Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring MPLS and H-VPLS (Rev. 03)
Multiprotocol Label Switching
© 2008 Foundry Networks, Inc
Page 6 of 113
Figure 3: MPLS Label Forwarding
Figure 3
illustrates the label-forwarding process, using the above-described label assignments. The
steps are as follows:
1.
A packet arrives at the ingress edge router LER 1, which examines the packet IP address to
determine which LSP to use. LER 1 label table indicates that destination address prefix 172.16
should be assigned label 3 and should be sent out port 0.
2.
Having been sent out LER 1 port 0, the packet now arrives at the middle LSR. The LSR looks
up label 3 in its forwarding table and discovers that label 9 should be attached to the packet
and that it should be forwarded out port 0. It swaps label 3 with label 9 before forwarding the
packet.
3.
Now the packet arrives at the next hop, LER 2, which looks at the label and determines from
its routing table that label 9 indicates port 0. Since LER 2 is the egress LER, there is no further
need for a label. The existing label is discarded and the packet is forwarded onto IP network
172.16.
Label swapping as described in step 3 takes place at core LSRs, not ingress and egress LERs. The
swap operation consists of looking up the incoming label to determine the outgoing label and the
output port.
Label Advertisement Modes
The MPLS architecture specifies two ways for labels to be distributed: downstream on-demand
label distribution and downstream unsolicited label distribution.
Downstream on Demand
A Forwarding Equivalent Class (FEC) is a set of packets that the router treats in the same way
regardless of the information in the network layer header of these packets. For example, all packets
with a destination address prefix of 10.0.1/24 form a FEC.
In downstream on demand mode, an LSR explicitly requests from its next hop for a particular FEC,
a label binding for that FEC. This is known as downstream on demand label distribution..
Downstream Unsolicited
In downstream unsolicited label distribution, an LSR is allowed to distribute bindings to LSRs that
have not explicitly requested them.