H3C Technologies H3C S12500-X Series Switches - Manual (Page 255)

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The MPLS L2VPN determines whether the primary PW fails according to the LDP session status or the
connectivity detection result. The backup PW is used when one of the following conditions exists:
• The public tunnel of the primary PW is deleted, or the link detection mechanism detects that the
public tunnel has failed.
• The primary PW is deleted because the LDP session between PEs goes down, or the link detection
mechanism detects that the primary PW has failed.
• A manual PW switchover is performed.
Multi-segment PW
A multi-segment PW includes multiple concatenated static or LDP PWs. Creating two PWs for a
cross-connect on a PE can concatenate the two PWs. Upon receiving a packet from one PW, the PE
removes the tunnel ID and PW label of the packet, adds the PW label of the other PW, and forwards the
packet over the public tunnel. Only static and LDP PWs can form a multi-segment PW.
As shown in
Figure 65 , you can create a multi-segment PW between PE 1 and PE 4 by concatenating PW
1 and PW 2 on PE 2, and PW 2 and PW 3 on PE 3.
Figure 65 Multi-segment PW
Multi-segment PWs include intra-domain multi-segment PWs and inter-domain multi-segment PWs.
Intra-domain multi-segment PW
An intra-domain multi-segment PW has concatenated PWs within an AS. You can create an intra-domain
multi-segment PW between two PEs that have no public tunnel to each other.
As shown in
Figure 66 , there is no public tunnel between PE 1 and PE 4 but there is a pubic tunnel
between PE 1 and PE 2 and a public tunnel between PE 2 and PE 4. By creating a PW between PE 1 and
PE 2 (PW 1) and a PW between PE 2 and PE 4 (PW 2) and then concatenating the two PWs on PE 2,
you can create an intra-domain multi-segment PW between PE 1 and PE 4.
Intra-domain multi-segment PWs can fully unitize existing public tunnels to reduce end-to-end public
tunnels.
CE 1
PE 1 PE 4
PE 2 PE 3
PW 1
PW
3
CE 2
PW 2
MPLS or IP backbone
PW