a forwarding state and a switching loop occurs. A switching loop is more serious than a routing loop because unlike low end routers, even a modest switch can forward broadcast frames at the full line speed. This can fully saturate a link and further prevent BPDUs from being exchanged. Thus, the loop is maintained. How is it possible for the switch to stop receiving BPDUs while the port is up? The most obvious answer is that the STP has been turned off. Another common cause is the unidirectional link. A link is considered unidirectional when: Consider the scenario in Figure , the arrows indicate the flow of STP BPDUs. During normal operation, bridge B is designated on the link B-C. Bridge B sends BPDUs down to C, which is blocking the port. The port is blocked while C is seeing BPDUs from B on that link. Now, consider what happens if the link B-C fails in the direction of C. C will stop receiving traffic from B, but B will still receive traffic from C. C will stop receiving BPDUs on the link B-C, and will age the information received with the last BPDU. This will take up to 20 seconds, depending on the maxAge STP timer. Once the STP information is aged out on the port, it will transition from blocking state to listening, learning, and eventually to forwarding STP state. This will create a forwarding loop, as there will be no blocking port in the triangle A-B-C. Packets will cycle along the path (B still receives packets from C) taking more and more bandwidth until the links will be filled up completely bringing the network down. Web Links http://www.cisco.com/warp/customer/ 473/spanning_tree1.swf Note: The user must have a valid CCO account to view this additional information.
Content 4.3 Troubleshooting Switched Ethernet Networks 4.3.2 Troubleshooting STP loops Once an STP loop has been identified it is important to isolate the reason behind the loop. One of the first things to check is if the Spanning Tree Protocol is running on each of the switches. A switch should only have STP disabled if it is not part of a physically looped topology. Unless there is a very good reason for turning STP off, every switch should have STP enabled. The IOS command to verify STP operation is: Switch#show spanning-tree It will be clear from the output of the show spanning-tree command if STP is not running. It is important to remember that spanning tree operates on a per-vlan basis by default on Cisco devices (PVST/PVST+) and executing a show spanning-tree command will report on VLAN 1 only. Use the show spanning-tree [vlan ID] command to verify STP operations for other VLANs. Should STP not be operating, it can be enabled using the spanning-tree vlan ID command.
Content 4.3 Troubleshooting Switched Ethernet Networks 4.3.3 Preventing STP loops In order to detect the unidirectional links before the forwarding loop is created, Cisco designed and implemented the UDLD protocol. UDLD is a Layer 2 protocol that works with the Layer 1 mechanisms to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports. When UDLD is enabled, it works with autonegotiation to prevent physical and logical unidirectional connections from forming and causing other protocols to malfunction. UDLD works by exchanging protocol packets between the neighboring devices. In order for UDLD to work, both devices on the link must support UDLD and have it enabled on respective ports. Each switch port configured for UDLD will send UDLD protocol packets containing the port's own device/port ID, and the neighbor's device/port IDs seen by UDLD on that port. Neighboring ports should see their own device/port ID (echo) in the packets received from the other side. If the port does not see its own device/port ID in the incoming UDLD packets for a specific duration of time, the link is considered unidirectional. This echo-algorithm allows detection of several issues: Once the unidirectional link is detected by UDLD, the respective port is disabled and the following message is printed on the console: UDLD-3-DISABLE: Unidirectional link detected on port 1/2. Port disabled Port shutdown by UDLD remains disabled until it is manually reenabled, or until errdisable timeout expires if configured. The following commands detail configuring UDLD on Catalyst switches running Cisco IOS. By default, UDLD is disabled. First, UDLD needs to be enabled globally by issuing the following command: Switch(config)#udld enable By default, this command will enable UDLD on all interfaces. UDLD can be enabled on a specific interface by issuing the following command: Switch(config-if)#udld enable The interface command overrides the global configuration command. UDLD can be verified with the following command: Switch#show udld interface See Figure for an example of the output produced. Should it be necessary to override the UDLD configuration on individual interfaces, this can be accomplished from the interface configuration mode: Switch(config)#interface fastethernet 0/12
Switch(config-if)#udld enable In an environment that supports UDLD, enabling this functionality has the potential to prevent STP loops and the loss of network connectivity that results. Cisco recommends that UDLD should be enabled wherever possible.
Content 4.3 Troubleshooting Switched Ethernet Networks 4.3.4 Troubleshooting incorrect STP root configuration Switches have become the basic building block of even the simplest networks. They form a seamless replacement for hubs, and come with STP enabled by default. Link redundancy can be implemented without any switch configuration being necessary. In fact it is possible to construct a highly redundant switched network without ever having heard of the Spanning-Tree Protocol (STP). Poorly configured Layer 2 designs usually result from a lack of knowledge of the Spanning Tree Protocol, or a misunderstanding of its operation. Examine the switched network topology in Figure . At first glance, it appears to be an efficient design. A few pings to devices in the network reveals full connectivity. However, upon closer inspection the traffic flow in this topology reveals suboptimal traffic patterns. To really analyze the network topology it is necessary to look at the spanning tree states of each of the ports. In Figure it is apparent that the network has unfortunately chosen the Marketing department switch as the STP root. Spanning tree has placed the Data Center port leading to the Telephone Sales Center in a blocking state. Consequently the majority of traffic must take the longest path to the Data Center. With the Data Center at one end of the Spanning Tree, the link between the switch in Reception and Data Center must carry the entire traffic load of the organization. The symptoms of suboptimal Layer 2 design are usually those of congestion and are often only noticed when traffic levels increase. In a worst case, the following are symptoms of poor Layer 2 design: Troubleshooting suboptimal Layer 2 design can be a challenge. Traditional tools such as traceroute operate at Layer 3 and will reveal nothing about the Layer 2 topology. It is a common error to overlook Layer 2 as the culprit and incorrectly troubleshoot the higher layers when faced with congestion caused by poor Layer 2 design. Another common mistake made by inexperienced network administrators and