Monthly Archives: October 2012

Auto QoS and ASIC / Port mappings on a 6500

Today I came across an interesting qwerk which I felt worth sharing. If you enable auto-qos on a 6500 port, the QoS features will be applied to every port which shares the relevant ASIC. Exactly how this plays out will depend on the architecture of the switch, we need to take a closer look. In this excellent post James Ventre says:

“In the 6500 platform, [the ASIC port mapping is] easily displayed with ‘show interface capabilities’… The portion we’re interested in is labeled ‘Ports-in-ASIC’.”

In my case, I have a WS-X6748-SFP in slot 3 of my 6500 and I’m interested in port 1:

my-6500# show interfaces gig 3/1 capabilities | i ASIC
 Ports-in-ASIC (Sub-port ASIC) : 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35,37,39,41,43,45,47 (1,3,5,7,9,11,13,15,17,19,21,23)

According to this post, there are three levels of ASIC on a 6748 line card:

  1. Two JANUS Bus ASICs
  2. Two SSA Fabric ASICs
  3. Four ROHINI Port ASICs
The section in parenthesis refers to the ASIC we are interested in. The backplane on a sup-720 offers 40G per line card. My 6748 has 48 ports, managed by four ROHINI or Sub-port ASICs, each connected at 10Gbps (as an aside, this gives an oversubscription ratio of 1.2:1 as 12 Gigabit port has access a 10G connection to the backplane).
When I enabled auto-qos on interface Gig 3/1, I saw the following on the remaining (default config) ports sharing its ROHINI ASIC:
interface GigabitEthernet3/n ! where n is odd and < 24
 no ip address
 shutdown
 wrr-queue cos-map 2 1 1 2
 wrr-queue cos-map 3 5 3 4
 wrr-queue cos-map 3 7 6 7
 rcv-queue cos-map 1 2 1
 rcv-queue cos-map 1 3 2
 rcv-queue cos-map 1 4 3
 rcv-queue cos-map 1 5 4
 rcv-queue cos-map 1 6 5
 rcv-queue cos-map 1 7 6
 rcv-queue cos-map 1 8 7

So there you have it – no need to panic if a load of config appears on your box from a single auto-qos command. However, do be aware of your switch architecture before trying to squeeze everything you can out of an old line card.