BGP Weight Attribute

A Cisco router can use the BGP weight, in that router to influence that one router’s choice of outbound route. To do so, when a router receives a BGP update, that router can set the weight either selectively, per route, using route map or for all routes learned competing routes, choosing the route with the bigger weight.

lets see the configuration:-

GOAL:

R1#show ip interface brief

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 10.1.1.1 YES NVRAM up up

Serial3/0 1.1.1.1 YES NVRAM up up

Serial3/3 4.1.1.2 YES NVRAM up up

Loopback0 11.0.0.1 YES NVRAM up up

Loopback1 11.0.1.1 YES NVRAM up up

Loopback2 11.0.2.1 YES NVRAM up up

Loopback3 11.0.3.1 YES NVRAM up up

R1(config)#router bgp 65111

R1(config-router)#neighbor 1.1.1.2 remote-as 65111

R1(config-router)#neighbor 4.1.1.1 remote-as 65444

R1(config-router)#neighbor 1.1.1.2 next-hop-self

R1(config-router)#network 10.0.0.0

R1(config-router)#network 1.0.0.0

R1(config-router)#network 4.0.0.0

R1(config-router)#network 11.0.0.0 mask 255.255.255.0

R1(config-router)#no auto-summary

R1(config-router)#no synchronization

R2(config)#router bgp 65111

R2(config-router)#neighbor 1.1.1.1 remote-as 65111

R2(config-router)#neighbor 2.1.1.2 remote-as 65333

R2(config-router)#neighbor 1.1.1.1 next-hop-self

R2(config-router)#network 20.0.0.0

R2(config-router)#network 2.0.0.0

R2(config-router)#network 1.0.0.0

R2(config-router)#network 12.0.0.0 mask 255.255.255.0

R2(config-router)#no auto-summary

R2(config-router)#no synchronization

R3(config)#router bgp 65333

R3(config-router)#neighbor 2.1.1.1 remote-as 65111

R3(config-router)#neighbor 3.1.1.2 remote-as 65444

R3(config-router)#network 30.0.0.0

R3(config-router)#network 3.0.0.0

R3(config-router)#network 2.0.0.0

R3(config-router)#network 13.0.0.0 mask 255.255.255.0

R3(config-router)#no synchronization

R3(config-router)#no auto-summary

BGP table version is 21, local router ID is 14.0.3.1

Status codes: s suppressed, d damped, h history, * valid, > best, i — internal,

r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,

x best-external, a additional-path, c RIB-compressed,

Origin codes: i — IGP, e — EGP, ? — incomplete

RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path

*> 1.0.0.0 4.1.1.2 0 0 65111 i

* 3.1.1.1 0 65333 65111 i

* 2.0.0.0 4.1.1.2 0 65111 i

*> 3.1.1.1 0 0 65333 i

*> 3.0.0.0 0.0.0.0 0 32768 i

* 3.1.1.1 0 0 65333 i

*> 4.0.0.0 0.0.0.0 0 32768 i

* 4.1.1.2 0 0 65111 i

* 3.1.1.1 0 65333 65111 i

*> 10.0.0.0 4.1.1.2 0 0 65111 i

* 3.1.1.1 0 65333 65111 i

*> 11.0.0.0/24 4.1.1.2 0 0 65111 i

* 3.1.1.1 0 65333 65111 i

*> 12.0.0.0/24 4.1.1.2 0 65111 i

Network Next Hop Metric LocPrf Weight Path

* 3.1.1.1 0 65333 65111 i

* 13.0.0.0/24 4.1.1.2 0 65111 65333 i

*> 3.1.1.1 0 0 65333 i

*> 14.0.0.0/24 0.0.0.0 0 32768 i

*> 20.0.0.0 4.1.1.2 0 65111 i

* 3.1.1.1 0 65333 65111 i

* 30.0.0.0 4.1.1.2 0 65111 65333 i

*> 3.1.1.1 0 0 65333 i

*> 40.0.0.0 0.0.0.0 0 32768 i

Type escape sequence to abort.

Tracing the route to 30.1.1.1

VRF info: (vrf in name/id, vrf out name/id)

1 3.1.1.1 168 msec 60 msec 24 msec

To reach the 30.1.1.1 router 4 by default using 3.1.1.1 interface because it has less numbers of AS.

but we want to router 4 go via 4.1.1.1 interface to reach all the networks.

R4(config)#router bgp 65444

R4(config-router)#neighbor 4.1.1.2 weight 20000

Type escape sequence to abort.

Tracing the route to 30.1.1.1

VRF info: (vrf in name/id, vrf out name/id)

1 4.1.1.2 8 msec 56 msec 4 msec

2 1.1.1.2 [AS 65111] 52 msec 40 msec 36 msec

3 2.1.1.2 [AS 65111] 52 msec 40 msec 64 msec

BGP routing table entry for 30.0.0.0/8, version 22

Paths: (2 available, best #1, table default)

Advertised to update-groups:

4.1.1.2 from 4.1.1.2 (11.0.3.1)

Origin IGP, localpref 100, weight 20000, valid, external, best

rx pathid: 0, tx pathid: 0x0

3.1.1.1 from 3.1.1.1 (13.0.3.1)

Origin IGP, metric 0, localpref 100, valid, external

rx pathid: 0, tx pathid: 0

BGP table version is 29, local router ID is 14.0.3.1

Status codes: s suppressed, d damped, h history, * valid, > best, i — internal,

r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,

x best-external, a additional-path, c RIB-compressed,

Origin codes: i — IGP, e — EGP, ? — incomplete

RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path

*> 1.0.0.0 4.1.1.2 0 20000 65111 i

* 3.1.1.1 0 65333 65111 i

*> 2.0.0.0 4.1.1.2 20000 65111 i

* 3.1.1.1 0 0 65333 i

*> 3.0.0.0 0.0.0.0 0 32768 i

* 3.1.1.1 0 0 65333 i

*> 4.0.0.0 0.0.0.0 0 32768 i

* 4.1.1.2 0 20000 65111 i

* 3.1.1.1 0 65333 65111 i

*> 10.0.0.0 4.1.1.2 0 20000 65111 i

* 3.1.1.1 0 65333 65111 i

*> 11.0.0.0/24 4.1.1.2 0 20000 65111 i

* 3.1.1.1 0 65333 65111 i

*> 12.0.0.0/24 4.1.1.2 20000 65111 i

Network Next Hop Metric LocPrf Weight Path

* 3.1.1.1 0 65333 65111 i

*> 13.0.0.0/24 4.1.1.2 20000 65111 65333 i

* 3.1.1.1 0 0 65333 i

*> 14.0.0.0/24 0.0.0.0 0 32768 i

*> 20.0.0.0 4.1.1.2 20000 65111 i

* 3.1.1.1 0 65333 65111 i

*> 30.0.0.0 4.1.1.2 20000 65111 65333 i

* 3.1.1.1 0 0 65333 i

*> 40.0.0.0 0.0.0.0 0 32768 i

Originally published at https://www.internetworks.in.