The purpose of this document is to show how to modify ingress buffers on the Cisco Nexus 7000 (N7k) Cisco Nexus 7000 48-Port 1 and 10 Gigabit Ethernet F2-Series Module (F2) and Cisco Nexus 7000 Enhanced F2-Series 48-Port Fiber 1 and 10 Gigabit Ethernet Module (F2e) linecards for Virtual Lane 3 (VL3).
Also, you will see the amount of ingress buffering capacity you gain for VL3 after modifying these values.
Using Fiber Channel over Ethernet (FCoE) multihop connections between Datacenters over distances greater than 2 kilometers can result in input drops. By default, the F2/F2e linecards have 0 pages in the latency buffer to queue packets after pause is sent and this will lead to input drops on long distance FCoE multihop interfaces.
The latency buffer is defined as follows:
PL_STOP - HWM (PL_Pause) = LB (Latency Buffer)
You'll notice the values mentioned above are displayed as pages. Each page is roughly 384 bytes.
Notice below, the ingress buffer capacity of VL3 with the default FCoE QoS policy:
Note: Each clipper asic has 6MB of buffer capacity. There are 4 ports per clipper, therefore this equates to ~1.5MB buffer capacity per port. With 99/1 you will see ~.9MB is allocated to VL3 latency buffer and the remainder is used by HWM for each VL(majority to VL3). When adding each VLs HWM with the LB of VL3 you'll see it equates to ~1.35MB buffer capacity.