H3C Technologies H3C SR8800 - Manual (Page 167)

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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.
• 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
For PEs in the same VSI, you can configure the peer PE addresses or use an automatic discovery
mechanism. Currently, LDP and BGP are used to automatically discover VSI peer PEs.
For a PW to be created, a PW signaling protocol is needed to assign a multiplex distinguishing flag (that
is, VC label) and advertise the assigned VC flag 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, PWs can be established between PEs to form a fully meshed network to provide VPLS
services. LDP and BGP can be used as PW signaling protocols.
VPLS can be one of the following based on the PW signaling protocol used:
• LDP VPLS—Uses LDP as the signaling protocol. This mode is also called the Martini mode.
• BGP VPLS—Uses BGP extension as the signaling protocol. This mode is also called the Kompella
mode.
NOTE:
For more information about the Martini mode and Kompella mode, see the chapter “Configuring MPLS
L2VPN.”
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.
• 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.
• 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.
• 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 a PW label and a tunnel label into
the packet before forwarding the packet. Otherwise, the PE directly adds a PW label and a tunnel
label 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.