Troubleshooting guide

8-2
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 8 Troubleshooting Tag and MPLS Switching Connections
Troubleshooting Tag Switching Example
A tag switching network consists of tag edge routers and tag switch routers, as shown in Figure 8-1. Tag
edge routers are located at the edge of a tag switching network. They use standard routing
protocols—such as Open Shortest Path First (OSPF)—to create routing tables that identify routes
through the network. Based on the routing tables, tag edge routers use the Tag Distribution Protocol
(TDP) to apply and distribute tags to other tag edge routers or tag switch routers. Tag switch routers are
located at the core of a tag switching network. They receive TDP information from the tag edge routers
and build their own forwarding database. Tag switch routers then switch the packets based on the tags
only (without looking at the Layer 3 header).
How Tag Switching Works
When a tag edge router at the entry point of a tag switching network receives a packet for forwarding the
following occurs:
1. The router analyzes the network layer header and performs any applicable network layer services
such as security, accounting, or quality of service (QoS) classification.
2. The router chooses a route for the packet based on the information in its routing table, applies a tag,
and forwards the packet to the next-hop tag switch router.
3. The tag switch router receives the tagged packet and switches the packet from switch router to
switch router based on the tag only. The switch routers do not reanalyze the network layer header;
they look only at the short, fixed-length tag.
4. The packet reaches the tag edge router at the exit point of the tag switched network, where the tag
is removed and the packet is delivered.
Troubleshooting Tag Switching Example
In the example network in Figure 8-1, the primary campus network backbone is made up of two
ATM switch routers connected to two Cisco routers:
• AdminFl1Rt1—Tag switching router located in the administration building
• AdminFl1Ls1—Tag switching switch router located in the administration building
• EngFl1Ls1—Tag switching switch router located in the engineering building
• EngFl1Rt1—Tag switching router located in the engineering building