Best Path BGP for dual link between two AS - how force only one path

[_|brt.drml|_]

Question:

In BGP I understand how to influence routing information comming from my peers. However, every example I have learned in the course was , dual-homed... 

However, now I stumble on this situation.

I have two routers, both connected with two 'links'. Both router are different AS.

In my BGP table I receive routers from my AS.

I emulated one link as a low bandwidth and high delay link. 

I emulated one link as my high speed link.

What is the best option in BGP to influence the route selection? 

I thought local-preference or med, but this only influence the route I install and use... It does not influence my 'interface' decision 

Any ideas are welcome. 

Thank you.

B

[Flavio Miranda]

Hi

 Take a look on this two parameters:

maximum-paths and bgp dmzlink-bw

https://blog.ipspace.net/2021/06/ucmp-bgp-link-bandwidth.html 

[MHM Cisco World]

"I thought local-preference or med, but this only influence the route I install and use... It does not influence my 'interface' decision"
influences the route and hence influence the next-hop and finally effect the interface lead to this next-hop.

so LP and MED is what looking for force Edge router to use on interface.

[_|brt.drml|_]

Yes but:

The router receives information from one ISP . So influencing, is not influencing the 'interface'...

I receive on both interface the neighbour information... How can I configure the router to tell not to use the slowest link for BGP routing...



R1:LINK_SLOW : R2

R1:LINK_FAST : R2



How can I influence what 'interface' to take?

If I look in the topology table of BGP I can see all possible metrics (Mnemonic: I like oranges ...)



Or did I misunderstood you? Already thank you for the reply. Appreciate it.

[David Ruess]

Hello,

Can you provide a diagram and indicate which link you would like the routes to take?

Also to clarify you want to choose the higher speed link to reach the ISPs? 

-David

[_|brt.drml|_]

file provides the schema example. 

I hope it is clear enough. Do have a look and it is iBGP in our situation....

So in short: slow link, fast link. 

Situation is that two tunnels exist with my central routers. Both tunnels receive information from those routers. 

The question is: How can I ignore for BGP the tunnel with the slowest link?

The configuration provides other networks to that site through MP-BGP. And the bonus is, how to 'direct' traffic... 

Solutions i think off:

- IP SLA - shut down a tunnel, but this affects my other RP... so not a good solution.

- IP route vrf **** 0.0.0.0 0.0.0.0 tunnel_high_speed [but this seems to not direct my traffic at all..]

The above push with 'dmz-bw' metric was a good idea. But, need to test this first in my lab. 

Many thanks , 

B 

[MHM Cisco World]

AS1 include all 

R1 and R2 & R3, 
if you run EIGRP between router why you need iBGP ?

Anyway 

R1-R2 is iBGP 
R1-R3 is iBGP 

to config the LP and MED "I will check using MED since MED is never use inside AS where it use in eBGP but i will make look" we need to sure that both R2 and R3 receive same subnet from MPLS and forward it to R1.
R1 then will select best path. 
if each R2 and R3 forward different subnet R1 must use slow link.

So are both R2 & R3 forward same subnet to R1?

[_|brt.drml|_]

We use this to differentiate clearly between services. The different networks are separated and come together with mp-bgp. It is also used to add or limit services on our routers in those areas.

Why not eigrp, to not clutter the routing table with many routes and separate them clearly with vrf.

[_|brt.drml|_]

R2/R3 are acting as our backup, so if one fails the other gets all traffic and serves the 'network'. This means they all receive the networks. We just manage different id's. Works fine.

[_|brt.drml|_]

We use this to differentiate clearly between services. The different networks are separated and come together with mp-bgp. It is also used to add or limit services on our routers in those areas.

Why not eigrp, to not clutter the routing table with many routes and separate them clearly with vrf.

[paul driver]

Hello

---

@_|brt.drml|_ wrote:

I receive on both interface the neighbour information... How can I configure the router to tell not to use the slowest link for BGP routing...
R1:LINK_SLOW : R2
R1:LINK_FAST : R2

---

Depending on what ios you are using and as this is a single router with two bgp peerings then you can use weight attribute, either to influence all traffic for received routes or on specific routes..

Example1
router bgp x
neighbor (R2link1) weight 40000 < preferred for al routes
neighbor (R2link2) weight 20000



Example2
route-map R2link1-Weight permit 10
set weight 40000


route-map R2link2-Weight permit 10
set weight 20000


router bgp x
neighbor (R2link1) route-map R2link1-Weight in  < preferred for all routes
neighbor (R2link2) route-map R2link2-Weight in



Example3
access-list 1 permit 3.3.3.0 0.0.0.255  < preferred  link1
access-list 2 permit 33.33.33.0 0.255.255.255 < preferred link2



route-map R2link1-Weight permit 10
match ip address 1
set weight 40000
route-map R2link1-Weight permit 99 <  prefer all existing and future routes
set weight 30000


route-map R2link2-Weight permit 10
match ip address 2
set weight 50000

route-map R2link2-Weight permit 99

router bgp x
neighbor (R2link1) route-map R2link1-Weight in
neighbor (R2link2) route-map R2link2-Weight in 

[_|brt.drml|_]

But in the route-map we have found yesterday that you can set the interface...



Route-map foo

Match ACL

Set interface 'interface'



It looks like a valuable solution....

With the testing my colleague could see traffic generated over the preferred interface and the other stayed empty...

[paul driver]

Hello

---

@_|brt.drml|_ wrote:

But in the route-map we have found yesterday that you can set the interface...

Route-map foo
Match ACL
Set interface 'interface'

It looks like a valuable solution....
With the testing my colleague could see traffic generated over the preferred interface and the other stayed empty.

---

What you are showing there would be used with Policy Based routing(PBR) which could be viable but I would say it isn't as dynamic than using bgp attributes also PBR it will ONLYi nfluence in one direction, you would still have to append bgp attributes to influence the return traffic from R2.

[gurbinder.kabbay]

if I understood your question correctly, then you can achieve this through weight configuration. to give the preference to a particular interface on same Router.

Thanks.

Gurbinder

[MHM Cisco World]

Sorry for may late relay but I found solution two days ago but I was so so busy.
friend R2 and R3 "edge router connect to MPLS SP" must interconnect to each other, 
when the return traffic come from MPLS SP2 and want to take Slow Link it will not it will go through interconnect link to other Edge router and from there it will take Fast Link.