“WEIGHT” is Cisco Proprietary Attribute and it’s not included in the BGP updates. Weight can be set between 0-65535 (default value is 0) and higher value is more preferable. Weight is used within individual router, means its locally significant. When BGP choosing best path, Weight is considered first above all the other attributes.
BGP router set the weight to 0 on learned routes from peers and 32768 for locally generated routes.
In the below design and configuration output, all routes are configured with minimum configuration and we will focus on INBL and INTERNET routers to understand weight attribute in more detail.
So the INBL (AS-7090) and INTERNET (AS-8090) router is peering with ISP1 (AS-8070) and ISP2 (AS-9090) routers.INTERNET router advertising two loopback interfaces (18.104.22.168 and 22.214.171.124) and two connected interfaces (126.96.36.199 and 188.8.131.52) using network command.
ISP1, ISP2 and INBL routers advertising connected interfaces using redistribute connected command.
As per below output from INBL router, notice the selected routes which shows details about Weight attribute. As networks 184.108.40.206/32, 220.127.116.11/32, 18.104.22.168/30, 22.214.171.124/30 are learned from BGP peers, so Weight is set to 0 and networks 10.1.70.0/30, 126.96.36.199/30 and 192.168.1.0 are locally generated, so Weight is set to 32768.
Check the asterisk and caret symbol following every route; (*) symbol denotes valid route and (>) symbol denotes best route which is installed in routing table.
As BGP considers Weight Attribute first, networks 188.8.131.52 and 184.108.40.206 are reachable via 220.127.116.11 means from ISP2.
To verify, check trace from INBL router. INBL#traceroute 18.104.22.168 source loopback1 Type escape sequence to abort. Tracing the route to 22.214.171.124 VRF info: (vrf in name/id, vrf out name/id) 1 126.96.36.199 56 msec 56 msec 52 msec 2 188.8.131.52 [AS 9090] 52 msec 52 msec 56 msec
Now we will change the default value from 0 to 100 for all routes.
After configuration on INBL router, Weight has been changed to 100 on all learned routes from peers. Now we will check trace output again.
INBL#trace 184.108.40.206 source loopback1 Type escape sequence to abort. Tracing the route to 220.127.116.11 VRF info: (vrf in name/id, vrf out name/id) 1 10.1.70.2 32 msec 52 msec 52 msec 2 18.104.22.168 [AS 8070] 68 msec 48 msec 52 msec
In the above configuration, we have changed the Weight for all routes. Now we will use ISP1 (22.214.171.124) to reach only 126.96.36.199/32 and 188.8.131.52/32 networks.
Configuration as below:
Now check below output, as Weight has been changed for network 184.108.40.206/32 and 220.127.116.11/32 to 200 and traffic is routing towards 18.104.22.168 (ISP1).