Addressing Dissimilar Deskew Counters in Trunk Group among GDC switches:
Affected Items: SAN Core Switches (BRCD 5300) ISL Trunk (port#0 and port#1 of switches trunked)
Summary:
Our core-SAN switches ISL-ports: it appears there is a descrepency in cable length for the fiber used to
form ISL-Trunk-Group between core switches (connecting 3rd and 4th floor core switches). Our goal is to
make sure that deskew numbers do match-up on both ISL-ports in this trunk-group.
Analysis:
In a ISL Trunk-Group, the switch automatically sets/assigns the minimum deskew value of the ISL with
the least latency (shortest round-trip time) to 15 deskew units. The deskew for the remaining ISLs is
calculated in relation to the ISL with the least latency. The deskew value is a representation of an ISL
transmission capabilities. Differences in deskew can be caused by signal degradation which affects the
transmission time of frames through the link. Differencesin deskew can also be caused by excessive
differences in cable length or line quality. Deskew values are displayed in the trunkshow output.
We notice signifiant difference in our deskew values. Also observed: significant difference between
port thruput in this trunk-group! This difference in ISL/port thruput has been observed over a period of
time.
Deskew Counter: This value is a metric used to identify the differences in cabling distances / signalquality between trunking links within a trunking group. This metric is used to ensure in-order delivery
across trunking links within the same trunking group. The higher the deskew counter value, the longer
the cable lengths. If using the same cable lengths to connect two switches, expect to see the deskew
counters for each ISL in the trunk to have very similar values.
trunkShow: Use this to display detailed trunking information. Helpful for identifying deskew constant.
switchShow: Use this command to check status of switch ports and trunking. This command is available
in all versions of Fabric OS.
FL4R11-SWF-FA: > trunkshow –perf
1: 0-> 0 10:00:00:05:1e:ee:cf:7e 2 deskew 28 MASTER  13 unit difference compared to next ISL in Trunk
1-> 1 10:00:00:05:1e:ee:cf:7e 2 deskew 15
Tx: Bandwidth 16.00Gbps, Throughput 231.11Mbps (1.68%)
Rx: Bandwidth 16.00Gbps, Throughput 210.08Mbps (1.53%)
Tx+Rx: Bandwidth 32.00Gbps, Throughput 441.19Mbps (1.61%)
A Good Example, showing almost identical deskew counters:
Run portperfshow to show current ISL/trunk thruput on switches
FL4R11-SWF-FA: > portperfshow 0-1 -t 10
0
1
Total
========================
1.6m
0
55.6m
1
57.3m
 Notice the significant difference in thruput between port#0 and port#1
Total
========================
1.8m
0
25.0m
1
26.8m
 Ditto as above
Total
========================
1.7m
0
39.3m
1
41.0m
 Ditto as above
Total
========================
4.7m
44.7m
49.4m
 Ditto as above
Appendix: