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Switch Fabric- Troubleshooting tips

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Introduction

What is Switch Fabric?

The switch fabric is essentially the backplane for all ports and modules on the switch module. When a connection is made from a port on one module to a port on another module, it is made across the switch fabric. Physically, it is the combination of silicon, plastic, and metal that enables ports to connect and pass traffic between themselves.

 

The switch fabric can be blocking or non-blocking. Non-blocking fabric ensures that the total bandwidth of all ports that use the switch fabric do not exceed its capacity. What this means is that the density of the ports on the switch are such that their total capacity will never be greater than that of the switch fabric. Switches operating in non-blocking mode ensure that congestion will never occur on the switch, nor will ports ever want for bandwidth between each other.

 

A blocking switch has a port density capacity that exceeds the total capacity of the switch fabric. Control is possible by blocking traffic flow when the switch fabric capacity is exceeded or otherwise not available.

 

The switch fabric resides on the SE. When a port has to communicate with another port, it has the supervisor check its tables (Content Addressable Memory [CAM] for Layer 2 addresses and Ternary CAM [TCAM] for Layer 3 addresses) to determine what slot and port it needs. The supervisor then establishes the connection between the ports.The switch fabric can also reside on its own module (such as the Switch Fabric Module 2 (WS-X6500-SFM2) and the Switch Fabric Module (WS-C6500-SFM for the Catalyst 6500 Series), which enables the available capacity to be expanded without replacing the SE, or to expand beyond the capacity of the SE.

 

 

On Catalyst 6500, Switch fabric is a daughter card installed at Sup720 , it used to be a separate module, Switch Fabric module in its first implementation back in the days of Sup2. It is used to provide backplane connectivity between linecards. The default bandwidth available on the backplane of 6500 is 32 Gbps. This 32 Gbps is used by all slots for serial transmission of data. Therefore at any instant only two ports can be communicating.

 

With the addition of Switch fabric, the switch’s backplane changes from serially-accessed bus to crossbar fabric. By using crossbar fabric, many ports can be simultaneously transmitting and receiving data, providing a much higher throughput.

 

switch fabric.JPG

 

The crossbar fabric consists of 18 fabric channels, providing each linecard two fabric channels into the crossbar fabric. These channels can run at 8Gbps or 20Gbps depending upon the line card used. The CEF256 and dCEF256 series modules connect to fabric using 8 Gbps per channel and CEF720 series modules connect to it using 20 Gbps per channel.

.

Requirements

For a module to use switch fabric, it should be a fabric enabled module.

Troubleshooting Tips

1.    If the Fabric Switch Module does not work as expected, check the following:

 

a)    Check if the Fabric Switch Status is Active. To do this,

use the show fabric active command. This command will display the

current status of the Fabric Switch. Here is an example.

 

Switch# show fabric active 
Active fabric card in slot 5
No backup fabric card in the system
         If the system has backup fabric card, then:
 
              Switch #show fabric active
show fabric active:
Active fabric card in slot 5
Backup fabric card in slot 6

b)    Check the fabric status of switching modules in the device. To do this use the 

 

show fabric status [slot_number | all] command.  This command will display the fabric

status of one or all switching modules. Here is an example,

          Switch# show fabric status 

          slot    channel      speed               module               fabric

                                          status               status

                1          0             8G                   OK                   OK

                5          0             8G                   OK           Up- Timeout

                6          0             20G                  OK          Up- BufError

                8          0             8G                   OK                   OK

                8          1             8G                   OK                   OK

                9          0             8G                   Down- DDRsync        OK

          Switch#

 

c)    Check the fabric utilization of switching modules. To do this use the show fabric utilization [slot_number | all] command. This command will display the fabric utilization of one or all modules.

Here is an example,

 

           Switch# show fabric utilization all 

           slot    channel      speed    Ingress %     Egress %

                 1          0        20G            0            0

                 1          1        20G            0            0

                 2          0        20G            0           24

                 2          1        20G            0           24

                 3          0        20G           48            0

                 4          0        20G            0            0

                 4          1        20G            0            0

2. In certain rare condi tions out put of 'show fabric channel-counters'may show 

   incrementing number of rxErrors.

