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VCS throughput drops if a monitor session with source and destination on different rbridges is configured before the SPAN destination interface links up for the first time since the previous reload.

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TitleVCS throughput drops if a monitor session with source and destination on different rbridges is configured before the SPAN destination interface links up for the first time since the previous reload.
Symptoms
VCS throughput drops if a monitor session with source and destination on different rbridges is configured before the SPAN destination interface links up for the first time since the previous reload. This throughput drop can also be observed after a reload where the destination interface is physically down before, during, and immediately after the reload.

While this behavior is occurring, under
"show qos rcv-queue interface" for the ISL on the destination rbridge handling incoming mirrored traffic, packets dropped will continuously increase and In-use Bytes for TC6 will be at or near Max Bytes. The TC6 Max Bytes value for an interface matches the "qos rcv-queue limit" setting for the rbridge. This behavior is more easily visible in 6.0.x than in 7.x since 6.0x has a lower default setting for "qos rcv-queue limit". The 6.0.x default is 285 KB (291840 bytes) while the 7.x default is 2048 KB (2097152 bytes).

For example, while using 6.0.x software, where Te2/0/48 is an ISL interface and the monitor session has source "tengigabitethernet 1/0/36 destination tengigabitethernet 2/0/43 direction both":

VDX2# show qos rcv-queue interface tengigabitethernet 2/0/48
Interface TenGigabitEthernet 2/0/48 (member of ISL Trunk) Receive Queues
  In-use 291840 bytes, Max buffer 2017280 bytes
  2166 packets dropped
          In-use        Max
  TC       Bytes      Bytes
  -------------------------
    0          0     291840
    1          0     291840
    2          0     291840
    3          0     133120
    4          0     291840
    5          0     291840
    6     291840     291840
    7          0     133120
VDX2# show qos rcv-queue interface tengigabitethernet 2/0/48
Interface TenGigabitEthernet 2/0/48 (member of ISL Trunk) Receive Queues
  In-use 291840 bytes, Max buffer 2017280 bytes
  2486 packets dropped
          In-use        Max
  TC       Bytes      Bytes
  -------------------------
    0          0     291840
    1          0     291840
    2          0     291840
    3          0     133120
    4          0     291840
    5          0     291840
    6     291840     291840
    7          0     133120
VDX2#

 
Environment
  • VDX6740, VDX6740T
  • NOS 6.0.2a, 6.0.2h, 7.0.2b, 7.1.0b, 7.2.0a, 7.3.0a
  • Logical chassis VCS
  • Single link ISL between rbridges
  • SPAN (Explicit RSPAN configuration is not necessary to see this issue)
  • SPAN source and destination interfaces are on separate rbridges
  • SPAN monitor session destination interface has not linked up since the previous reload.
Cause
Due to a hardware limitation, the monitor session destination rbridge buffers mirrored traffic, judges that congestion is happening on the ISL where mirrored and other traffic is received, and discards both mirrored and other traffic.

 
Resolution
This behavior can be avoided by either removing the monitor session settings, bringing up the monitor session destination interface, increasing the number of links between rbridges to more than one, or configuring monitor session source and destination to be on the same rbridge.

While this throughput drop is happening, while the destination interface is still down, if the monitor session destination rbridge is a VDX6740T, and if "
speed 1000" is configured on the destination interface, then throughput drops stop happening. If "speed auto" is configured then throughput drops happen again.

While it is possible to observe this in both 6.0.x and 7.x software, it can be observed more easily in 6.0.x software due to the lower default 285 KB for rbridge setting "
qos rcv-queue limit". 7.x software has a default of 2048 KB. When using 6.0.x software it is possible to reduce the degree of throughput loss to the same level as 7.x software by configuring "qos rcv-queue limit 2048" under the settings for the destination rbridge.
 
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