CCNP TSHOOT Learning Guide/Chapter 4

This article is under development....

VLAN Troubleshooting

Layer 2 reviewing

ARP

• show mac-addresses

SW1#<input>show mac-address-table</input>
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 All    0100.0ccc.cccc    STATIC      CPU
 All    0100.0ccc.cccd    STATIC      CPU
<notice>...OUTPUT OMITTED...</notice>
   1    001b.0cba.4f99    DYNAMIC     Gi0/9
   1    001b.2a32.2e99    DYNAMIC     Gi0/5
   1    0021.5a7c.5549    DYNAMIC     Gi0/4
   1    00a0.d1e6.60b4    DYNAMIC     Gi0/22
  41    0014.f273.96c0    DYNAMIC     Gi0/7
<notice>...OUTPUT OMITTED...</notice>
  41    001b.0cb6.9141    DYNAMIC     Gi0/7
  41    001b.0cba.4fc1    DYNAMIC     Gi0/9
  41    001b.2a32.2ec1    DYNAMIC     Gi0/5
  12    001f.297d.3460    DYNAMIC     Gi0/2
  20    00a0.c543.035d    DYNAMIC     Gi0/6
  22    0003.47ca.d67c    DYNAMIC     Gi0/7
  22    000c.296c.1120    DYNAMIC     Gi0/12
  24    0016.769f.ff4e    DYNAMIC     Gi0/8
  28    0013.2085.a1fe    DYNAMIC     Gi0/9
  28    0013.2085.c447    DYNAMIC     Gi0/9
<notice>...OUTPUT OMITTED...</notice>

Troubleshooting layer 2

• Hardware issues (move cable to good port)
• VLAN configuration.
  • Inter-VLAN traffic must be routed
• Trunk configuration
  • Same trunkprotocol (DOT1Q,ISL)
  • Same Native VLAN

Verifying Layer 2 Forwarding

Often used commands

• show mac-address-table
• show vlan
• show interface trunk
• show interface switchport
• show platform forward interface - Example below
• traceroute mac - Example below

show platform forward interface example

Show how the switch will direct traffic

SW1#<input>show platform forward gigabitEthernet 0/1 vlan 1 0021.5a7c.5549 001b.2a32.2e99</input>
Ingress:
Global Port Number: 1, lpn: 3 Asic Number: 1
Source Vlan Id: Real 1, Mapped 1. L2EncapType 0, L3EncapType 3
Hashes: L2Src 0x00 L2Dst 0x09 L3Src 0x00 L3Dst 0x09
 Lookup                   Key-Used                  Index-Hit    A-Data
Classify 68_0070001B_2A322E99-00_00000021_5A7C5549     00FFC   00000000
InputACL 20_0070001B_2A322E99-00_00000021_5A7C5549     01FF8   01000000
L2LrnMsk FF_03FFFFFF_FFFFFFFF-00_000003FF_00000000
L2Learn  83_00010021_5A7C5549-93_00000401_00000000     01840   00000000
L2FwdMsk FF_03FFFFFF_FFFFFFFF
L2Fwd    83_0001001B_2A322E99                          0003E   000000C5
Station Descriptor: F005F001, DestIndex: F005, RewriteIndex: F001
==========================================
Egress: Asic 0, switch 1
Source Vlan Id: Real 1, Mapped 1. L2EncapType 0, L3EncapType 3
portMap 0x4, non-SPAN portMap 0x4

Output Packets:
------------------------------------------
GigabitEthernet0/5 Packet 1
 Lookup                   Key-Used                  Index-Hit    A-Data
OutptACL 30_0070001B_2A322E99-00_00000021_5A7C5549     01FFC   01000000

Port       Vlan      SrcMac          DstMac    Cos  Dscpv
Gi0/5      0001 0021.5a7c.5549  001b.2a32.2e99

traceroute mac

Did not work out all right.

SW1#<input>traceroute mac 001b.0c5d.42c2 0006.5bc5.2dfb</input>
l2trace error response received : 3 from 192.168.4.22
Layer2 path not through SW2 [192.168.4.22].
Layer2 trace aborted.