                       Switch#show fabric channel-counters

                       slot channel   rxErrors   txErrors    txDrops  lbusDrops

                            1       1          0          0          0          0

                            3       0          0          0          0          0

                            3       1          0          0          0          0

                            5       0          5          0          0          0

                            8       0         39          0          0          0

                            8       1          0          0          0          0
 

a)    RxRrror indicates that the module received corrupted packet(s) and dropped.

b)    The Fabric do NOT check CRC when forwarding frames between different fabric

      ports/channels.

c)    This could be due to the receiving module corrupting the frames or receiving

      corrupted frames from any fabric-enabled module in the switch.

 

The following actions can be taken to solve these errors:

               

a)    Reseat the module with rxErrors. Reloading the linecard in question might stop

      the errors for some time, but the errors might eventually come back.

b)    If empty slot is available in the chassis move the affected line card to empty

      slot.

c)    If no empty slots available, swap the linecard that counts rxErrors with other

      linecard within the chassis (with no issue) or good known working linecard.

d)    Swap the active and standby supervisors (i.e. move supervisor from slot 5  to

      slot 6 and vice versa. Sup failover.

e)    Replace the affected linecard.

 

If the output of "show fabric status" command is showing "not-hot" for linecards under

hotStandby support.

 

    Switch#show fabric status

    slot  channel speed module   fabric   hotStandby  Standby  Standby

                     status   status      support  module   fabric

    3        0   20G     OK       OK   Y(not-hot)

    3        1   20G     OK       OK   Y(not-hot)

    4        0   20G     OK       OK   Y(not-hot)

    4        1   20G     OK       OK   Y(not-hot)

    5        0   20G     OK       OK          N/A

    5        1   20G     OK       OK          N/A

    6        0   20G     OK       OK          N/A

    6        1   20G     OK       OK          N/A

    7        0   20G     OK       OK   Y(not-hot)

    7        1   20G     OK       OK   Y(not-hot)

    8        0   20G     OK       OK   Y(not-hot)

    8        1   20G     OK       OK   Y(not-hot)

    9        0   20G     OK       OK   Y(not-hot)

    9        1   20G     OK       OK   Y(not-hot)

 

Reason: The standby fabric hot sync feature is only supported on the E version of the

        6500 chassis, and this system has a non-E version.

 

3.    If you see the error message as, “SP:  Linecard endpoint of Channel 7 lost Sync.

      To Lower fabric and trying to recover now!”.

 

Reason: The message caused by a line card not being fully or properly seated. To

        identify this line card - the capture of show fabric fpoe map command

 

        need to be analyzed. Here is an example,

 

 

Switch#show fabric fpoe map

slot channel   fpoe

  1     0    0

  1     1    9

  2     0    1

  2     1    10

  3     0    2

  3     1    11

  4     0    3

  4     1    12

  5     0    4

  6     0    5

  6     1    14

  7     0    6

  7     1    15

  8     0    7

  8     1    16

  9     0    8

  9     1    17

Workaround: The fpoe will be mapped to a specific line card slot. Once the suspect line

card is identified From the output of show fabric fpoe map, fpoe 7 points to the line

card in slot 8 and that is card that is causing the error messages.your next action

should be to schedule a removal and re-insert of that card to try to eliminate this

message from re-occurring.

 

4.    If the system switching performance drops from 30Mpps to 15Mpps.

 

Reason: When classic and fabric enabled modules are mixed in a chassis, the system

switching performance drops from 30Mpps to 15Mpps.

 

Older "Classic" modules in the 6500, models 61xx, 62xx, 63xx, 64xx, send all traffic

over the switch BUS backplane, to be forwarded by the supervisor.  Fabric enabled modules

only send the packet headers over the bus and the switch fabric can be utilized for forwarding the data portion of

the packet.

 

Workaround: Consider replacing any "Classic" modules with fabric enabled modules, in

order to increase system performance.

 

5.    To troubleshoot further, collect the following show command output before opening a TAC case.

 

     step 1. turn on service internal.

