Adding jumbo frame support into an existing Ethernet network

[brad-denham]

I have a remote site with 40 users that connect back to our main site via two point to point T-1’s. These users connect to an Exchange server (DMZ), Sybase databases on Sun servers (Internal network), and access the Internet via the main site.

I have installed a new WS-2970G-24TS-E switch into the network at this remote site. I have connected all of the designers (total of 10) Apple G5 workstations and two Apple Xserve’s to this switch. I have also configured the “system jumbo MTU” to 9000 bytes on the 2970. I have not yet enabled jumbo frames on the Xserve’s or the G5’s since I am unsure of what the effect on the network might be. I imagine it could range from dropped packets to crashing the router.

I would like to enable jumbo frame support on these devices since they transfer hundreds of gigabytes of data on their local network. But if I do this, what will be the affect when they attempt to visit web sites or connect to the Exchange server?

How have you guys worked with this type of scenario?

The address space for this site is a /25 of one of our class C’s.

Please see the attached Visio diagram that outlines the connection points throughout the network.

Thank you

[c.lemaire]

Brad,

Here is what I found on CCO :

"If Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces, jumbo frames received on a Gigabit Ethernet interface and sent on a 10/100 interface are dropped."

So as your uplink towards the router is configure in 100M, you will have lots of drops.

HTH

Regards,

Christophe

[brad-denham]

Hi Christophe,

I had read the same thing and I don't doubt that at all. What I was hoping to get from my post here is how some of you guys have worked around this challenge.

Thank you

[Marvin Rhoads]

TCP has a Path MTU Discovery process that should learn the supported MTU size between end stations and set that for communications between those addresses. see http://www.cisco.com/warp/public/105/pmtud_ipfrag.html for more information.

Hope this helps. Please rate helpful posts.

[bhedlund]

Best practice here is to segment your Jumbo Frame servers on their own VLANs for Jumbo supported systems only.

As a post here has already mentioned, Path MTU discovery will tell the systems on the Jumbo VLANs to keep the frames under 1500 when talking to a non-Jumbo VLAN.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2970/12225see/scg/swint.htm#wp1154596

Please rate all helpful posts.

Brad