Lsrs In The Core Of An Mpls Network; Lers At The Output Edge Of An Mpls Network - Cisco 7604 Configuration Manual
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Chapter 42
Configuring PFC3BXL or PFC3B Mode MPLS QoS
LSRs in the Core of an MPLS Network
This section describes how LSRs used at the core of an MPLS network process packets:
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Within the service provider network, there is no IP precedence field for the queueing algorithm to use
Note
because the packets are MPLS packets. The packets remain MPLS packets until they arrive at PE2, the
provider edge router.
LERs at the Output Edge of an MPLS Network
At the egress side of an MPLS network, LERs process packets as follows:
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The MPLS EXP bits allow you to specify the QoS for an MPLS packet. The IP precedence and DSCP
Note
bits allow you to specify the QoS for an IP packet.
OL-4266-08
Incoming MPLS-labeled packets (and 802.1p bits or IP ToS bits) from an edge LER (or other core
device) arrive at the core LSR.
The PFC3BXL or PFC3B receives the traffic from the input interface and uses the EXP bits to
perform classification, marking, and policing.
The PFC3BXL or PFC3B performs a table lookup to determine the next-hop LSR.
An appropriate label is placed (swapped) into the packet and the MPLS EXP bits are copied into the
label header.
The PFC3BXL or PFC3B forwards the labeled packets to the appropriate output interface for
processing.
The PFC3BXL or PFC3B also forwards the 802.1p bits or the IP ToS bits to the output interface.
The outbound packet is differentiated by the MPLS EXP field for marking or policing.
The labeled packets (marked with EXP) are sent to another LSR in the core MPLS network or to an
LER at the output edge.
MPLS-labeled packets (and 802.1p bits or IP ToS bits) from a core LSR arrive at the egress LER
(PE2) from the MPLS network backbone.
The PFC3BXL or PFC3B pops the MPLS labels (disposition) from the packets. Aggregate labels
are classified using the original 802.1p bits or the IP ToS bits. Nonaggregate labels are classified
with the EXP value by default.
For aggregate labels, the PFC3BXL or PFC3B performs a lookup on the IP address to determine the
packet's destination; the PFC3BXL or PFC3B then forwards the packet to the appropriate output
interface for processing. For non-aggregate labels, forwarding is based on the label. By default,
non-aggregate labels are popped at the penultimate-hop router (next to last), not the egress PE router.
The PFC3BXL or PFC3B also forwards the 802.1p bits or the IP ToS bits to the output interface.
The packets are differentiated according to the 802.1p bits or the IP ToS bits and treated accordingly.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
PFC3BXL or PFC3B Mode MPLS QoS
42-7
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