BGP Local Preference Attribute

BGP attribute local preference is the second BGP attribute and it can be used to choose the exit path for an autonomous system. Here are the details:

  • Local preference is the second BGP attribute.
  • You can use local preference to choose the outbound external BGP path.
  • Local preference is sent to all internal BGP routers in your autonomous system.
  • Not exchanged between external BGP routers.
  • Local preference is a well-known and discretionary BGP attribute.
  • Default value is 100.
  • The path with the highest local preference is preferred

Let me show you an example:

bgp local preference example

You can use local preference to configure your autonomous system to select a certain exit point. Instead of configuring weight on each router you can use local preference because it is exchanged on all internal BGP routers. By increasing the local preference to 800 we can make AS 1 send all traffic towards AS 2.

A well-known discretionary BGP attribute must be recognized by all BGP routers per RFC but its presence in a BGP update is optional.

Now let me show you how to configure local preference, here is the topology that we will use:

bgp local preference lab

In the picture above we have two autonomous systems. R1 will advertise network 1.1.1.0/24 towards AS 2 and R4 will have to make a choice when it wants to reach this network. It can go through router R2 or R3, we’ll see how local preference influence this.

R1(config)#router bgp 1
R1(config-router)#neighbor 192.168.12.2 remote-as 2
R1(config-router)#neighbor 192.168.13.3 remote-as 2
R1(config-router)#network 1.1.1.0 mask 255.255.255.0

This is the configuration of R1, nothing spectacular here.

R2(config)#interface loopback 0
R2(config-if)#ip address 2.2.2.2 255.255.255.0

R2(config)#router ospf 1
R2(config-router)#network 192.168.24.0 0.0.0.255 area 0
R2(config-router)#network 2.2.2.0 0.0.0.255 area 0
R3(config)#interface loopback 0
R3(config-if)#ip address 3.3.3.3 255.255.255.0

R3(config)#router ospf 1
R3(config-router)#network 192.168.34.0 0.0.0.255 area 0
R3(config-router)#network 3.3.3.0 0.0.0.255 area 0
R4(config)#interface loopback 0
R4(config-if)#ip address 4.4.4.4 255.255.255.0

R4(config)#router ospf 1
R4(config-router)#network 192.168.24.0 0.0.0.255 area 0
R4(config-router)#network 192.168.34.0 0.0.0.255 area 0
R4(config-router)#network 4.4.4.0 0.0.0.255 area 0

I’ll configure OSPF within AS2 to prepare it for IBGP.

R2(config)#router bgp 2
R2(config-router)#neighbor 192.168.12.1 remote-as 1
R2(config-router)#neighbor 3.3.3.3 remote-as 2
R2(config-router)#neighbor 3.3.3.3 update-source loopback0
R2(config-router)#neighbor 4.4.4.4 remote-as 2
R2(config-router)#neighbor 4.4.4.4 update-source loopback0
R2(config-router)#neighbor 4.4.4.4 next-hop-self
R3(config)#router bgp 2
R3(config-router)#neighbor 192.168.13.1 remote-as 1
R3(config-router)#neighbor 2.2.2.2 remote-as 2
R3(config-router)#neighbor 2.2.2.2 update-source loopback0
R3(config-router)#neighbor 4.4.4.4 remote-as 2
R3(config-router)#neighbor 4.4.4.4 update-source loopback0
R3(config-router)#neighbor 4.4.4.4 next-hop-self
R4(config)#router bgp 2
R4(config-router)#neighbor 2.2.2.2 remote-as 2
R4(config-router)#neighbor 2.2.2.2 update-source loopback 0
R4(config-router)#neighbor 3.3.3.3 remote-as 2
R4(config-router)#neighbor 3.3.3.3 update-source loopback 0

And above you can see the BGP configurations.

Now let’s find out what path R4 will use to reach network 1.1.1.0/24:

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Forum Replies

  1. hy, i’m something problem in my network, can you help me.
    example ;
    - added 2 router James
    - added 1 router CE
    - AS 1 and AS 2
    - Local preference : 700
    - routing router CE to router James A and B with OSPF
    how to configur Local preference in router Jim ?
    My topology ;

    router CE ==> router james A & B (AS :1) ======> (AS:2) router Jim & Jon

    Thanks

  2. You can use a route-map like I did above. For example attach it to router Jim on inbound updates and set the local preference there.

  3. i’ve done like route-map like did above, so ping & Lokal preference OK.
    but I am having problems in the CE router, I use OSPF routing.

    router-ce (config)#router ospf 10
    router-ce (config-router)#
    router-ce (config-router)#redistribute connected
    % Only classful networks will be redistributed
    R22(config-router)#
    

    why Only classful networks will be redistributed ?

    thanks

  4. Hi Parwis,

    By default, only classful networks are redistributed when using the “redistribute connected” command. You’ll need to include the “subnets” option if you want classless networks to be also redistributed. The command will be :

    redistribute connected subnets

  5. Hi,

    What would be scenario if i apply

    neighor 192.168.13.1 route-map LOCALPREF out instead of route-map LOCAL PREF in

    Regards,
    Vishal

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