R3303-HP HSR6800 Routers MPLS Configuration Guide

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{ If the peer PE does not require the ingress to rewrite the P-tag:
The PE keeps the P-Tag unchanged for the packet and then encapsulates the packet. If the packet
contains no P-tag, the PE adds a null label (the label value is 0) into the packet, and then
encapsulates the packet.
{ If the peer PE requires the ingress to rewrite the P-tag:
The PE changes the P-Tag to the VLAN tag (the tag might be a null tag) expected by the peer PE,
and then encapsulates the packet. If the packet contains no P-tag, the PE adds a VLAN tag
expected by the peer PE (the tag value might be 0) and then encapsulates the packet.
For a packet sent to a CE, the PE rewrites, removes, or retains the P-tag depending on the access
mode you have configured.
NOTE:
A
P-tag is also called a "service delimiter." An ISP may require users to add P-tags to distin
g
uish differen
t
user networks.
VC types for ATM links
For more information about ATM, see Layer 2—WAN Configuration Guide.
The VC type available for ATM links is ATM AAL5 transparent transport.
ATM AAL5 transparent transport is used to transparently transport ATM AAL5 Service Data Units (SDUs)
over a Packet Switched Network (PSN), such as an IP network, through an end-to-end virtual connection.
With this technology, two separate ATM networks can communicate with each other through a PSN.
As shown in Figure 43,
after receiving an ATM cell from CE 1, PE 1 takes the ATM AAL5 SDU out of the
ATM cell, encapsulates the ATM AAL5 SDU, and then sends the ATM AAL5 SDU to PE 2 through the VC
on the PSN. PE 2 removes the outer encapsulation, encapsulates the original ATM AAL5 SDU to an ATM
cell, and then sends the ATM cell to CE 2. In the communication process, CE 1 and CE 2 cannot sense
the existence of the PSN, as if they were communicating directly.
Figure 43 ATM AAL5 transparent transport
Control word
The control word field is between the VC label and the Layer 2 data. It carries information about the Layer
2 packet, such as the sequence number.
The control word filed has the following functions: