H3C Technologies H3C S7500E Series Switches - Manual (Page 166)

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the packet before forwarding the packet. Otherwise, the PE directly adds two levels of MPLS
labels into the packet and then forwards the packet. For a packet to be sent downstream, whether
the PE adds the service delimiter into the packet depends on your configuration. However,
rewriting and removing of existing tags are not allowed.
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In VLAN mode, every packet to the PW must carry a P-TAG. For a packet from a CE, if it contains
the service delimiter, the PE directly adds two levels of MPLS labels into the packet and sends the
packet out. Otherwise, the PE adds a null tag together with two levels of MPLS labels into the
packet and sends the packet out. For a packet to be sent downstream, the PE rewrites, removes,
or retains the service delimiter depending on your configuration.
According to the protocol, the packet encapsulation type of a PW is VLAN by default.
H-VPLS Implementation
Hierarchy of VPLS (H-VPLS) can extend the VPLS access range of a service provider and reduce
costs.
Advantages of H-VPLS access
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H-VPLS has lower requirements on the multi-tenant unit switch (MTU-s). It has distinct
hierarchies which fulfill definite tasks.
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H-VPLS reduces the logical complexity of the fully meshed network consisting of PEs and the
configuration complexity.
Two H-VPLS access modes
1) H-VPLS LSP access
Figure 4-3 H-VPLS LSP access
As shown in Figure 4-3 , UPE functions as the convergence device MTU-s and establishes only a
virtual link U-PW with NPE 1. It does not establish virtual links with any other peers.
Data forwarding in H-VPLS LSP access mode is as follows:
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Upon receiving a packet from a CE, UPE tags the packet with the MPLS label for the U-PW,
namely the multiplex distinguishing flag, and then sends the packet to NPE 1.
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When receiving the packet, NPE 1 determines which VSI the packet belongs to by the label and,
based on the destination MAC address of the packet, tags the packet with the multiplex
distinguishing flag for the N-PW, and forwards the packet.
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Upon receiving the packet from the N-PW, NPE 1 tags the packet with the multiplex distinguishing
flag for the U-PW and sends the packet to UPE, which forwards the packet to the CE.