Packet size on TX ring/Rx ring

[sarahr202]

Hi everybody,

I have started studying QOS a little deeper now:)

R1#show controllers ethernet 1/0
ADDR: 649B9654, FASTSEND: 6005A794, MCI_INDEX: 0

Omitted!!

rx ring entries=32, tx ring entries=64

Above we see RX ring can hold up to 32 packets and TX ring can hold up to 64.  But what is maximum packet size ? Does RX 32 means it can hold 32 packets of 64 bytes or 1500 bytes?

Does this size  factor in layer 2 overhead for example a normal untagged Ethernet II may add up to 18 bytes to IP packet so 64 byte packet  will be  82 bytes. 

Thanks

[acampbell · Accepted solution]

Hi Sarah,

May be have a read at these 2 links.

This one is quite old but still applies today.

1) http://www.cisco.com/c/en/us/support/docs/asynchronous-transfer-mode-atm/ip-to-atm-class-of-service/6142-txringlimit-6142.html

Then

2) http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/quality-of-service-qos/sol_ov_c22-708224.html

Regards

Alex

[sarahr202]

Good links, thanks Alex,

Actually already read the first one before I posted my question, but it did not say anything the packet size on TX ring.  But based on the link, I feel it works the same way it does for ATM interface.

I.e MTU of interface and layer 2 encapsulation being used on the interface will determine the packet size being referred to TX ring entries 64 .