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Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring MPLS and H-VPLS (Rev. 03)
Multiprotocol Label Switching
© 2008 Foundry Networks, Inc
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ingress point. An LSP starts at an
ingress
LER, crosses one or more LSRs, and ends at an
egress
LER.
When a packet arrives at the ingress, the LER does most of the work of handling the packet. It
looks at the packet IP address, determines a route, assigns an LSP, and attaches a label. Actually
the LER is free to check any part of the incoming packet in order to determine the right label to be
used. The packet is then forwarded into the LSP, where it is switched across a series of LSRs until
it reaches the egress LER. Note that while traveling within the MPLS cloud, the packet label is
changed at each MPLS hop. At the egress LER, the label is removed and the packet is forwarded
on its way using standard IP routing.
The default MPLS label assignment and label-forwarding process starts with each label switching
device learning about the network to which it is directly attached. Routing protocol such as Open
Shortest Path First (OSPF) is used to build routing tables. However, static configuration of routes
is also an option. MPLS devices then build label-forwarding tables by negotiating label mappings
with each peer, for each routing entry.
Figure 2: Label Propagation in MPLS
Figure 2
provides an overview of the label-forwarding process. Each LER requests mappings for
all its locally connected networks and receives a mapping for those networks from the neighboring
LSRs. Looking at LER 2 on the upper right, note that a label table has been built for the two
network prefixes. Label 9 is assigned to 172.16 and label 10 is assigned to 129.10. LER 2
distributes these label assignments to the LSR in the middle of the MPLS network. In this
exchange, if a packet for network 172.16 is sent to LER 2, label 9 should be attached to this packet.
If the packet is destined for network 129.10, the label attached should be 10.
The LSR in the middle of the network receives label assignments from both LER 2 and LER 3, and
then builds its own label table. Note that it creates a table entry for each network it knows about on
its out ports. It then assigns a label to each one of these entries and forwards the label assignment
to LER 1. The LSR is basically saying to use labels 3, 4, or 5 for networks 172.16, 129.10 and
210.100, respectively.