Multiprotocol Lable Switching (Mpls):
How A Packet Travels Along A Lsp
How a packet travels along a LSP
When an IP packet enters an LSP, the ingress router examines
the packet and assigns it a label based on its destination,
attaches the label to the IT packet. The label transforms
the packet from one that is forwarded based on its IP routing
information to one that is forwarded based on information
associated with the MPLS label. The basic configuration
of an MPLS packet is given below: As showing in the figure
above, the label value consists of 20 bits.
The packet is then forwarded to the next router in the
LSP. This router and all subsequent routers in the LSP do
not examine any of the IP routing information in the labeled
packet. Rather, they use the MPLS label attached to the
packet and look up information in their local MPLS forwarding
table. They then replace the old label with a new label
and forward the packet to the next router in the path. It
is important to note that the MPLS labels have only local
significance, and the label is replaced at each node within
the MPLS network.
When the packet reaches the egress router, the label
is removed, and the packet again becomes a native IP packet
and is forwarded based on its IP routing information to
Label switching: In a hop-by-hop router
configuration, packets enter a router, the router examines
the IP header, and then the router sends the packet to the
next hop based on the ultimate destination address. In a
label-switched network, the operation is different. Packets
are not forwarded on a hop-by-hop basis. Instead, paths
are established for particular source-destination pairs
In the topology of an IP-routed network, traffic from router
1 is forwarded to router 4, which then makes its own forwarding
decision, and so on, until the packets arrive at router 9.
In a label-switched network, a path from router 1 to router
9 is created so that all traffic from router 1 to router 9 takes
the same deterministic path. Because a preset path exists, individual
routing nodes don't need to do a forwarding lookup on the packets
as they enter the router.
Instead, each node must keep information only on the paths
that have been established through it (so switching tables tend
to be much smaller than routing tables). As packets from that
flow enter a router, the router can switch the packets on to
a predefined path toward its destination through the network.
If router 4 knows that for all traffic from router 1 to router
9, the next stop along the way is router 6, it can just forward
the packets to that predetermined hop without ever looking up
the route in its routing table.