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Brent Rockburn

MPLS training question

Hey guys,

I took an MPLS course online and it pointed something out I really didn't get, or at least need clarification on.

The person giving the course said that because of how the PE's interact with the P LSR's that is isn't necessary to run a routing protocol in the core on the P's and that savings in resource is a reason to go to MPLS.

Can I get some clarification?

Do the P MPLS LSR's rely only on labels for routing or are they running a routing protocol in the background?



Reza Sharifi
Hall of Fame Expert

Hi Brent,

P routers just like the PE routers run IGP, BGP and MPLS.  The only difference between the two is that P routers do not need to support VPN functionality.  So if you have this scenario:


You only create your VPN/VRF on the PE routers, and not on the P router, but you still need to enable IGP, BGP, and MPLS on the P router



Hello Brent,

Reza is actually incorrect when he says that the P router runs BGP, P routers only run MPLS and the Core IGP which it uses to perform packet forwarding updates between PE routers, and in most cases a Route Reflector.

Here is a quick decription of each element of a Service Provider IP/MPLS network:

P Router - A better term is Label Switch Router (LSR) which is a featureless box with the primary purpose of Label Switching and passing IGP routing updates for MP-BGP routing between PE routers.

PE Router - Provider Edge Router, which by its name is at the aggregation layer of the MPLS Core. This is where Customer Edge routers terminate their peering (whatever PE-CE protocol is selected). These routers run MultiProtocol BGP between each other PE router and Route Reflector (if its in the design) to exchange VPNV4 routing (VPN routes) between PEs.

CE Router - Customer Edge Router, is  the Customers CPE or WAN Edge router.

If you are intested in learning more on MPLS, I recommend you read the Cisco Press book "MPLS Fundamentals" by Luc De Ghein.




So, how do you configure a P router as a Router Reflector without running BGP?

Hi Reza,

You misunderstood what I was saying. I meant for PE routers and for a RR if its part of the solution, not that the P is a RR.



Luc De Ghein
Cisco Employee

Hi Brent,

Let me see if I can bring clarification.

The trainer said that running a routing protocol on the P routers was not necessary.

Let's have only a look at an MPLS VPN network, because that is what is referred to here, I think.

(Also, we only consider LDP and not TE as label distribution protocol.)

In that case, all PE and P routers need to run an IGP. When MPLS is deployed, MPLS still relies on

a routing protocol, which distributes the routes. LDP distributes the labels. LDP only assigns

labels to IGP routes in the routing table, hence an IGP is needed in the core. The routes from the

IGP and the labels from LDP are the two main ingredients needed to build the LFIB which is used

to forward the labeled packets.

Now, perhaps I know where the confusion comes from. I believe that in the training a hypothetical

alternate solution to MPLS VPN as we know it, was presented. Namely, that the VPN routes

could be distributed hop-by-hop from PE to P to PE routers by one IGP per VPN (and LDP

distributing the labels). This could be theoretically possible, but it would lead to scalability

issues because of the number of routing protocols. It also leads to an operational nightmare

for the people operating the MPLS network. Hence, that kind of MPLS VPN was never implemented.

I believe this could be the "savings in resource" that you mentioned.

In the real MPLS VPN solution, we run BGP between the PE routers which carry the VPN routes (and the VPN label),

hence the P routes do not need to run an IGP per VPN. They only need to run one IGP for the global routes.

BGP is not needed on the P routers.

Well, that's what I can come up with as an explanation. Anyway, you need to run an IGP on the P routers.



Hello Luc,

quite a good post rated as it deserves

Best Regards


shivlu jain

labels are generated for two things:-

1. one for igp routes

2. second for vpnv4 routes

to reach end to end we need labels for the remote PE which is only possible with the routing protocol. Otherwise its not possible.


Shivlu Jain