R2511-HP MSR Router Series MPLS Configuration Guide(V5)

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1. After receiving a packet from an interface, the PE searches for the VC bound to the interface.
2. The PE encapsulates the packet and sends the packet to the peer PE through the VC.
3. The peer PE removes the outer encapsulation to get the original PPP or HDLC packet, and then
forwards the packet to the user network.
VC encapsulation types for FR links
The following VC encapsulation types are available for an FR link:
• FR DLCI mode—After a PE receives an FR packet from the user network, it searches for the VC
according to the DLCI of the packet, and then forwards the packet to the VC. The FR DLCI mode is
also called the "one-to-one mapping" mode, because one FR virtual circuit corresponds to one VC.
• FR port mode—A PE forwards all FR packets received from an interface to the VC bound to the
interface. The FR port mode is also called the "many-to-one mapping" mode, because many FR
virtual circuits correspond to one VC.
VC encapsulation types for Ethernet links
The following VC encapsulation types are available for an Ethernet link:
• Ethernet—P-Tag is not transferred on a VC. If a packet from a CE contains a P-tag, the PE removes
the P-tag before encapsulating the packet. If the packet contains no P-tag, the PE directly
encapsulates the packet. Whether the PE adds a P-tag into a packet sent to a CE depends on the
access mode you have configured. However, rewriting and removing existing tags are not allowed.
• VLAN—Packets transmitted over a VC must carry a P-Tag.
{ If the peer PE does not require the ingress to rewrite the P-tag:
The PE keeps the P-Tag unchanged for a packet from a CE 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 your
configuration.
NOTE:
A
P-tag is also called a "service delimiter." An ISP might require users to add P-ta
g
s to distin
g
uish differen
t
user networks.
VC encapsulation types for ATM links
The encapsulation type 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 23,
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.