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3 Posts
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23218
November 13th, 2015 09:00
Is the Compellent rejects fragmented packets?
Hi
i'm trying to figure if it's funny issue on ours SC8000 6.6 OS or by design (and if so, why?).
Sending packets (ping -l) bigger than the MTU that configured on the server is returning a timeout.
regardless if i'm setting it to any 9014/4088/1650. these servers have no problem to ping each other in any scenario.
i crossed the example in pages 22-23 here http://en.community.dell.com/cfs-file/__key/telligent-evolution-components-attachments/13-4491-00-00-20-43-79-24/Windows_5F00_Server_5F00_2012_5F00_R2_5F00_Best_5F00_Practices_5F00_v2.pdf
It was nice to find. but the phrases there not give me defiant conclusion if when not using the -f parameter the fragmentation should work or not.
these are my result's when configured to 9014:
non-fragmented bigger than server MTU to Compellent - failed (expected)
C:\>ping 10.1.158.13 -f -l 10000
Pinging 10.1.158.13 with 10000 bytes of data:
Packet needs to be fragmented but DF set.
non-fragmented smaller than server MTU to Compellent - works (expected)
C:\>ping 10.1.158.13 -f -l 8968
Pinging 10.1.158.13 with 8968 bytes of data:
Reply from 10.1.158.13: bytes=8968 time<1ms TTL=64
fragmented to Compellent - failed (is that expected???)
C:\>ping 10.1.158.13 -l 10000
Pinging 10.1.158.13 with 10000 bytes of data:
Request timed out.
fragmented to another server - works (expected)
C:\>ping 10.1.158.26 -l 10000
Pinging 10.1.158.26 with 10000 bytes of data:
Reply from 10.1.158.26: bytes=10000 time=2ms TTL=128
Thanks
Gidi
Gidi M
3 Posts
1
April 21st, 2016 11:00
Hi
So. It's always nice to come back to the forum with the answer.and not leave the next one crossing it in the blank. so here it is.
YES. the Compellent not going to answer to fragmented ping. but we had a different behavior that we couldn't explain, even with the -f flag. instead of getting the response that the packet need to be fragmented, we got the ping timeout for packets size between 8968-8972:
C:\>ping 10.1.158.13 -f -l 8969
Pinging 10.1.158.13 with 8969 bytes of data
Request timed out.
So we saw this as a black-hole.we we're really concerned when finding this out, but when taking packet trace i found something interesting. on any ping reply above 62Bytes the Compellent also return four Bytes of garbage at the end (well it's look like it's putting some more of the abc after the frame)
We tested it on another system that has different configuration, on this one we had no black hole. another quick packet trace (down at the bottom), we saw that the server does receive up to1918 bytes frames (the card configured for 9014 but still accept higher).
so - two issues, both seems to be with pings-to-the Compellent
1. Compellent adds four bytes garbage to pings reply's over 62 Bytes packets.
2. Compellent allow pings reply over 9014Bytes that are dropped by our NIC (buts seems to be only for ping).
because we scooped it pretty well only to ICMP. i haven't continue to try and captured anywhere else on the wire.
Our configuration:
SC8000 Compellent OS 6.6.5.19, Chelsio T320 Dual Port LP Vlan tagged, connected to Nexus 5k. cos policy for all the ports mtu 9216 connected to intel X520 card configured with 9014 MTU.
.
Mellanox card on another system that does receive the over 9014bytes frame, BTW that system also not receives answers to fragmented pings)
Gidi M
3 Posts
0
December 4th, 2015 18:00
can someone try to reproduce on his system?