 

     switch# configure terminal

     switch(config)# service internal


     step 2. collect the requested logs.


     terminal length 0

     show fabric active

     show fabric channel-counters

     show fabric drop

     show fabric errors

     show fabric errors threshold

     show fabric fpoe map

     show fabric status

     show fabric utilization

     show tech-support

     remote login switch

     terminal length 0

     show fabric error

     show fabric state-machine channel state

     show fabric state-machine channel event_trace 11

     show fabric resync

     show fabric timeout

     show platform hardware capacity fabric

     exit


     step 3. turn off service internal


     switch # configure terminal

     switch(config)# no service internal

Troubleshooting Example

 

1.    Fabric Time out Error Message:

%FABRIC-SP-[module-number]-TIMEOUT_ERR: Fabric in slot [dec] reported timeout error for

channel [dec] (Module [dec], fabric connection [dec])

 

Description

 

The error message indicates that firmware code on the fabric detected that the input or

output buffer was not moving. To recover from this condition, the system will automatically

resynchronize the fabric channel.

Troubleshooting Steps

  1. Issue the command “hw-module reset” to soft-reset the module. After the module is up again,
  2. capture the output of the command “show module” and the command “show diagnostic module all”.

Sample Output Of “show module”

Show Module

Mod   Ports          Card Type                                  Model                Serial No.

---   ----- -------------------------------------- ------------------ --------- ------------------------------------

  2     24     CEF720 24 port 1000mb  SFP         WS-X6724-SFP     SAL0AAAAAAA

  3     24     CEF720 24 port 1000mb  SFP         WS-X6724-SFP     SAD0AAAAAAA

  5      2      Supervisor Engine 720    (Hot)          WS-SUP720-3B    SAD0AAAAAAA

  6      2      Supervisor Engine 720    (Active)      WS-SUP720-3B    SAD0AAAAAAA

  7      4      CEF720 4 port 10-Gigabit Ethernet   WS-X6704-10GE   SAL1AAAAAAA

  8      4      CEF720 4 port 10-Gigabit Ethernet   WS-X6704-10GE   SAL1AAAAAAA

 

Sample Output of “show diagnostic module all”

 

Switch#show   diagnostic module all

Current   bootup diagnostic level: minimal

Module   6: Supervisor Engine 720 (Active)

  Overall Diagnostic Result for Module 6 :   PASS

  Diagnostic level at card bootup: minimal

  Test results: (. = Pass, F = Fail, U =   Untested)

    1) TestScratchRegister ------------->   .

    2) TestSPRPInbandPing -------------->   .

    3) TestTransceiverIntegrity:

      Port    1  2

      ----------

            U    U

    4) TestActiveToStandbyLoopback:

      Port    1  2

      ----------

            U    U

    5) TestLoopback:

      Port    1  2

      ---------

    6) TestNewIndexLearn --------------->   .

    7) TestDontConditionalLearn -------->   .

    8) TestBadBpduTrap ----------------->   .

    9) TestMatchCapture ---------------->   .

   10) TestProtocolMatchChannel -------->   .

   11) TestFibDevices ------------------>   .

   12) TestIPv4FibShortcut ------------->   .

   13) TestL3Capture2 ------------------>   .

   14) TestIPv6FibShortcut ------------->   .

   15) TestMPLSFibShortcut ------------->   .

   16) TestNATFibShortcut -------------->   .

   17) TestAclPermit ------------------->   .

   18) TestAclDeny --------------------->   .

   19) TestQoSTcam --------------------->   .

   20) TestL3VlanMet ------------------->   .

   21) TestIngressSpan ----------------->   .

   22) TestEgressSpan ------------------>   .

   23) TestNetflowInlineRewrite:

      Port    1  2

      ----------

            U    U

   24) TestFabricSnakeForward ---------->   .

   25) TestFabricSnakeBackward --------->   .

   26) TestTrafficStress --------------->   U

   27) TestFibTcamSSRAM ---------------->   U

   28) TestAsicMemory ------------------>   U

   29) TestAclQosTcam ------------------>   U

   30) TestNetflowTcam ----------------->   U

   31) ScheduleSwitchover -------------->   U

   32) TestFirmwareDiagStatus ---------->   .

