Troubleshooting guide

CHAPTER
8-1
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
8
Troubleshooting Tag and MPLS
Switching Connections
This chapter provides troubleshooting information for connectivity and performance problems in tag
switching and MPLS Label Distribution Protocol (LDP) environments. For more information on tag
switching and MPLS, refer to the “Configuring Tag Switching and MPLS” chapter in the ATM Switch
Router Software Configuration Guide.
Before you begin, make sure that all physical port connections are working correctly. See Chapter 5,
“Troubleshooting Switch Router ATM Interface Connections.”
This chapter contains the following sections:
• Tag Switching Overview, page 8-1
• Troubleshooting Tag Switching Example, page 8-2
• Initial Troubleshooting of Tag Switching, page 8-3
• Troubleshooting TDP Neighbors, page 8-8
• Troubleshooting Tag Switching on VP Tunnels, page 8-9
• Troubleshooting Tag Switching Using debug Commands, page 8-12
• MPLS Overview, page 8-12
• Troubleshooting MPLS Connections, page 8-19
• Troubleshooting MPLS VPN, page 8-26
Tag Switching Overview
Tag switching is a high-performance packet-forwarding technology that assigns tags to multiprotocol
frames for transport across packet-based or cell-based networks.
In conventional Layer 3 forwarding, as a packet traverses the network, each router extracts forwarding
information from the Layer 3 header. Header analysis is repeated at each router (hop) through which the
packet passes.
In a tag switching network, the Layer 3 header is analyzed just once. It is then mapped into a short,
fixed-length tag. At each hop, the forwarding decision is made by looking at the value of the tag only;
there is no need to reanalyze the Layer 3 header. Because the tag is a fixed-length, unstructured value,
looking it up is fast and simple.