3560 4507 MTU

[Brendan Marmont]

I'd like to enable Jumbo frames. We have devices 4507 and 3560.

4507 - Set the MTU on interface level. ok with this.

3560 - cisco documentation specifies "Note: If Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces, jumbo frames that ingress on a Gigabit Ethernet interface and egress on a 10/100 interface are dropped." - what does this mean exactly?

If client (B) adapters set at 1500, will the transaction be setup 1500 end to end negotiating lowest packet? or If a server is set at 9000 and a conversation begins with client B, will the packets be dropped when it hit the egress interface?

I guess my query is, whats the affect of having a mixed environment, are we to go through every end point and set max mtu?

Thanks

[Joseph W. Doherty]

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Posting

I think what the warning is trying to describe, at L2, if max MTUs differ between ports (same device or different interconnected devices) , frames will be dropped.  At L3, on same device, they can be fragmented.

In a mixed environment, you want to insure, all L2 supports the same max MTU.

[Brendan Marmont]

Thanks for your reply.

If I enable 'system mtu jumbo 9000' on the 3560G, all GB interfaces globally will use this setting.

I connect remotely via a in-path steelhead with MTU 1500, If I apply the command to the 3560 it reloads the switch, what issues if any will arise?

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Are you asking about the impact of reloading the switch or the situation where the switch supports jumbo, but an "in-line" (e.g. you Steelhead) does not?  Assuming the latter, it should work fine until a frame is sent to the Steelhead bigger than 1500.  I would expect the Steelhead to drop such frames.

As I mentioned in my first post, when you enable jumbo, you want to insure all ports, at L2, end-to-end, support it.  Fragmentation is only supported at L3 hops.

Something else to keep in mind, there's no real standard for jumbo, so one port might support 9000 and the other side 9200.  This will be a problem if any frames, from the 9200 side, are sent larger than 9000.

As TCP will agree with the other side on MSS, and MSS is normally kept within a frame, you can insure this will avoid problems as long as one end host's MTU doesn't exceed MTU from itself along the L2 path, until the its first L3 hop.