Troubleshooting guide

8-17
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 8 Troubleshooting Tag and MPLS Switching Connections
MPLS Overview
MPLS Example
This section provides a description of a packet being transmitted across an MLPS enabled network, and
of the process used to switch the packets.
When a packet is received at an MPLS ingress interface, the interface driver uses the IDB (interface
descriptor block) to start the following MPLS processes on the packet:
• Packet encapsulation is checked and verified
• The packet is checked for QoS or policing limitations.
• Label and ingress interface data are used to check the TFIB trying to determine the egress label and
interface number.
Note If an MPLS header and label are not found in the packet, the lookup process reverts to the Layer 3
process. See Chapter 11, “Troubleshooting Layer 3 Network Connections.”
• The TTL field is updated and the label is either replaced with the next-hop label or, if this is the
MPLS edge exit LSR, is deleted (“popped”) from the stack.
• The packet is transmitted to the next hop.
Figure 8-4 shows a packet as it traverses a network from its source on network 130.0.0.0 to its destination
on network 180.0.0.0.