ASIC to switch port mapping / oversubscription

Interface/Module Connectivity Problems

Connectivity Problem or Packet Loss with WS-X6548-GE-TX and WS-X6148-GE-TX Modules used in a Server Farm

When you use either the WS-X6548-GE-TX or WS-X6148-GE-TX modules, there is a possibility that individual port utilization can lead to connectivity problems or packet loss on the surrounding interfaces. Especially when you use EtherChannel and Remote Switched Port Analyzer (RSPAN) in these line cards, you can potentially see the slow response due to packet loss. These line cards are oversubscription cards that are designed to extend gigabit to the desktop and might not be ideal for server farm connectivity. 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. Table 4 in the Cisco Catalyst 6500 Series 10/100- & 10/100/1000-Mbps Ethernet Interface Modules shows the different types of Ethernet interface modules and the supported buffer size per port.
Oversubscription happens due to multiple ports combined into a single Pinnacle ASIC. The Pinnacle ASIC is a direct memory access (DMA) engine that transfers packets between backplane switching bus and the network ports. If any port in this range receives or transmits traffic at a rate that exceeds its bandwidth or utilizes a large amount of buffers to handle bursts of traffic, the other ports in the same range can potentially experience packet loss. The buffer assignment on these modules is documented in Buffers, Queues & Thresholds on Catalyst 6500 Ethernet Modules.

For WS-X6748-GE-TX module, use following command to find out ASIC to switch port mapping:


Switch1#show interfaces capabilities module 3
GigabitEthernet1/3/1
  Model:                 WS-X6748-GE-TX
  Type:                  10/100/1000BaseT
  Speed:                 10,100,1000,auto
  Duplex:                half,full
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  Ports-in-ASIC (Sub-port ASIC) : 1-24 (1-12) <------ First 24 ports on one fabric, of which, the first half of the ports maps to one ASIC and the other half maps to the second ASIC. Combined with the output below, 20G bandwidth for each of the two fabric on the module or 10G shared per 12 ports. 

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  Remote switch uplink:  no
  Port-Security:         yes
  Dot1x:                 yes

Switch1#sh 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