May 23 2008

PE-CE Routing MPLS VPN

PE-CE Routing OSPF – MPLS

Introduction

If you are serious about using OSPF as your PE to CE protocol on the edge of the MPLS cloud then you should familiarise yourself with RFC 4577 (OSPF as the Provider/Customer Edge Protocol for BGP/MPLS IP Virtual Private Networks).

MPLS – PE-CE routing scenario

Starting from the left in the MPLS toplogy below. Rip is redistributed into OSPF on CE1, therefore 10.5.1.0/24 is seen as an external route on the MPLS PE1. 10.4.1.0/24 however is advertised into a non-zero ospf area and is thus recieved at PE1 as an inter-Area route. Both of these routes are advertised across the MPLS cloud by MBGP. MBGP then redistributes these routes into OSPF on PE2. OSPF then advertises these routes across the MPLS PE-CE link to CE2. CE2 sees these routes as follows:-

15.1.1.0/24: External route LSA Type 5
10.4.1.0/24: Summary route LSA Type 3

From the right, CE2 advertises 10.6.1.0/24 to PE2 as an intra-area route ie LSA Type 1/2. PE2 then acts as an ABR for area2 and advertises 10.6.1.0/24 across then MPLS Cloud to PE1. PE1 advertises 10.6.1.0/24 across the PE-CE link to CE1. CE1 sees this route as follows:-

10.6.1.0/24: Summary route LSA Type 3

The Key here is understanding which router acts as an ABR and forwards generates summary routes.

Type 1/2 LSAs ie intra-area routes can be forwarded nativly across the MPLS cloud. However that is outside of the scope of this document and will be covered in a later document.

Topology

Tutorial

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Components Used

The topology below was configured on the following software and hardware:-

IOS: c7200-k91p-mz.122-31.SB11.bin:

Hardware: 7200VXR chassis emulated using GNS3 and dynamips.

Configurations

ce1.cfg
pe1.cfg
pe2.cfg
ce2.cfg

Further Reading

RFC 4364 – BGP/MPLS IP Virtual Private Networks (VPNs)
RFC 4577 – OSPF as the Provider/Customer Edge Protocol for BGP/MPLS IP Virtual Private Networks (VPNs)
RFC 4576 – Using a Link State Advertisement (LSA) Options Bit to Prevent Looping in BGP/MPLS IP Virtual Private Networks (VPNs)

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By zismail • PE-CE Routing • 1 • Tags: , , ,