Cisco 7604 Configuration Manual page 354

PFC3BXL and PFC3B Mode MPLS Label Switching
MPLS to MPLS
At the core of an MPLS network, the PFC3BXL or PFC3B uses the topmost label to perform a lookup in the
FIB TCAM. The successful lookup points to an adjacency that swaps the top label in the packet with a new
label as advertised by the downstream label switch router (LSR). If the router is the penultimate hop LSR
router (the upstream LSR next to the egress LER), the adjacency instructs the PFCBXL to pop the topmost
label, resulting in either an MPLS packet with the remaining label for any VPN or AToM use or a native IP
packet.
MPLS to IP
At the egress of the MPLS network there are several possibilities.
For a native IP packet (when the penultimate router has popped the label), the PFC3BXL or PFC3B
performs a route lookup in the FIB TCAM.
For a MPLS VPN packet, after the Interior Gateway Protocol (IGP) label is popped at penultimate router,
the VPN label remains. The operation that the PFC3BXL or PFC3B performs depends on the VPN label
type. Packets carrying aggregate labels require a second lookup based on the IP header after popping the
aggregate label. For a nonaggregate label, the PFC3BXL or PFC3B performs a route lookup in the FIB
TCAM to obtain the IP next hop information.
For the case of a packet with an IGP label and a VPN label, when there is no penultimate hop popping
(PHP), the packet carries the explicit-null label on top of the VPN label. The PFC3BXL or PFC3B looks
up the top label in the FIB TCAM and recirculates the packet. Then the PFC3BXL or PFC3B handles
the remaining label as described in the preceding paragraph, depending on whether it is an aggregate or
nonaggregate label.
Packets with the explicit-null label for the cases of EoMPLS, MPLS, and MPLS VPN an MPLS are
handled the same way.
MPLS VPN Forwarding
There are two types of VPN labels: aggregate labels for directly connected network or aggregate routes, and
nonaggregate labels. Packets carrying aggregate labels require a second lookup based on the IP header after
popping the aggregate label. The VPN information (VPN-IPv4 address, extended community, and label) is
distributed through the Multiprotocol-Border Gateway Protocol (MP-BGP).
Recirculation
In certain cases, the PFC3BXL or PFC3B provides the capability to recirculate the packets.
Recirculation can be used to perform additional lookups in the ACL or QoS TCAMs, the NetFlow table,
or the FIB TCAM table. Recirculation is necessary in these situations:
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Packet recirculation occurs only on a particular packet flow; other packet flows are not affected.The
rewrite of the packet occurs on the modules; the packets are then forwarded back to the PFC3BXL or
PFC3B for additional processing.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
24-4
To push more than three labels on imposition
To pop more than two labels on disposition
To pop an explicit null top label
When the VPN Routing and Forwarding (VRF) number is more than 511
For IP ACL on the egress interface (for nonaggregate (per-prefix) labels only)
Chapter 24 Configuring PFC3BXL and PFC3B Mode Multiprotocol Label Switching
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