Figure 12: Multisegment Pseudowire: Example
A pseudowire is a tunnel between two PE nodes. There are two types of PE nodes:
• A Switching PE (S-PE) node
• Terminates PSN tunnels of the preceding and succeeding pseudowire segments in a multisegment
pseudowire.
• Switches control and data planes of the preceding and succeeding pseudowire segments of the
multisegment pseudowire.
• A Terminating PE (T-PE) node
• Located at both the first and last segments of a multisegment pseudowire.
• Where customer-facing attachment circuits (ACs) are bound to a pseudowire forwarder.
Every end of a multisegment pseudowire must terminate at a T-PE.
Note
A multisegment pseudowire is used in two general cases when:
• It is not possible to establish a PW control channel between the source and destination PE nodes.
For the PW control channel to be established, the remote PE node must be accessible. Sometimes, the
local PE node may not be able to access the remote node due to topology, operational, or security
constraints.
A multisegment pseudowire dynamically builds two discrete pseudowire segments and performs a
pseudowire switching to establish a PW control channel between the source and destination PE nodes.
• Pseudowire Edge To Edge Emulation (PWE3) signaling and encapsulation protocols are different.
The PE nodes are connected to networks employing different PW signaling and encapsulation protocols.
Sometimes, it is not possible to use a single segment PW.
A multisegment pseudowire, with the appropriate interworking performed at the PW switching points,
enables PW connectivity between the PE nodes in the network.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
81
Implementing Point to Point Layer 2 Services
Multisegment Pseudowire
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