MPLS

Nov 19 2008

Inter-AS MPLS VPN Option A

Option A is the simplist of the three inter-AS MPLS options, however its simplicity has drawbacks in that it doesn’t scale too well.

Option A required two bordering PE routes to share an EBGP session. VPN customers VPNv4 prefixes are converted into IPv4 prefixes and advertised across the EBGP session.

Once they are received on the other side, they are converted back into VPNv4 prefixes and advertised accross the MPLS VPN cloud.

A sample setup is as below.

If there are 100 Inter-As MPLS VPNS configured using option A, the service providers will have to configure 100 EBGP sessions between their neighboring PEs.

By zismail • BGP, CCIE SP, Inter-AS, MPLS VPN • 2 • Tags: BGP, CCIE SP, Inter-AS, MPLS VPN

Nov 19 2008

Inter AS MPLS VPN – Option what?

When people talk about Inter AS MPLS VPN they often mention option A, B or C. I searched high a low trying to make sense of this naming convention, thinking it was some official terminology.

I concluded whether rightly or wrongly, that this is referring to paragraph 10 in RFC 4364 BGP/MPLS IP Virtual Private Networks (VPNs) (This RFC Obsoletes the well known RFC 2547).

Paragraph 10 describes 3 ways to connect an MPLS VPN using multiple service providers.

The 3 ways are described in 3 sub paragraphs a, b, and c. Which I believe became known as option A, option B and option C.

To further confuse matters, there are some references* namely option 2a, 2b and 2c which are suboptions for RFC 4364 option b.

*see Cisco press – MPLS configuration on Cisco IOS Software

By zismail • CCIE SP, Inter-AS, MPLS VPN • 0 • Tags: CCIE SP, Inter-AS, MPLS VPN

Nov 18 2008

OSPF sham-links give the technical administrator an option to use either the back door link or the WAN link by presenting Type 1 LSAs over the WAN cloud. CEs then have Type 1 LSAs via 2 routes and as they are the same type of LSA you can manipulate the cost to either use the back door or the WAN link.

By zismail • CCIE SP, MPLS VPN, OSPF, PE-CE Routing • 0 • Tags: CCIE SP, MPLS VPN, OSPF

Nov 18 2008

MPLS VPN – using Site of Origin (SOO)

When an MPLS VPN customer is dual homed, SOO can be used to prevent routing loops as well as sub optimal routing.

Toplology as below.

Using Site of Origin community with BGP

I’ve used EIGRP between CE1 and CE3 to demonstrate the sub optimal routing.(its only suboptimal if you want to route traffic between sites using the backdoor link, which isnt always the case)

By zismail • CCIE SP, MPLS VPN, PE-CE Routing • 0 • Tags: BGP, CCIE SP, MPLS VPN, PE-CE Routing, SOO

Nov 17 2008

MPLS VPN using BGP as the PE-CE Routing Protocol

Topology is as below.

MPLS VPN using EBGP as the PE-CE routing protocol

By zismail • BGP, CCIE SP, MPLS VPN, PE-CE Routing • 1 • Tags: BGP, MPLS VPN, PE-CE Routing

Nov 17 2008

MPLS VPN using EIGRP as the PE-CE routing Protocol

Topology is as below.

By zismail • CCIE SP, EIGRP, MPLS VPN, PE-CE Routing • 1 • Tags: EIGRP, MPLS VPN, PE-CE Routing

Oct 25 2008

ISIS header

Below is a diagram of the ISIS header. The different ISIS packets ie hellos and SNPs have slightly different headers, however the first 8 bytes are identical.

Click here to see a power point show on the isis header.

By zismail • CCIE SP, Headers, MPLS • 0 • Tags: CCIE, Headers, ISIS

Oct 25 2008

MPLS Header

This is what an MPLS label looks like. The MPLS label contains a 20-bit label, dont get confused between the different uses of the word label.

The 3 bit EXP bits are used for QoS purposes. You can use them to define different tunnel modes, which I’ll talk about on another day.

The S bit identifies the bottom label of the stack.

The TTL field is similar to the IP TTL, ie if the MPLs TTL reaches zero the MPLS frame is discarded.

The MPLS frame is inserted between the layer 2 frame header and the layer 3 header.

Click here if you want to see a powerpoint version of the MPLS frame header

By zismail • CCIE SP, Headers, MPLS • 0

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

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