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Any Questions?. Chapter 3-Troubleshooting LAN Switching. Generalized Troubleshooting Methodologies Troubleshooting the LAN Switching Data Plane Predicting Normal Operation of the LAN Switching Data Plane. Troubleshooting.

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  1. Any Questions?

  2. Chapter 3-Troubleshooting LAN Switching • Generalized Troubleshooting Methodologies • Troubleshooting the LAN Switching Data Plane • Predicting Normal Operation of the LAN Switching Data Plane

  3. Troubleshooting • Analyzing/predicting normal operation: The description and prediction of the details of what should happen if the network is working correctly, based on documentation, configuration, and show and debug command output. • Problem isolation: When some problem might be occurring, find the component(s)that do not work correctly as compared to the predicted behavior, again based ondocumentation, configuration, and show and debug command output. • Root cause analysis: Identify the underlying causes of the problems identified in theprevious step, specifically the causes that have a specific action with which the problemcan be fixed. Pg 110

  4. Any Questions?

  5. Normal Operations • Data Plane • Decisions for each packet • Control Plane • Support features • Routing Protocols • VTP Pg 111

  6. Data Plane Analysis • Step 1 Think about PC1’s IP address and mask, the IP address and mask of PC2, and PC1’s logic to realize that PC2 is in another subnet. This causes PC1 to choose to send the packet to its default gateway (R1). • Step 2 Consider R1’s forwarding logic for matching the packet’s destination IP address with R1’s routing table, with the expectation that R1 chooses to send the packet to R2 next. • Step 3 On R2, consider the same routing table matching logic as used on R1 in the previous step, using R2’s routing table. The matching entry should be a connected route on R2. • Step 4 This step relates to PC2’s reply packet, which uses the same basic logic as Step 1. Compare PC2’s IP address/mask with PC1’s IP address, noting that they are in different subnets. As a result, PC2 should send the packet to its default gateway, R2. Pg 112-113

  7. Data Plane Analysis • Step 5 Consider R2’s forwarding logic for packets destined to PC1’s IP address, with the expectation that the matching route would cause R2 to send these packets to R1 next. • Step 6 The final routing step, on R1, should show that a packet destined to PC1’s IP address matches a connected route on R1, which causes R1 to send the packet directly to PC1’s MAC address. Pg 112-113

  8. Control Plane Analysis • Each control process has debug and show commands • Routing • Sh ip prot • Sh ip route • VTP • Sh vtp status • VLANS • Sh vlan • Sh interfaces status Pg 113

  9. Any Questions?

  10. Normal Operations • Step 1 Examine the data plane as follows: • a. Determine the major Layer 3 steps—including origin host to default router, each router to the next router, and last router to the destination host—in both directions. • b. For each Layer 2 network between a host and router or between two routers, analyze the forwarding logic for each device. • Step 2 Examine the control plane as follows: • a. Identify the control plane protocols that are used and vital to the forwarding process. • b. Examine each vital control plane protocol for proper operation; the details of this analysis differ for each protocol. • c. Defer any analysis of control plane protocols that do not affect the data plane’s correct operation until you clearly see a need for the protocol to answer that question (for example, CDP). Pg 114

  11. Problem Isolation • Where is the breakdown • How far do my packets get • Step 1 Begin by examining the Layer 3 data plane (IP forwarding), comparing the results to the expected normal behavior, until you identify the first major routing step that fails. • Step 2 Further isolate the problem to as few components as possible: • a. Examine functions at all layers, but focusing on Layers 1, 2, and 3. • b. Examine both data plane and control plane functions. Pg 114

  12. Root Cause Analysis • Why is the problem happening • Not a symptom • Step 1 Continue isolating the problem until you identify the true root cause, which in turn has an obvious solution. • Step 2 If you cannot reduce the problem to its true root cause, isolate the problem as much as is possible, and change something in the network, which will hopefully change the symptoms and help you identify the root cause. Pg 115-116

  13. Troubleshooting Switching Data Plane • Step 1 Determine the VLAN in which the frame should be forwarded, as follows: • a. If the frame arrives on an access interface, use the interface’s access VLAN. • b. If the frame arrives on a trunk interface, use the VLAN listed in the frame’s trunking header. • Step 2 If the incoming interface is in an STP Learning or Forwarding State in that VLAN, add the source MAC address to the MAC address table, with incoming interface and VLAN ID (if not already in the table). • Step 3 If the incoming interface is not in an STP Forwarding State in that VLAN, discard the frame. • Step 4 Look for the destination MAC address of the frame in the MAC address table, but only for entries in the VLAN identified at Step 1. If the destination MAC is found or not found, follow these steps: • a. Found: Forward the frame out the only interface listed in the matched address table entry • b. Not found: Flood the frame out all other access ports in that same VLAN that are in an STP Forwarding State, and out all trunk ports that list this VLAN as fully supported (active, in the allowed list, not pruned, STP Forwarding) Pg 117-118

  14. LAN Switching Troubles • Step 1 Confirm the network diagrams using CDP. • Sh cdp neighbors • Sh cdp neighbors details • Sh cdp entry name Pg 119-120

  15. LAN Switching Troubles • Step 2 Isolate interface problems. • Show interfaces • Show interfaces status • Check speed, duplex and trunking Pg 121-127

  16. LAN Switching Troubles • Step 3 Isolate filtering and port security problems. • Show running config • Check port security settings • Show port security interface Pg 127-132

  17. LAN Switching Troubles • Step 4 Isolate VLANs and trunking problems. Pg 132-136

  18. LAN Switching Troubles • Step 4 Isolate VLANs and trunking problems. • Show mac address-table • Show interfaces trunk • Switchport mode command • Access • Trunk • Dynamic desirable|auto • Switchport access vlan ID Pg 132-136

  19. Any Questions?

  20. Example PC 1, 2, and 3 can’t ping R1 at 2.2.2.9 Pg 137

  21. Normal operation of LAN data plane Pg 147

  22. Any Questions?

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