Orhan Ergun 2 Comments

MPLS Layer 2 and Layer 3 VPN

RFC 2547 defines standard MPLS VPN to carry customer prefixes over the MPLS backbone.

In February 2006 RFC 4364 was published for Inter AS VPNs which is known as Multi AS VPNs. RFC 4364 obsoleted RFC 2547 and  defined many other applications for MPLS VPNs such as CSC which is also known as Carrier Supporting Carrier with the Cisco terminology and Carrier of Carrier with the Juniper definition.

With basic Layer 3 MPLS VPN , Enterprise customers can carry their prefixes from multiple sites over SP backbone . It is multi point to multipoint connection. With the ATOM based MPLS solution which is Cisco’s E-Line solution , customer sites are connected as point to point and with VPLS multipoint to multipoint.

Basic difference with the VPLS and IP/VPN from customer point of view , with VPLS all attached sites share the same L3 network.Service Provider acts as a big switch for the customer. IP/MPLS VPNs use different IP address at each site.

With the IP/VPN also known as BGP or L3 VPN , customer runs IP routing protocol or static route with the Service provider and Customer equipment which is known as CE don’t see other CE as connected like in VPLS or ATOM based MPLS.

Depending on expectations of the customer from the Service Provider , for the MPLS L3/VPN case, customer can run any of the IGP routing protocols including EIGRP, OSPF, IS-IS , BGP or static route. You may want to talk with your Service Provider before you decide since some Service Providers don’t service every routing protocols. Most of them if not all supports BGP.

If customer wants very granular policy control, dual homed site connectivity, and customer network stuff well trained , best choice would be BGP.

In the past fast convergence was an issue with BGP and maybe still with the vanilla BGP configuration, recent enhancements allow BGP to converge super fast thanks to BGP Fast Reroute Mechanism which is BGP PIC.

All IGP protocol’s metric information can be carried over SP MPLS backbone end to end. In this case SP core behaves differently. For OSPF there is Superbackbone and for ISIS there is L3 backbone concepts. This is out of the scope of this post so I will not explain further.But if you want to learn and interested please comment, so I definitely write about them.

One another caveat for PE-CE protocol , for almost all protocol , if customer has backdoor link to another customer site, loop or suboptimal path usage may occur. We prefer generally MPLS link when it is necessary to have low latency , secure , reliable connection compare to Internet based option.

If customer has backup Internet link (Not MPLS but maybe DSL, 3G/LTE, Satellite,Microwave, Cable) and its requirement is low latency , predictable delay variation which is called as jitter , reliable and secure ( Relative ) connection, probably wants to use MPLS connection as primary and Internet connection as a backup although LTE is much cheaper and provides very high bandwidth nowadays and started to take its place as a primary connection on some networks or part of the network such as remote offices.