Routing Issue - BGP

[Jit Oza]

Hi Guys,

Need you help to solve my BGP issue. I have attached network topology with subnets at different sites as an example.

Trace route from Remote site 1 and 2 to London(e.g. 10.124.0.0/16)is going via Melbourne. And from Melbourne using different ISP 2 to London. That increases the latency.

What I want is traffic from Remote site 1 and 2 should go via Singapore to London as marked with green line. Does not want that traffic via Melbourne. 

Remote site 1 and 2, Melbourne and Singapore they all in same AS. While Melbourne to London is on different AS. 

Any help really appreciated. Let me know if any questions.

Jit

[Edwin Portillo]

My Friend, 

You could consider the following:

iBGP----> remote site 1 - Singapore - london (same AS). 

eBGP--->remote site 1 - Melbourne - london (diferent AS).

Link: https://www.networkcomputing.com/data-centers/bgp-basics-internal-and-external-bgp 

[Jit Oza]

Dear Edwin,

I cant change it. They all connected via MPLS with different AS. and not possible to put them in one AS between Singapore and London as they are different service providers.

remote site 1 to Singapore - eBGP 

remote site 1 to Melbourne - eBGP

Melbourne to London - eBGP.

[Edwin Portillo]

Hermano, 

El atributo de peso es un atributo definido por Cisco. Este atributo utiliza el peso para seleccionar un mejor camino. El peso se asigna localmente al enrutador. El valor solo tiene sentido para el enrutador específico. El valor no se propaga ni se transmite a través de ninguna de las actualizaciones de ruta. Un peso puede ser un número de 0 a 65,535. Las rutas que origina el enrutador tienen un peso de 32,768 de forma predeterminada, y otras rutas tienen un peso de 0.

Las rutas con un valor de peso mayor tienen preferencia cuando existen varias rutas al mismo destino. 

RTC # router bgp 300 vecino 1.1.1.1 remoto-como 100 vecino 1.1.1.1 peso 200 
! --- La ruta a 175.10.0.0 desde RTA tiene un peso de 200.
vecino 2.2.2.2 remoto-como 200 vecino 2.2.2.2 peso 100 
! --- La ruta a 175.10.0.0 desde RTB tiene un peso de 100.

[Jit Oza]

Hi Edwin,

London subnet is advertised only at London site. So 10.124.0.0 is London specific. 

Even in singapore its not advertised as well. 

I tried via LP but it did not work. 

So 10.124.0.0/16 is learnt only via Melbourne not from any other site.

I hope this makes sense.

Jit

[Giuseppe Larosa]

Hello Jit,

in order to understand better your network scenario you should provide some more details:

a) what kind of MPLS services are you using ? are all the sites connected with MPLS L3 VPN service?

if yes, what makes Melbourne site different so that is able to advertise the London prefix 10124.0.0/16 and Singapore is not able to do the same.

Note:

In MPLS L3 VPN the routing is not under your control, but under the service provider control. Your CE routers peer with eBGP not between them directly but with the PE nodes of the service provider(s).

If Singapore learns of London prefix 10.124.0.0/16 from the backbone (from the SP PE node) it will not re-advertise it to other sites.

So likely Melbourne site is learning the London prefix over an eBGP session that is not with a PE of a L3 VPN service and for this reason is allowed to propagate it to other sites.

b) You can check on remote site 1, remote site 2 how many paths to 10.124.0.0/16 exist with

show ip bgp 10.124.0.0/16

if only one path exists via Melbourne you are currently tied to it.

>> So 10.124.0.0/16 is learnt only via Melbourne not from any other site.

Hope to help

Giuseppe

[Jit Oza]

Hi  Giuseppe,

Its MPLS L3 VPN service. 

To answer to your questions,

Site 1, site 2, Melbourne and Singapore they all connected via same MPLS network, which is Telstra. 

However, to go to London from Melbourne is peering with EIGRP router then BT link via eBGP.

 So like this, Melbourne (Nexus <-> EIGRP <-> Router then BT router (as provider) with eBGP).

I hope this helps.

Jit

[Jit Oza]

Hi,

Anyone for the solution.

[Jon Marshall]

Does Singapore peer with London using EBGP ? 

What does the BGP routing table on Melobourne and Singapore look like for that London subnet ? 

Jon

[Giuseppe Larosa]

Hello Jon,

the original poster Jit has described his network scenario in a previous post:

>>

Site 1, site 2, Melbourne and Singapore they all connected via same MPLS network, which is Telstra.

>>>> However, to go to London from Melbourne is peering with EIGRP router then BT link via eBGP.

So like this, Melbourne (Nexus <-> EIGRP <-> Router then BT router (as provider) with eBGP).

In the current network scenario, Site1, Site2, Singapore and Melbourne are VRF sites of a single MPLS L3 VPN managed by Telstra.

The Melbourne site advertises the London subnet to the local PE node in eBGP because it learns it via EIGRP or via BGP (that learns it via a DIFFERENT eBGP session with a BT router that is not part of the Telstra L3 VPN).

for this reason from Site1, Site 2 they send traffic to Melbourne to reach the London subnet.

From Jit's network diagram in first post of this thread we see that there is also an eBGP session in Singapore that can be used to reach London.

However, being an MPLS L3 VPN service the best path choice is left to Telstra PE nodes that connects to Site1 and Site2.

I mean that even if Singapore is advertising the London subnet to the local PE node, from the point of view of PE nodes serving Site1 and Site2 they may prefer the route coming from the Melbourne's PE node.

For example because the Melbourne PE node has a lower IGP metric inside the Telstra backbone in comparison with the IGP metric to PE node in Singapore.

To achieve the desired target of having Site1 and Site2 using the Singapore route to London Jit needs to work with Telstra tech support asking them to rise the BGP local preference on the Singapore PE node for the London prefix.

It is not something that Jit can achieve by himself as routing is managed by Telstra.

Hope to help

Giuseppe

[Jit Oza]

Hi Giuseppe,

I dont think so its Telstra issue, but I will double check with them. Whatever we route to Telstra, they pass it to MPLS. 

[Giuseppe Larosa]

Hello Jit,

let us say that you need a customization of the routing for your MPLS L3 VPN service.

ask to Telstra to perform the following:

>>To achieve the desired target of having Site1 and Site2 using the Singapore route to London Jit needs to work with Telstra tech support asking them to rise the BGP local preference on the Singapore PE node for the London prefix.

A MPLS L3 VPN uses a peer model for routing: your devices exchange eBGP routing updates with the Telstra PE nodes and not directly between them.

From Site1 CE node , your router, how many paths do you see for the London subnet?

My guess is that you see only one path via the local PE node of Telstra.

The local PE node of Telstra can have two paths over VPNv4 to reach the London prefix and makes a best path choice based on backbone routing criteria: if all BGP attributes are the same it will pick up the closest remote PE node and it choices to go to Melbourne.

Hope to help

Giuseppe

[Jon Marshall]

Hi Giuseppe

Thanks for the clarification and good to see you back on the forums :) 

Jon

[Jit Oza · Accepted solution]

Hi Guys,

I have found the solution. As mentioned earlier that we have BT presence in Singapore and Melbourne. So i just redistributed EIGRP learned route with route-map with London subnet only and advertised to BGP. And that way now my traffic from different sites going to Singpore and to London.

Thank you all. 

Jit

[Giuseppe Larosa]

Hello Jit,

nice to know that you have solved this issue.

Thanks for your feedback.

Best Regards

Giuseppe