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Basic MPLS Tutorial

May 4th, 2011 30 comments

MPLS is a new forwarding mechanism called “label switching” in which packets are forwarded based on labels. However, hosts are unaware about labeled packets so routers will need to add a label when entering “MPLS area” and remove that label after leaving there.

The idea of label switching is to have only the first router do an IP lookup and assign a label, then all future routes in the network can “cheat” by doing exact match “switching” based on a label. This would reduce load on the core routers, where high-performance was the most difficult to achieve, and distribute the routing lookups across lower speed edge routers.

In a traditional IP network:
* Each router performs an IP lookup (“routing”), determines a next-hop based on its routing table, and forwards the packet to that next-hop.
* Rinse and repeat for every router, each making its own independent routing decisions, until the final destination is reached.
MPLS does “label switching” instead:
* The first device does a routing lookup, just like before.
* But instead of finding a next-hop, it finds the final destination router.
* And it finds a pre-determined path from “here” to that final router.
* The router applies a “label” (or “shim”) based on this information.
* Future routers use the label to route the traffic without needing to perform any additional IP lookups.
* At the final destination router, the label is removed and the packet is delivered via normal IP routing.

Therefore in an MPLS network, data packets are assigned labels. Packet-forwarding decisions are made solely on the contents of this label, without the need to examine the packet itself.

Read more…

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MPLS Basic Terminologies

April 13th, 2011 1 comment

* AS – autonomous system. A collection of networks that share the same routing protocol and that are under the same system administration.

*ASBR – autonomous system boundary router. A router that connects and exchanges information between two or more autonomous systems.

* BGP – Border Gateway Protocol. The exterior border gateway protocol used to exchange routing information between routers in separate autonomous systems. BGP uses Transmission Control Protocol (TCP). Because TCP is a reliable protocol, BGP does not experience problems with dropped or fragmented data packets.

* CE router – customer edge router. The customer router that connects to the provider edge (PE) router.

* eBGP – External Border Gateway Protocol. A BGP session between routers in different autonomous systems (ASs). When a pair of routers in different ASs are more than one IP hop away from each other, an EBGP session between those two routers is called multihop EBGP.

* iBGP – Internal Border Gateway Protocol. A BGP session between routers within the same autonomous system.

* IGP – Interior Gateway Protocol. Internet protocol used to exchange routing information within an autonomous system. Examples of common Internet IGPs include Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), and Routing Information Protocol (RIP).

* LDP—Label Distribution Protocol. A standard protocol between MPLS-enabled routers to negotiate the labels (addresses) used to forward packets. The Cisco proprietary version of this protocol is the Tag Distribution Protocol (TDP).

* LER – label edge router. The edge router that performs label imposition and disposition.

* LSR – label switch router. The role of an LSR is to forward packets in an MPLS network by looking only at the fixed-length label.

* NLRI – Network Layer Reachability Information. BGP sends routing update messages containing NLRI, which describes the route. In this context, an NLRI is a prefix. A BGP update message carries one or more NLRI prefixes and the attributes of a route for the NLRI prefixes. The route attributes include a BGP next hop gateway address, community values, and other information.

* P router – provider router. The core router in the service provider network that connects to provider edge (PE) routers. In a packet-switched star topology, a router that is part of the backbone and that serves as the single pipe through which all traffic from peripheral networks must pass on its way to other peripheral networks.

* PE router – provider edge router. The label edge router (LER) in the service provider network that connects to the customer edge (CE) router.

* RR – route reflector. A router that advertises or reflects IBGP learned routes to other IBGP peers without requiring a full network mesh.

* VPN – Virtual Private Network. A group of sites that, as a result of a set of administrative policies, can communicate with each other over a shared backbone.

* VPNv4 addresses – When multiple VPNs use the same address space, the VPN addresses are made unique by adding a route distinguisher to the front of the address.

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