Solved

MPLS and BGP

Posted on 2011-02-14
4
719 Views
Last Modified: 2012-05-11
Junior engineer here, studying MPLS.
This quesiton is regarding a typical MPLS backbone network where you have multiple VRFs on PEs and the PE's peer with a RR.
I want to make sure I understand the role of MPLS and the role of MP-BGP in this process.

Am I correct in assuming that the PE peering with the RR's purpose is to advertise routes for a VRF via MP-BGP? So the actual lables are not getting forwarded via BGP, but rather by LDP?

In other words, when a packet from VRF-BLUE arrives at the PE-1 destined to reach VRF-BLUE on PE-2, the packet will get encapsulated into a label but what protocol will it use to get transported across the MPLS network to the other side?

Labels are
0
Comment
Question by:trojan81
  • 2
  • 2
4 Comments
 
LVL 32

Expert Comment

by:harbor235
ID: 34901242

>Am I correct in assuming that the PE peering with the RR's purpose is to advertise routes for a VRF >via MP-BGP? So the actual lables are not getting forwarded via BGP, but rather by LDP?

The purpose is to get around the full mesh of MP-BGP peering that would need to take place otherwise. RRs allow you to peer your PEs directly with a RR to get al the necessary routes via a single (should also use a backup RR) peering relationship.  LDP uses routing entries to assign a label and then distribute via LDP, they work together.

>In other words, when a packet from VRF-BLUE arrives at the PE-1 destined to reach VRF-BLUE on >PE-2, the packet will get encapsulated into a label but what protocol will it use to get transported >across the MPLS network to the other side?

The frame does not get encapsualted into a label, A SHIM header is added and is sandwiched between the layer 2 and layer 3  header information of the out going frame. So when a frame with a SHIM header reaches a MPLS speaking router it does not need to look a destination route it jsut pushes, pops, or swaps a label. At the egress router all labels should have been removed and a destination route lookup is perfromed and the packet is delivered.

harbor235 ;]
0
 

Author Comment

by:trojan81
ID: 34903988
Harbor,

You are correct about the purpose of RR. I meant what function is it serving in regards to the PE? I beleve the PE is sending VRF routing updates via MP-BGP to the RR.

I made a typing misake by saying the frame gets "encapsulated". When the frame from the CE hits the first PE and get's a label affexed, is it getting all the lables that it needs to reach the EGRESS PE?

Original primary question about how the label is moved is still somewhat unclear...It sounds like MPLS, LDP, and MP-BGP all do their part to move this label to the final destination.
0
 
LVL 32

Accepted Solution

by:
harbor235 earned 500 total points
ID: 34906181

The PE exchanges VPNV4 routes with the RR utilizing MP-BGP, which includes all necessary vrf routing information. In essence this is the function of the RT, RD makes each route unique and RT is an extended BGP community that enables MP-BGP to import and export VRF routes to the proper destination VRF. So you are correct, so in gneneral term the RR is a route server.

When LDP converges, just like a routing protocol but is dependant on routing protocol information, the LDP domain has all labels in place for end to end forwarding for all prefixes it knows about (via the routing table info). As the frame is forwarded it recieves a label (one or more) for forwarding to the next MPLS speaking device, there is a LFIB (label forwarding information base) thats tells it what the incoming label is and what the outgoing label should be, so labels are pushed, swapped, and poped on a per MPLS device basis. It is much like frame relay and dlci's. Make sensse?

So LDP, MP-BGP, and teh core IGP all play a part in the ultimate forwarding of an LSP (label switch path). So yes that is correct, something also to think about is that for VRF to VRF communication labels must be stacked, that is there is one label to forward traffic across the MPLS core and another label to forward the traffic into the correct VRF.

Hope this all makes sense,

harbor235 ;}



 
0
 

Author Closing Comment

by:trojan81
ID: 34940388
well done
0

Featured Post

How to run any project with ease

Manage projects of all sizes how you want. Great for personal to-do lists, project milestones, team priorities and launch plans.
- Combine task lists, docs, spreadsheets, and chat in one
- View and edit from mobile/offline
- Cut down on emails

Join & Write a Comment

This article is focussed on erradicating the confusion with slash notations. This article will help you identify and understand the purpose and use of slash notations. A deep understanding of this will help you identify networks quicker especially w…
Data center, now-a-days, is referred as the home of all the advanced technologies. In-fact, most of the businesses are now establishing their entire organizational structure around the IT capabilities.
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

758 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

18 Experts available now in Live!

Get 1:1 Help Now