Verifying That The Host Is Configured For High Mtu Or Mss With The Cli - Cisco 9134 - MDS Multilayer Fabric Switch Troubleshooting Manual

Chapter 20 Troubleshooting IP Storage Services
S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m

Verifying that the Host Is Configured for High MTU or MSS with the CLI

To get the real benefit of an increased MTU and higher Fibre Channel frame size, the path between the
iSCSI client and the IPS iSCSI interface (as well as the host NIC) has to be capable of supporting this
high MTU.
If you do not have access to the host, one way to see if the host is also configured for high MTU/MSS
(as well as the path in the middle) is to check the split packets field in the show ips stats tcp display.
However this is a generic display for all TCP sessions. That is, if you have some hosts with high
MTU-capable NICs, and some others without, it may be difficult to assess which is which. (See
Example
Example 20-5 Sample Output for Low Packet Split Count
MDS_Top# show ips stats tcp interface gigabitethernet 2/1 detail (truncated output)
TCP Statistics for port GigabitEthernet2/1
...
Afterward, traffic starts flowing from the FC storage towards the server that is connected via iSCSI to
the IPS.
Example 20-6 Sample Output for Large Packet Split Count
MDS_Top# show ips stats tcp interface gigabitethernet 2/1 detail
TCP Statistics for port GigabitEthernet2/1
...
OL-9285-05
20-5.)
TCP send stats
10 segments, 240 bytes
5 data, 5 ack only packets
0 control (SYN/FIN/RST), 0 probes, 0 window updates
0 segments retransmitted, 0 bytes
0 retransmitted while on ethernet send queue, 0 packets split
TCP Active Connections
Local Address
10.48.69.250:3260
10.48.69.250:3260
0.0.0.0:3260
TCP send stats
715535 segments, 943511612 bytes
712704 data, 2831 ack only packets
0 control (SYN/FIN/RST), 0 probes, 0 window updates
0 segments retransmitted, 0 bytes
0 retransmitted while on ethernet send queue, 345477 packets split
Remote Address State
10.48.69.233:1026 ESTABLISH
10.48.69.233:1040 ESTABLISH
0.0.0.0:0 LISTEN
Cisco MDS 9000 Family Troubleshooting Guide, Release 3.x
iSCSI TCP Performance Issues
Send-Q Recv-Q
0 0
0 0
0 0
20-57