Friday, December 9, 2016

Performing Time Domain Reflectometer (TDR) on a Cisco Switch

I was troubleshooting some lightweight APs that were connected on a Cisco 3650 PoE switch and two of them can't seem to associate with the WLC. I've checked the WLC config is correct since three APs were registered and wireless clients got associated to them. So I troubleshoot the switch to see if there's any Layer 1 or Layer 2 issues but it can't detect the said APs. I tried to move the AP to another port on the switch but it's still down when I do a show cdp neighbor command.

Switch#show interface status

Port      Name               Status       Vlan       Duplex  Speed Type
Gi1/0/1   | AP |             notconnect   1            auto   auto 10/100/1000BaseTX
Gi1/0/2   | AP |             connected    1          a-full a-1000 10/100/1000BaseTX
Gi1/0/3   | AP |             connected    1          a-full a-1000 10/100/1000BaseTX
Gi1/0/4   | AP |             notconnect   1            auto   auto 10/100/1000BaseTX
Gi1/0/5   | AP |             connected    1          a-full a-1000 10/100/1000BaseTX

Switch#show cdp neighbor
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
sw03         Gig 1/1/2         169              S I   WS-C3560- Gig 0/1
ap03         Gig 1/0/3         164              R T   AIR-CAP16 Gig 0.1
ap02         Gig 1/0/2         173              R T   AIR-CAP16 Gig 0.1
ap05         Gig 1/0/5         143              R T   AIR-CAP16 Gig 0.1

Switch#show power inline

Module   Available     Used     Remaining
          (Watts)     (Watts)    (Watts)
------   ---------   --------   ---------
1           390.0       77.0       313.0
Interface Admin  Oper       Power   Device              Class Max
                            (Watts)                           
--------- ------ ---------- ------- ------------------- ----- ----
Gi1/0/1   auto   on         15.4    Ieee PD             3     30.0
Gi1/0/2   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0
Gi1/0/3   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0
Gi1/0/4   auto   on         15.4    Ieee PD             3     30.0
Gi1/0/5   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0


Switch#show version

<OUTPUT TRUNCATED>

Switch Ports Model              SW Version        SW Image              Mode  
------ ----- -----              ----------        ----------            ----  
*    1 52    WS-C3650-48PS      03.06.04.E        cat3k_caa-universalk9 INSTALL


There's a built-in Time Domain Reflectometer (TDR) feature on a Cisco switch which is similar to what a cable tester does. The switch detected an "open" circuit where the problematic APs are connected. This could mean the cable is broken or there's no pin contact (due to improper crimp). Below is a summary and meaning for the cable pair status output:


Switch#test cable ?
  tdr  test TDR on interface

Switch#test cable tdr ?
  interface  Specify an interface for test

Switch#test cable tdr interface ?
  GigabitEthernet  GigabitEthernet IEEE 802.3z

Switch#test cable tdr interface GigabitEthernet1/0/1
TDR test started on interface Gi1/0/1
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.

Switch#show cable-diagnostics ?
  tdr  show TDR result

Switch#show cable-diagnostics tdr ?
  interface  Specify an interface for result

Switch#show cable-diagnostics tdr interface ?
  GigabitEthernet  GigabitEthernet IEEE 802.3z

Switch#show cable-diagnostics tdr interface GigabitEthernet1/0/1
TDR test last run on: September 06 06:00:24

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/1   auto  Pair A     5    +/- 10 meters N/A         Normal             
                Pair B     4    +/- 10 meters N/A         Normal             
                Pair C     5    +/- 10 meters N/A         Normal             
                Pair D     3    +/- 5  meters N/A         Open 

Switch#test cable tdr interface GigabitEthernet1/0/2   // AP IS NORMALLY DETECTED ON THIS PORT; THE WORKING PORT WILL BOUNCE IF TDR IS PERFORMED
TDR test started on interface Gi1/0/2
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.
Switch#
Sep  6 06:02:28.021: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/2,
changed state to down
Sep  6 06:02:29.023: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/2, changed state to down
Sep  6 06:02:31.913: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/2, changed state to up
Sep  6 06:02:32.911: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/2,
changed state to up

Switch#show cable-diagnostics tdr interface GigabitEthernet1/0/2     
TDR test last run on: September 06 06:02:26

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/2   1000M Pair A     17   +/- 10 meters N/A         Normal             
                Pair B     15   +/- 10 meters N/A         Normal             
                Pair C     15   +/- 10 meters N/A         Normal             
                Pair D     15   +/- 10 meters N/A         Normal   


I moved the APs to another port but it's still down.

Switch#show power inline                                          

Module   Available     Used     Remaining
          (Watts)     (Watts)    (Watts)
------   ---------   --------   ---------
1           390.0       77.0       313.0
Interface Admin  Oper       Power   Device              Class Max
                            (Watts)                           
--------- ------ ---------- ------- ------------------- ----- ----
Gi1/0/1   auto   on         15.4    Ieee PD             3     30.0
Gi1/0/2   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0
Gi1/0/3   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0
Gi1/0/4   auto   off        0.0     n/a                 n/a   30.0
Gi1/0/5   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0
Gi1/0/6   auto   on         15.4    Ieee PD             3     30.0

Switch#test cable tdr interface GigabitEthernet1/0/6
TDR test started on interface Gi1/0/6
A TDR test can take a few seconds to run on an interface
Use 'show cable-diagnostics tdr' to read the TDR results.

Switch#show cable-diagnostics tdr interface GigabitEthernet1/0/6
TDR test last run on: September 06 06:18:46

Interface Speed Local pair Pair length        Remote pair Pair status
--------- ----- ---------- ------------------ ----------- --------------------
Gi1/0/6   auto  Pair A     42   +/- 10 meters N/A         Normal             
                Pair B     39   +/- 10 meters N/A         Normal             
                Pair C     40   +/- 5  meters N/A         Open     // SAME TDR RESULT ON PORT G1/0/4 
                Pair D     42   +/- 10 meters N/A         Normal


So I asked a tech to replace the cable run from the patch panel in the main server room going to the wall jack where the AP is mounted. I was able to detect the other AP by just moving the cable to another port on the patch panel.

Switch#show power inline

Module   Available     Used     Remaining
          (Watts)     (Watts)    (Watts)
------   ---------   --------   ---------
1           390.0       77.0       313.0
Interface Admin  Oper       Power   Device              Class Max
                            (Watts)                           
--------- ------ ---------- ------- ------------------- ----- ----
Gi1/0/1   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0    
Gi1/0/2   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0
Gi1/0/3   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0
Gi1/0/4   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0
Gi1/0/5   auto   on         15.4    AIR-CAP1602E-E-K9   3     30.0

Switch#show cdp neighbor
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,
                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
ap03         Gig 1/0/3         165              R T   AIR-CAP16 Gig 0.1
ap02         Gig 1/0/2         129              R T   AIR-CAP16 Gig 0.1
ap01         Gig 1/0/1         174              R T   AIR-CAP16 Gig 0.1
ap05         Gig 1/0/5         171              R T   AIR-CAP16 Gig 0.1
ap04         Gig 1/0/4         175              R T   AIR-CAP16 Gig 0.1

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