MPLS defines protocol that make different paradigm for a way routers forward packets. rather than forwarding packets based on the packets destination IP address, MPLS defines how router can forward packets based on MPLS label .

What we are going to see in this section:

How MPLS work?

Traditional IP routing

Basic MPLS feature

MPLS terminology





































Let’s take an example to understand better.







MPLS Packet Forwarding

host 10.1.1.1 generates and send an unlabeled packet destined to host 20.1.1.1. router 1, with no MPLS feature configured, forwards the unlabeled packet based on the destination IP address, traditional IP routing, without any labels. MPLS router e receives the unlabeled packet and decides, as part of the MPLS forwarding process to impose(push) a new label (value 20) into the packet and forwards the packet to router 3. MPLS router 3 revives the labeled packet. router 3 swaps the label for new label value (30) and then forward the packet to router 4. MPLS router receives the labeled packet, removes (pops) the label, and forwards the packet toward router 5 non-MPLS router 5 forward the unlabeled packet based on the destination IP address, as normal .















Traditional IP routing

Routing protocol are used to distribute layer routing information. A forwarding decision is made based on:

Packet header

Local routing table

Routing lookup are independently perform at every hop

Basic MPLS features

MPLS is a forwarding mechanism in which packets are forwarded based on labels. MPLS packets can run other layer 2 technology such as ATM,Fame relay .PPP, Ethernet. MPLS leverage both IP routing and CEF switching.

MPLS terminology

LSR (label switch router) any router that push label onto packet, pops label from packet, or simple forwards labeled packet. in other wards LSR forward packets based on labels and swap labels.





Edge LSP (E-LSR) Edge LSR in the MPLS network process both labeled and unlabeled







Labels IP packets (Imposes labels) and forwarded them into MPLS domain

Forwards IP packets out of the MPLS domain



A sequence of labels to reach a destination is called an lSP.

Benefits of MPLS

MPLS support multiple applications including:

Uncast and multicast IP routing

MPLS decrease forwarding overhead on core routers.

BGP free core

MPLS can support forwarding of non-IP protocol

VPN

TE

QOS

Atom

MPLS label







MPLS uses a 32-bit label header that is inserted between l2 & l3 of OSI

20-bit label

3-bit experiment field

1-bit bottom of stuck indicator

8-bit time to live field

A single label corresponds to single route and shares them with MPLS neighbor (using LDP protocol)

MPLS label stack

Usually only one labels is assigned to a packet, but multiple labels in a label stack are supported.

These scenarios may produce more than one label:





MPLS VPN (two labels):

The top points to the egress routers, and the second label identifies the vpn.

MPLSTE (two or more labels):





The top label points to the endpoint of the traffic engineering tunnel and the second label point to the point destination.







MPLS VPNs combined with MPLSTE

Three or more labels).







