Jumbo frames on ProCurve 5400zl

[BOVC]

Hi,

Can anyone tell whether or not I should enable jumbo frames on my server VLAN? There are both devices that support jumbo frames and devices that don't.
Can the switch change the pacet size when sending to devices that do not support Jumbo Frames or are on 100Mb ports?

Thanks in advance,
Rasmus

[Sergej Gurenko]

AFAIK if the switch does not support Jumbo Frames it will just silently drop oversized frame. That's it. No ICMP message, no fragmentation. Just a black hole.

[Sergej Gurenko]

Just remembered,

Hosts can still detect MTU restrictions if EnablePMTUBHDetect is enabled (on the host level)

[Kevin Richter_1 · Accepted solution]

The answer to this one appears in the Management and Configuration Guide fo the 5400zl series ProCurve switches at ftp://ftp.hp.com/pub/networking/software/6200-5400-3500-Mgmt-and-Config-Jan-06.pdf

On pages 13-33 and 13-34 (323 & 324 of 552 in the current version of the .pdf) there is a heading titled "Outbound Jumbo Traffic" which is part of the section on "Operating Notes for Jumbo Traffic-Handling." Depending upon your configuration, Jumbo packets can be forwarded by the 5400zl to ports where non-Jumbo enabled devices are connected. The 5400zl does not fragment the Jumbo packet down into 1518/1522 byte packets so any downstream device receiving the Jumbo packets will discard the packets as giants unless it is capable of and configured to receive Jumbo packets.

Ports operating at 100Mbps do not support Jumbo packets. Inbound Jumbo packets will be dropped and the Giants counter incremented (with an associated message in the Event Log). Only ports operating at 1 Gbps or higher can transmit outbound Jumbo packets.

Enabling Jumbo frames on your server VLAN with devices which don't support Jumbos is not recommended.

[Matt Hobbs]

I think the 9300/9400 series have the ability to fragment jumbo frames down to the standard MTU if required.