 

In case the output doesn’t come as expected, physically pull out and reseat the module firmly in the chassis to hard-reset the module. After the module is up again, capture the output of the command “show module” and “show diagnostic module all”

Here is an example of failed diagnostic test for module 1

Module 1: Catalyst 6000 supervisor 2 (Active)  SerialNo : 

  Overall Diagnostic Result for Module 1 : MINOR ERROR

  Diagnostic level at card bootup: minimal

  Test results: (. = Pass, F = Fail, U = Untested)

    1) TestSPRPInbandPing --------------> F

    2) TestTransceiverIntegrity: 

2.     Overruns on some ports on Card 5 (WS-X6548-GE-TX)

Switch1# show interface counters 

Noticed "overruns" on 4 interfaces. They were not incrementing 

GigabitEthernet5/1 is up, line protocol is up (connected)

  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseT

     0 input errors, 0 CRC, 0 frame, 26 overrun, 0 ignored  

GigabitEthernet5/6 is up, line protocol is up (connected)

  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseT

     0 input errors, 0 CRC, 0 frame, 14 overrun, 0 ignored            

GigabitEthernet5/9 is up, line protocol is up (connected)

  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseT

     0 input errors, 0 CRC, 0 frame, 159 overrun, 0 ignored

GigabitEthernet5/12 is up, line protocol is up (connected)

  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseT

     0 input errors, 0 CRC, 0 frame, 269 overrun, 0 ignored

Also will enabling switch fabric rectify the overruns?

 

Solution:

Enabling switch fabric will not rectify overruns because after the installation of a Switch Fabric Module in a Cisco Catalyst 6500 series switch, the traffic is forwarded to and from modules in different modes which doesn't necessarily facilitate resolution for overruns. The traffic is forwarded in one of these of these modes:

a)    Flow-through mode: In this mode, data passes between the local bus and the supervisor engine bus. This mode is used for traffic to or from modules that are not fabric-enabled.

b)    Truncated mode: Only truncated data (the first 64 bytes of the frame) goes over the switch fabric channel if both the destination and the source are fabric-enabled modules. If either the source or destination is not a fabric-enabled module, the data goes through the switch fabric channel and the data bus. The Switch Fabric Module does not get involved when traffic is forwarded between modules that are not fabric-enabled.

c)    Compact mode:  A compact version of the DBus header is forwarded over the switch fabric channel, which delivers the best possible switching rate. Modules that are not fabric-enabled do not support the compact mode and generate cyclic redundancy check (CRC) errors upon receipt of frames in compact mode. This mode is used only when no such modules are installed in the chassis.

Let’s understand what overrun is:-

Overrun - The number of times the receiver hardware was unable to hand received data to a hardware buffer

Common Cause - The input rate of traffic exceeded the ability of the receiver to handle the data.

From the given example, the module used is WS-X6548-GE-TX:

This module is 8:1 oversubscribed. The ports on this module go to servers. On these modules there is a single 1-Gigabit Ethernet uplink from the port ASIC that supports eight ports. These cards share a 1 Mb buffer between a group of ports (1-8, 9-16, 17-24, 25-32, 33-40, and 41-48) since each block of eight ports is 8:1 oversubscribed. The aggregate throughput of each block of eight ports cannot exceed 1 Gbps. These line cards are oversubscription cards that are designed to extend gigabit to the desktop and might not be ideal for server farm connectivity. For more information refer to:-

Troubleshooting Switch Port and Interface Problems

To resolve this issue of overrun, move the high volume servers to ports on different asic groups, so that the traffic flow through the 8 ports of every asic group does not exceed 1 Gbps. Alternatively look for other ideal design recommendations on line cards that have better oversubscription ratio.

For Best Practices, please refer to Oversubscription and Density Best Practices.

Related Information

For a module to use switch fabric, it should be a fabric enabled module.

Comments
Cisco Employee

Excellent documents

Cisco Employee

Thanks!

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