wight attribute in BGP Best Path Selection

[vainatarajan]

Why does Cisco use Weight for BGP path selection process whereas others don't use it? Is there a specific significance for it ?

[Predrag Jovic]

Since it is locally significant to BGP router you can use it to load balance traffic in scenario where you have 1 router and 2 BGP neighbors and want ot load balance traffic. Configure higher weight for specific neighbor on some NLRI that is advertised from both neighbors and traffic will be forwarded according to configured weight attribute.

Configuration example:

access-list 10 permit 192.168.0.0 0.0.0.255
access-list 20 permit 172.16.0.0 0.0.0.255

!

route-map ISP1 permit 10
  match ip address 10
  set weight 500
route-map ISP1 permit 20

!

route-map ISP2 permit 10
  match ip address 20
  set weight 500
route-map ISP2 permit 20

!

router bgp 65500
  neighbor 10.0.0.1 route-map ISP1 in
  neighbor 10.10.10.1 route-map ISP2 in

The same principle can be used to route traffic on any BGP router in the network, however it will create additional administrative overhead (just as routing traffic with static routes).

[Julio E. Moisa]

Hi

There are many ways to manipulate path for BGP paths, they are known as BGP attributes or in some cases BGP parameters. There is a list of BGP attributes and it has an order. Weight is a Cisco only parameter, it is local significant for the router where it is configured, basically it is ideal when you are not using iBGP on your network infrastructure basically used on the border routers having eBGP peering only in order to select a best way over other. The higher Weight value is always preferred. 

If you have an iBGP network I suggest to use Local Preference because the LP value is transmitted on the local AS. 

I suggest the following link for additional information:

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

http://netcerts.net/bgp-path-attributes-and-the-decision-process/

Hope it is useful

:-)

[Richard Burts]

The original post asks why did Cisco create this unique vendor specific attribute. I believe that the answer is that Cisco wanted to go beyond the standard features identified in the RFC which specifies this routing protocol and which every vendor must implement to provide a unique capability.

Weight is very similar to Local Preference. They both provide a way for a router to prefer certain prefixes learned from one neighbor over those prefixes learned from other neighbors. If you have a single router running BGP to multiple neighbors then it would not matter whether you use Weight or Local Preference to prefer certain prefixes from one of the neighbors. You would get the same results with either method of establishing preference. But if you have multiple routers in your network running BGP (and with multiple routers running BGP you should be running IBGP between your routers) then there is a difference. If router A learns a prefix ans assigns a Local Preference to it, then it will advertise that prefix to router B with that Local Preference and both routers will have the same degree of preference for that route. But what if router A leanrs a prefix and wants to assign a preference, but you do not necessarily want router B to have that same preference? In that case the unique Cisco attribute of Weight can accomplish what you need. This is the reason why Cisco created the Weight attribute in BGP.

HTH

Rick