Re: BGP Weight Attribute

nap.deguzman · ‎03-20-2015

Guys,

I just need some clarification on the use of the bgp weight attribute. Let us say I have this setup. If both R1 and R2 are receiving the same set of routes from R3-R6, can I use weight attribute to influence the received routes so that R1 will have the best routes? Does weight works on different AS?

Jon Marshall · ‎03-20-2015

BGP weight is local to the router you apply it to.

It is not exchanged between any BGP peers.

So it is only used to prefer some routes over others on the same router.

Jon

Cisco Freak · ‎03-21-2015

As Jon said, weight is local to the router.

In this setup, you can use weight to influence if R3 or R4 is better in AS 65512. Similarly you can you it to influence whether R5 or R6 is better in AS 65513.

However, you can't use weight to do comparison between R1 and R2. They are running EIGRP between them.

CF

Ravindra Simpi · ‎03-23-2015

Absolutely. Cisco Freak and Jon Marshall are correct.

Also, you cannot compare between BGP attributes if R1 and R2 are not BGP peers. If you want to take complete control of routes being advertised by R3, R4, R5 and R6, you would be in better position if you run iBGP between R1 and R2.

Once BGP starts to run between R1 and R2, they both will get information about routes advertised by R3, R4, R5 and R6 and you will be able to reap many benefits of BGP protocol.

Peace and health,

Ravindra

nap.deguzman · ‎03-23-2015

Thanks for the clarifications Guys!

Ravindra Simpi · ‎03-23-2015

Please rate the Post so other will know when an answer has been found.

Ravindra Simpi · ‎03-23-2015

Please rate the Post so that others will know when an answer has been found.

Edgar c Francis · ‎02-15-2019

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.

Weight is cisco proprietary attribute, weight tells how to exit the autonomous system. Path with the highest weight is more desirable range is 0 through 65,535, by default 0 for learned routes, 32,768 for locally injected routes. weight is partial attribute, set on inbound route update.

lets 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 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

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:

1

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

https://www.internetworks.in/

EDGER C FRANCIS

Lazarus_telecomTech · ‎12-19-2023

Here's a video that might help to clarify how the Weight attribute works in the whole scheme of things as far as the BGP Best Path Algorithm goes: