Change Local Preference for a specific network

[Henrikp92]

Hello!

I have a question about Local Preference.

As you can see at the picture below i have 2 routers connected via two fastethernet links. Both routers running BGP. R1 (AS 65100) and R2 (AS65000).

R1 got a loopback interface with the IP address 1.1.1.1 255.255.255.255

R2 got two loopback interfaces 2.2.2.2 255.255.255.255 and 3.3.3.3 255.255.255.255.

Let's say that i want the host 3.3.3.3 to go over fa0/1 to reach 1.1.1.1 and rest of the network to go through fa0/0. What can I do to make this possible?

I know how to set Local Preference on an interface... but then all networks will take that specific path.

route-map LP_FA0/1 permit 10

set local-preference 200

neighbor 192.168.2.1 route-map LP_FA0/1 in

[Peter Paluch]

Hello Henrik,

You simply modify the route-map so that it applies only to selected prefixes. For example:

ip prefix-list LocalPref permit 3.3.3.3/32
!
route-map LP_FA0/1 permit 10
 match ip address prefix-list LocalPref
 set local-preference 200
!
route-map LP_FA0/1 permit 20

And afterwards, you use this route-map just like you do now. The prefix-list is similar to ACLs and performs matching on prefixes and masks. The seemingly empty block 20 of your route-map is necessary to allow all other prefixes without modification (if it was not present in the route-map, all other prefixes would be dropped completely).

Best regards,

Peter

[Henrikp92]

Hello Peter!

Thanks for the fast reply!

I tried the prefix-list like you said, but it still doesn't work. If i try to ping 1.1.1.1 from 3.3.3.3 it takes Fa0/0 instead of Fa0/1.

And as you can see below the Local Preference is empty now.

R2#show ip bgp

   Network          Next Hop            Metric  LocPrf    Weight    Path

*  1.1.1.1/32       192.168.2.1            0                    0            65100    i

*>                     192.168.1.1             0                    0            65100    i

*> 2.2.2.2/32       0.0.0.0                  0                32768          i

*> 3.3.3.3/32       0.0.0.0                  0                 32768         i

This is from my config

neighbor 192.168.2.1 route-map LP_FA0/1 in

!

ip prefix-list LocalPref seq 5 permit 3.3.3.3/32

!

route-map LP_FA0/1 permit 10

match ip address prefix-list LocalPref

set local-preference 200

!

route-map LP_FA0/1 permit 20

Thanks,

Henrik

[Raju Sekharan]

Hi Henri

Your destination IP is 1.1.11/32. So you should have applied the local-preference for 1.1.1.1
Modify config as below. This should work


neighbor 192.168.2.1 route-map LP_FA0/1 in
!
ip prefix-list LocalPref seq 5 permit 1.1.1.1/32
!
route-map LP_FA0/1 permit 10
 match ip address prefix-list LocalPref
 set local-preference 200
!
route-map LP_FA0/1 permit 20

[Henrikp92]

Hello Rajs

Thanks for the reply!

Please, correct me if i'm wrong

But then all traffic to 1.1.1.1 will be filtred through Fa0/1. I just want all traffic that comes from 3.3.3.3 should go through Fa0/1. Shouldn't matter which destination the packet has. Only want the routers to look at the source address, in this case 3.3.3.3.

Just want a rule to say. "All traffic from 3.3.3.3 should go through Fa0/1, rest go through fa0/0.".

I have a MED route-map aswell that says that all traffic that comes from R1 should go via Fa0/0 to get into my AS. But that's what i want anyway. But that one was a bit simpler because then i didnt have to match any ACL or Prefix lists.
I just created the route-map, wrote a set statement for metric and then applied it on fa0/1 to get a higher metric.

[Raju Sekharan]

Hi Henrik,

I don't think you can implement your requirement by modifying BGP attributes

If you want to route based on the source and destination, you can go for policy-based routing using local-policy

I can help with configs if you are looking for that option

/Raju

[Raju Sekharan]

Normal routing is always based on destination.

So if you want to take routing decision using source IP, go for PBR

/Raju

[Henrikp92]

Ok I may have an idea then. Thanks I'll get back to you with my results!

[AhmedSonba]

Hello Henrik

I think what you are speaking about can be done by using Route Maps

for example please the the below :

Configuring the Route-Map for routing traffic by source
Step1: Define the traffic with an access-List 
RouterA(config)#access-list  10 3.3.3.3 255.255.255.255.

Step 2: Create a Route-Map with an action 
RouterA(config)# route-map Traffic_to_fastetherent0/1

RouterA(config-route-map)#match ip 10

RouterA(config-route-map)#set next hop 192.168.2.1

Step 3: Apply the Route-Map to the router interface that the traffic enters into 
RouterA(config)#interface fastetherent 0/1 
RouterA(config-int)#ip policy Traffic_to_fastetherent0/1

Traffic from the 3.3.3.3  host  will be forced to the router 192.168.2.1 network .

For more information about Route Maps please refer tot he following link :

http://www.ciscopress.com/articles/article.asp?p=1569333&seqNum=4

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t2/feature/guide/gt_brmcs.html

Hope it will Help

Ahmed Sonba

[Henrikp92]

Thanks for the reply Ahmed!

I'm gonna try it out tomorrow when i'm at school again.

Thanks,

Henrik

[Henrikp92]

Hey Ahmed,

I tried your solution. But it didn't work either.

And to be honoest I don't really understand why.

Because what you wrote, if the traffic match the source address for 3.3.3.3 it should change the next-hop to 192.168.2.1. But it don't...

I tried an prefix list instead of an ACL aswell... but same resualt..

I'm gonna try something else before I talk with my teacher. It's weekend now so he won't look in his mail today I think, so have to check with him tomorrow.

But i apperciate that you all took your time to help me.

Thanks!

Henrik Josefsson

[Raju Sekharan]

If the trafifc is originated from the router you have configured policy-routing, you need to configure

ip local policy route-map < route-map name>

/Raju

[Henrikp92]

Hello Rajs,

I tried what you suggest with the policy-routing, but nothing changed. The traffic still went through fa0/0.

I'm just gonna put my conf for R2 here and maybe you can tell me what i have done wrong.

interface Loopback0

ip address 2.2.2.2 255.255.255.255

!

interface Loopback1

ip address 3.3.3.3 255.255.255.255

!

interface FastEthernet0/0

ip address 192.168.1.2 255.255.255.252

duplex auto

speed auto

!

interface FastEthernet0/1

ip address 192.168.2.2 255.255.255.252

ip policy route-map TRAFFIC_TO_FA0/1

duplex auto

speed auto

!

!

!

router bgp 65000

no synchronization

bgp log-neighbor-changes

network 2.2.2.2 mask 255.255.255.255

network 3.3.3.3 mask 255.255.255.255

neighbor 192.168.1.1 remote-as 65100

neighbor 192.168.2.1 remote-as 65100

neighbor 192.168.2.1 route-map MED_FA0/1 out

no auto-summary

!

ip prefix-list TRAFFIC_FROM_3 seq 5 permit 3.3.3.3/32

!

route-map MED_FA0/1 permit 20

set metric 20

!

route-map TRAFFIC_TO_FA0/1 permit 10

match ip address prefix-list TRAFFIC_FROM_3

set ip next-hop 192.168.2.1

Question number 2 (more like a sidequestion)

I think i have confused myself when we're talking about setting "inbound" or "outbound" for route-maps.

neighbor 192.168.2.1 route-map MED_FA0/1 out <--- here i've got a "outbound" for an example... but does anyone have any good tips how i should think when i'm applying route-maps, ACL etc.

What I have been told... I should think that I'm the router. And in this case... i'm sending OUT MED packets so my neighbor knows how to send back packets. But then I got confused with the Local Preference where I use the "IN" statement instead. But if I'm right, inbound means that all packets comming IN to the router will be filtered. But in this case, Local preference statement is generated on this router.

Thanks

Henke

[cadet alain · Accepted solution]

Hi,

1) you can't use a prefix-list for PBR, you need to use an ACL

2) if traffic is originated from a loopback then it is considered local traffic and so you need to use local PBR:

    ip local policy route-map command

3) you said you wanted traffic from 3.3.3.3 to take f0/0 so you must set next-hop to 192.168.1.1 per you initial topology.

Side question:

when influencing outbound traffic path then apply route-map inbound to receive the attribute

when influencing inbound traffic path then apply route-map outbound to send your attribute to neighbour so it influences how it routes towards your prefixes

Regards.

Alain

Don't forget to rate helpful posts.

[Henrikp92]

Hey Cadet!

Thanks a lot. I tried it and it worked.

And thanks for the information about PBR and the side question.

Best regards

Henke