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This office has a network consisting of Cisco Catalyst and Juniper EX L3 switch. Cisco Router and Juniper L3 switch share a OSPF area in between them.

Hosts on Juniper side are running FTP servers and are keep running into packet loss that causes extra overhead to resend lost packets. Not sure what the problem is.

I thought MTU difference between the switches might be causing the frames to be lost. But I've heard MTU size difference is minor and can be ignored, but is this practical?

Did anyone get around adjusting MTU size for the inter-vendor switches? Or am I just imagining this problem?

One of output from Cisco to Juniper:

980 bytes from icmp_seq=0 ttl=255 time=11.49 ms
980 bytes from icmp_seq=1 ttl=255 time=3.765 ms

MTU on both sides are 1500.


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Did anyone get around adjusting MTU size for the inter-vendor switches? Or am I just imagining this problem? - Did you check the MTU on both switches? - One of output from Cisco to Juniper - Output of what? – joeqwerty Jun 6 '14 at 16:51
You're asking how to solve something that you don't even know exists. Go check it first then come back and report to us whether or not the MTU is actually different between the two. – joeqwerty Jun 6 '14 at 18:23
I think you should put a little more effort before asking this type of question. You are asking for help but haven't even checked the configuration of both switches. If you want a concise and good quality answer, the question needs to meet the same level of quality. – Alex Jun 6 '14 at 18:23
Q: "How do I solve a problem I don't know if I have?" A: "Go find out if you actually have the problem." – joeqwerty Jun 6 '14 at 18:24
Edited the question. Sorry. – SndLt Jun 6 '14 at 20:01
up vote 2 down vote accepted

I don't think there is an MTU issue because OSPF adjacency wouldn't be established if MTU doesn't match. Also, MTU mismatch doesn't cause packet loss on Ethernet. Show interface to check if there is any uncommon statistics.

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I see. Thank you. "MTU mismatch doesn't cause packet loss on Ethernet" is something I just learned. – SndLt Jun 6 '14 at 20:06
Unless DF-bit is set, MTU mismatch just causes fragments and low performance. If DF is set, oversized packets will be dropped. However, DF-bit is rarely used. – Minggang Jun 6 '14 at 20:11

TCP applications will perform path mtu discovery and find the lowest mtu in the path. Except for network guys trying to find the highest MTU there isn't an application I'm aware of that actually uses the DF'but, mostly because of PMTUD.

Also, are you sure the MTU's are actually different? Juniper devices list the MTU on a trunked interface as 1536 to account for the 802.1q header. Cisco devices increase the MTU internally but it does not appear in the command outputs last I checked. Unless someone has gone in and changed the defaults they are the same.

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