Juniper JUNOS OS 10.3 - SOFTWARE, Руководство пользователя

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The pop operation removes a label from the beginning of the packet. Once the label is
removed, the TTL is copied from the label into the IP packet header, and the underlying
IP packet is forwarded as a native IP packet
The swap operation removes an existing MPLS label from an IP packet and replaces it
with a new MPLS label, based on the following:
• Incoming interface
• Label
•
Label forwarding table
Figure 84 on page 3363 shows an IP packet without a label arriving on the customer-edge
interface (
ge-0/0/1
) of the ingress PE switch. The ingress PE switch examines the packet
and identifies that packet’s destination is the egress PE switch. The ingress PE switch
applies label 100 to the packet and sends the MPLS packet to its outgoing MPLS core
interface (
ge-0/0/5
). The MPLS packet is transmitted on the MPLS tunnel through the
provider switch, where it arrives at interface
ge-0/0/5
with label 100. The provider switch
swaps label 100 to label 200 and forwards the MPLS packet through its core interface
(
ge-0/0/7
) to the next hop on the tunnel, which is the egress PE switch. The egress PE
switch receives the MPLS packet through its core interface (
ge-0/0/7
), removes the
MPLS label and sends the IP packet out of its customer-edge interface (
ge-0/0/1
) to a
destination that is beyond the scope of the tunnel.
Figure 84: MPLS Label Swapping
Figure 84 on page 3363 shows the path of a packet as it passes in one direction from the
ingress PE switch to the egress PE switch. However, the MPLS configuration also allows
traffic to travel in the reverse direction. Thus, each provider edge switch operates as both
an ingress switch and an egress switch.
Ultimate and Penultimate Hop Popping
EX Series switches support ultimate and penultimate hop popping (that is, popping off
the MPLS label) as follows:
•
With circuit cross-connect (CCC), ultimate hop popping (UHP) is enabled by default
and label 0 (IPv4 Explicit Null Label) is advertised. With UHP, the egress PE switch is
responsible for popping the MPLS label at the termination of the CCC.
•
With IP and MPLS, penultimate hop popping (PHP) is enabled by default. With PHP,
the penultimate provider switch is responsible for popping the MPLS label and
forwarding the traffic to the egress PE switch. The egress PE switch then performs an
IP route lookup and forwards the traffic. This reduces the processing load on the egress
PE switch, because it is not responsible for popping off the MPLS label.
3363 Copyright © 2010, Juniper Networks, Inc.
Chapter 121: MPLS—Overview