Jumbo frame Question

George Bonser gbonser at seven.com
Thu Nov 25 23:14:02 CST 2010

> Hi
> Does anyone have experience on design / implementing the Jumbo frame
> enabled network?
> I am working on a project to better utilize a fiber link across east
> coast
> and west coast with the Juniper devices.
> Based on the default TCP windows in Linux / Windows and the latency
> between
> east coast and west coast (~80ms) and the default MTU size 1500, the
> maximum throughput of a single TCP session is around ~3Mbps but it is
> too
> slow for us to backing-up the huge amount of data across 2 sites.

There are a lot of stack tweaks you can make but the real answer is
larger MTU sizes in addition to those tweaks.  Our network is completely
9000 MTU internally. We don't deploy any servers anymore with MTU 1500.
MTU 1500 is just plain stupid with any network >100mb ethernet.

> The following is the topology that we are using right now.
> Host A NIC (MTU 9000) <--- GigLAN ---> (MTU 9216) Juniper EX4200 (MTU
> 9216)
> <---GigLAN ---> (MTU 9018) J-6350 cluster A (MTU 9018) <--- fiber link
> across site ---> (MTU 9018) J-6350 cluster B (MTU 9018) <--- GigLAN
> >
> (MTU 9216) Juniper EX4200 (MTU 9216) <---GigLAN ---> (MTU 9000) NIC -
> Host
> B
> I was trying to test the connectivity from Host A to the J-6350
> A
> by using ICMP-Ping with size 8000 and DF bit set but it was failed to
> ping.
> Does anyone have experience on it? please advise.
> Thanks :-)

You might have some transport in the path (SONET?) that can't send 8000.
I would try starting at 3000 and working up to find where your limit is.

Your description of "fiber link across site" is vague. Who is the
vendor, what kind of service?  

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