3Com S7906E - Switch, Руководство по настройке

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PEs are logically fully meshed (so are PWs), that is, each PE must create for each VPLS
forwarding instance a tree to all the other PEs of the instance.
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Each PE must support horizontal split to avoid loops, that is, a PE cannot forward packets via PWs
of the same VSI, because all the PEs of a VSI are directly connected. In other words, packets from
PWs on the public network side cannot be forwarded to other PWs; they can only be forwarded to
the private network side.
Peer PE discovery and PW signaling protocol
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For PEs in the same VSI, you can configure the remote PE addresses or use an automatic
discovery mechanism. Currently, LDP and BGP are used to automatically discover VSI peer PEs
and function as the PW signaling protocol to create PWs.
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The PW signaling protocol is designed to assign multiplex distinguishing flags (that is, VC labels)
and advertise the assigned VC flags to the peer. In addition, the PW signaling protocol advertises
VPLS system parameters such as PW ID, control word, and interface parameters. With the PW
signaling protocol, fully meshed PWs can be established between PEs for VPLS services.
VPLS Packet Encapsulation
Packet encapsulation on an AC
The packet encapsulation type of an AC depends on the user VSI access mode, which can be VLAN or
Ethernet.
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VLAN access: The Ethernet header of a packet sent by a CE to a PE or sent by a PE to a CE
includes a VLAN tag that is added in the header as a service delimiter for the service provider
network to identify the user. The tag is called a P-Tag.
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Ethernet access: The Ethernet header of a packet upstream from the CE or downstream from the
PE does not contain any service delimiter. If a header contains a VLAN tag, it is the internal VLAN
tag of the user and means nothing to the PE. This kind of internal VLAN tag of the user is called a
U-Tag.
You can specify the VSI access mode to be used.
Packet encapsulation on a PW
The packet encapsulation type of a PW, also called the PW transport mode, can be either Ethernet or
VLAN.
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In Ethernet mode, P-TAG is not transferred on the PW. For a packet from a CE, if it contains the
service delimiter, the PE removes the service delimiter and adds two levels of MPLS labels into 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.