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L2 troubleshooting - how to find an egress port? 


                 vlan10                              vlan20

R1  [gig0/0]    -------   [Fa1/0/9] sw1 [Fa1/0/12] -------- [gig0/1] R2
     10.0.0.10                                              20.0.0.20  
   e8b7.4842.4c58                                          e8b7.4842.45c9
                                 [fa1/0/13]

                                      |
                                      |trunk
                                      |
                             fa0/0.10 | fa0/0.20
                             10.0.0.1 | 20.0.0.1
                               04c5.a43f.d6d0
                                      R3

Today I would like to talk about one command which can help us during troubleshooting of switching. As you see above scenario contains three routers located in two different VLANs. Let’s try basic connectivity:
 
r1#ping 20.0.0.20

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 20.0.0.20, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
r1#

r2#ping 10.0.0.10

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.10, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
r2#

Let’s check mac address table on SW1:
 
SW1#sh mac address-table | i  e8b7.4842.4c58
  10    e8b7.4842.4c58    DYNAMIC     Fa1/0/9

SW1#sh mac address-table | i  04c5.a43f.d6d0
   1    04c5.a43f.d6d0    DYNAMIC     Fa1/0/13
  10    04c5.a43f.d6d0    DYNAMIC     Fa1/0/13
  20    04c5.a43f.d6d0    DYNAMIC     Fa1/0/13

SW1#sh mac address-table | i  e8b7.4842.45c9
  20    e8b7.4842.45c9    DYNAMIC     Fa1/0/12
SW1#

So, now I’d like to check what is the flow for packet sourced from Fa1/0/9 with mac address: e8b7.4842.4c58 and destination: e8b7.4842.45c9.
 
SW1#show platform forward fastEthernet 1/0/9 e8b7.4842.4c58 e8b7.4842.45c9
Ingress:
Global Port Number: 11, lpn: 10 Asic Number: 0
Source Vlan Id: Real 10, Mapped 2. L2EncapType 0, L3EncapType 3
Hashes: L2Src 0x0F L2Dst 0x0A L3Src 0x0F L3Dst 0x0A
 Lookup                   Key-Used                  Index-Hit    A-Data
Classify 68_02F0E8B7_484245C9-00_0000E8B7_48424C58     017FC   00000000
InputACL 20_02F0E8B7_484245C9-00_0000E8B7_48424C58     01FF8   01000000
L2LrnMsk FF_03FFFFFF_FFFFFFFF-00_000003FF_00000000   
L2Learn  83_0002E8B7_48424C58-C3_0000080B_00000000     00D26   00000045
L2FwdMsk FF_03FFFFFF_FFFFFFFF
L2Fwd    83_0002E8B7_484245C9                          01810   00000046
Station Descriptor: 03010000, DestIndex: 0303, RewriteIndex: F002
==========================================
Egress: Asic 0, switch 1
        CPU queues: 14.
Source Vlan Id: Real 10, Mapped 2. L2EncapType 0, L3EncapType 3
portMap 0x4400, non-SPAN portMap 0x4400 

Output Packets:
------------------------------------------
FastEthernet1/0/9 Packet 1
 Lookup                   Key-Used                  Index-Hit    A-Data
OutptACL 30_02F0E8B7_484245C9-00_0000E8B7_48424C58     01FFC   01000000
Dropped due to failed deja vu check.
------------------------------------------
FastEthernet1/0/13 Packet 2
 Lookup                   Key-Used                  Index-Hit    A-Data
OutptACL 30_02F0E8B7_484245C9-00_0000E8B7_48424C58     01FFC   01000000

Port       Vlan      SrcMac          DstMac    Cos  Dscpv
Fa1/0/13   0010  e8b7.4842.4c58  e8b7.4842.45c9  

SW1#

As we see the output port for this packet is Fa1/0/13.
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