Understand the Importance of BGP Weight Path Attribute in Network.

Over 16 years, SPOTO helped tens of thousands of candidates achieve their Cisco CCNA, CCNP, CCIE, CISSP certification. Subscribe us and get more news. The following is a technical article that will help you understand the BGP weight attribute in a network.

Cisco routers can use BGP weights to influence routers' choice of outbound routes. To this end, when the router receives the BGP update, the router can use the route map to selectively set a weight for each route, or set a weight for all learned contention routes, and select a route with a larger weight.

The weight is a proprietary attribute of Cisco, and the weight tells us how to exit the autonomous system. The most important path for the most weighted path is 0 to 65,535. By default, the learning path is 0 and the local injection path is 32,768. The weight is a partial attribute that is set when the inbound route is updated.

let us see the configuration:-

Topology:-

GOAL:

·configure the topology as per the diagram.

·configure the basic iBGP and EBGP peering using directly connected interfaces.

·advertise all the interfaces as per the topology

·configure the next-hop address should be the next router address

·configure router 4 to prefer exit path via router1 to reach all the networks.

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

R2#show ip interface brief

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 20.1.1.1 YES NVRAM up up

Serial3/0 1.1.1.2 YES NVRAM up up

Serial3/1 2.1.1.1 YES NVRAM up up

Loopback0 12.0.0.1 YES NVRAM up up

Loopback1 12.0.1.1 YES NVRAM up up

Loopback2 12.0.2.1 YES NVRAM up up

Loopback3 12.0.3.1 YES NVRAM up up

R3#show ip interface brief

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 30.1.1.1 YES NVRAM up up

Serial3/1 2.1.1.2 YES NVRAM up up

Serial3/2 3.1.1.1 YES NVRAM up up

Loopback0 13.0.0.1 YES NVRAM up up

Loopback1 13.0.1.1 YES NVRAM up up

Loopback2 13.0.2.1 YES NVRAM up up

Loopback3 13.0.3.1 YES NVRAM up up

R4#show ip interface brief

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 40.1.1.1 YES NVRAM up up

Serial3/2 3.1.1.2 YES NVRAM up up

Serial3/3 4.1.1.1 YES NVRAM up up

Loopback0 14.0.0.1 YES NVRAM up up

Loopback1 14.0.1.1 YES NVRAM up up

Loopback2 14.0.2.1 YES NVRAM up up

Loopback3 14.0.3.1 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

R4(config)#router bgp 65444

R4(config-router)#neighbor 3.1.1.1 remote-as 65333

R4(config-router)#neighbor 4.1.1.2 remote-as 65111

R4(config-router)#network 40.0.0.0

R4(config-router)#network 4.0.0.0

R4(config-router)#network 14.0.0.0 mask 255.255.255.0

R4(config-router)#network 3.0.0.0

R4(config-router)#no synchronization

R4(config-router)#no auto-summary

R1#show ip bgp summary

BGP router identifier 11.0.3.1, local AS number 65111

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

1.1.1.2 4 65111 14 13 14 0 0 00:06:21 7

4.1.1.1 4 65444 14 9 14 0 0 00:02:07 7

R2#show ip bgp summary

BGP router identifier 12.0.3.1, local AS number 65111

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

1.1.1.1 4 65111 13 14 14 0 0 00:06:56 7

2.1.1.2 4 65333 11 12 14 0 0 00:04:29 6

R3#show ip bgp summary

BGP router identifier 13.0.3.1, local AS number 65333

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

2.1.1.1 4 65111 13 12 14 0 0 00:05:27 9

3.1.1.2 4 65444 11 14 14 0 0 00:03:59 9

R4#show ip bgp summary

BGP router identifier 14.0.3.1, local AS number 65444

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

3.1.1.1 4 65333 14 12 21 0 0 00:04:25 10

4.1.1.2 4 65111 11 16 21 0 0 00:04:05 9

R4#sh ip bgp

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

R4#traceroute 30.1.1.1

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 fewer 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

R4(config-router)#end

R4#clear ip bgp * soft

R4#traceroute 30.1.1.1

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

R4#show ip bgp 30.1.1.1

BGP routing table entry for 30.0.0.0/8, version 22

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

Advertised to update-groups:

Refresh Epoch 2

65111 65333

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

Refresh Epoch 3

65333

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

R4#show ip bgp

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

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