Output Drops in the Millions - Traffic Sent to Alternate BLK Interface

[Ed Willson]

I've been having an issue with a switch that seems to be trying to send 1-2 million packets/hour to each interface that is an alternate interface on another switch.  The spanning tree topology hasn't changed on 4 hours now (that's because I did it), and there are only 5 switches in this equation.  Any idea why this switch would be trying so hard to send packets out an interface that it should know won't be listening?

Thanks,

   Ed

This is the switch that is having the issue:

FRS-Van-Swi-CCore#show span

VLAN0001

  Spanning tree enabled protocol rstp

  Root ID    Priority    32769

             Address     0008.30ea.7c80

             Cost        4

             Port        2 (GigabitEthernet1/0/2)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     2894.0f1a.3e80

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type

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

Gi1/0/1             Desg FWD 4         128.1    P2p

Gi1/0/2             Root FWD 4         128.2    P2p

Gi1/0/4             Desg FWD 4         128.4    P2p

Gi2/0/1             Desg FWD 4         128.57   P2p

Gi2/0/2             Altn BLK 4         128.58   P2p

Gi2/0/4             Desg FWD 4         128.60   P2p

VLAN0001 is executing the rstp compatible Spanning Tree protocol

  Bridge Identifier has priority 32768, sysid 1, address 2894.0f1a.3e80

  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6

  Current root has priority 32769, address 0008.30ea.7c80

  Root port is 2 (GigabitEthernet1/0/2), cost of root path is 4

  Topology change flag not set, detected flag not set

  Number of topology changes 115 last change occurred 04:05:14 ago

          from GigabitEthernet1/0/4

  Times:  hold 1, topology change 35, notification 2

          hello 2, max age 20, forward delay 15

  Timers: hello 0, topology change 0, notification 0, aging 300

GigabitEthernet2/0/1 is up, line protocol is up (connected)

  Hardware is Gigabit Ethernet, address is 2894.0f1f.fb01 (bia 2894.0f1f.fb01)

  Description: FRS-Van-Swi-ACore G2/1/1

  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive not set

  Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseLX SFP

  input flow-control is off, output flow-control is unsupported

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:52, output 00:00:25, output hang never

  Last clearing of "show interface" counters 00:29:00

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 894810

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 2000 bits/sec, 5 packets/sec

     1813 packets input, 142281 bytes, 0 no buffer

     Received 1813 broadcasts (1813 multicasts)

     0 runts, 0 giants, 0 throttles

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

     0 watchdog, 1813 multicast, 0 pause input

     0 input packets with dribble condition detected

     7591 packets output, 632091 bytes, 0 underruns

     0 output errors, 0 collisions, 0 interface resets

     0 unknown protocol drops

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier, 0 pause output

     0 output buffer failures, 0 output buffers swapped out

[sansarav720e]

Dear Ed,

            STP does not produce any problem on your switch  it looks fine, Check for your Physical link settings duplex & speed on both side of your switch .

Do see any output drop on any other switch port , try to shut this port for a while and try to observe drop on another switch port on same switch ..

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 894810

[Ed Willson]

SANTHOSHKUMAR,

       I did check speed/dulex first off.  I don't see any drops on any other ports, just ports that have the other side set to alternate by STP.  I did try shutting the alternate port on the other switches to see what would happen, and the drops stopped.  So I turned them back on, and the drops started again.  Today there was no one in that office, so I reloaded the problem switch, it had only been up a few weeks, but after that it seems to be doing okay. 

At the moment everything seems to be fine, but the exact same thing happened last time I added a switch to this network.  I know the STP is going to re-access its topology, but the same behavior was seen last time too.  I have another 9 switches to add in the next couple weeks, IDF cabinets in bays, so I'll be able to dial in the cause I think shortly.

Thanks,

    Ed

[Ed Willson]

[sansarav720e · Accepted solution]

Dear Ed ,

             If you dont want have any problem pretaining to STP in future or during addition of any switch , Best practise is to keep spanning priorty equal to 8192  or less 32768 on your Core switch or Distrubtion switch which is connecting to all other switch .

or

you can excute below command on your core switch

spanning-tree vlan 1 root primary (which bring your spanning priority to  24576 from 32678 )   .

Please share your otulined LAN diagram  .for better advise . If you have multiple VLAN on your switch, better advise to use PVLAN spanning tree for each VLAN .

[Ed Willson]

After I added additional switches it does seem that it was STP convergence that caused the problem. Thanks!