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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Figure 4: VPLS
As already mentioned, virtual circuits (VC) are used to provide means for transporting customer
Ethernet packets. In the VPLS world, each pair of VCs (one for each direction) is called a
pseudowire (PW). A full mesh of PWs must exist among PEs within the same VPLS instance. In
order to prevent loops, a PE must not forward traffic from one pseudowire to another in the same
VPLS instance. Note that this does not apply to traffic received on a PE user port which is
considered an access port of the VPLS service. If a packet with an unknown destination MAC
address arrives at such a port, the PE must flood this packet to all pseudowires and user ports (if
any), pertaining to the VPLS instance. This last rule is very important as it allows a VPLS instance
to be further deployed into Hierarchical VPLS without having to spread the full mesh of
pseudowires outside the provider core network.
While traveling along a PW, packets contain a stack of two labels. Both labels are added by PEs at
the time the packets enter the MPLS core. The core routers (LSRs) use the outer label (transport
label) to switch the packet until it gets to the far-end PE. LSRs do not know that the packet belongs
to a given VPLS instance as they only take into account the outer label. This can be regarded as a
feature that provides an additional level of security for user traffic. The other (inner) label, Virtual
Connection label, is put to use at the far-end PE. This label identifies the VPLS instance with
which the packet belongs, i.e, it is used as a service delimiter. Once the PE becomes aware of the
VPLS it can proceed with switching the packet based on the destination MAC address.
VPLS Packet Formats
To traverse the MPLS core, Ethernet packets have to be encapsulated in MPLS packets.
1
Figure 5
provides an overview of MPLS encapsulation by showing the packets that flow along different
stretches of the network.