An MH-PW is a set of two or more contiguous PW segments between two U-PEs. Switching
labels at PW label layer is required. Figure 6-5 shows an MH-PW that functions as a single
point-to-point PW.
Figure 6-5 Networking diagram of an MH-PWE3
CE1
CE2
U-PE1 U-PE2S-PE
PW200
P
PW100
l The forwarding mechanism of a U-PE on an MH-PW is the same as that on an SH-PW.
The only difference is that the MH-PW requires switching the control and data planes of
the preceding and succeeding PW segments on the S-PE.
l To carry out an MH-PW, the S-PE connects two PWs on both of PW endpoints and
exchanges labels. There are three combinations of two PWs:
– Dynamic and dynamic switching: Both PWs are dynamic PWs. The PWs on both sides
of the S-PE are set up by using signaling. The remote labels are sent from two
neighboring endpoints (U-PE or S-PE) to the S-PE by using signaling.
– Static and static switching: Both PWs are static PWs.
– Dynamic and static switching: One of the PWs is set up with signaling, while the other
is not.
l The preceding types of PW switching support the Control Word (CW) and Virtual Circuit
Connectivity Verification (VCCV). The CW and VCCV are sent from two U-PEs to the
S-PE through labels.
BFD for PW
The Bidirectional Forwarding Detection (BFD) can fast detect a PW between the local PE and
remote PE to enable PW Fast Reroute (FRR). This lessens the impact of any link faults on
services.
l Static BFD for PW
After being encapsulated by PWs, BFD control packets are transmitted on PWs. PWs
distinguish control packets and data packets by using CWs. BFD packets are encapsulated
by using the CWs of PWs. On an MH PW, the intermediate SPE only forwards BFD packets,
but does not send the BFD packets to its CPU for processing.
l Dynamic BFD for PW
1. The Up and Down states of a PW can trigger the dynamic creation and deletion of a
BFD session. When the status of a PW that needs to be detected is Up, the local device
notifies information about its neighbor and detection parameters to the BFD module.
The BFD module then sets up a session to detect the link between the local device and
its neighbor.
2. BFD session negotiation can be implemented by adding the BFD Discriminator TLV
field to VCCV ping packets.
Quidway S7700 Smart Routing Switch
Configuration Guide - VPN 6 PWE3 Configuration
Issue 01 (2011-07-15) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
545
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