Ethernet Switches & Routers

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New Contributor
Posts: 4
Registered: ‎08-25-2010

enable jumbo frame support per port

On my Fastiron SX 800s & 1600s, I need to enable jumbo frame support, and would like to do so per port, as the documentation recommends. The global "jumbo" command exists, and is functional, and enables jumbo frames on all ports when activated. My issue is that when in config mode on any given interface,  the "ip mtu <size>" command is not available to me, that is the system does not recognize the mtu keyword when trying to configure. Is there some other feature that needs to be enabled before "ip mtu <size>" becomes available?

Super Contributor
Posts: 1,087
Registered: ‎12-13-2009

Re: enable jumbo frame support per port

Hi Russel,

     I far as I can see,  port based IP MTU is only supported when running layer 2 code and routing code.  Do a show version to see what code you are running.  (mine is running SXL04001b.bin = SuperX base layer 3 ver 4.0.0.1b

SXR would be full layer 3,

So if you are running layer 3 then you can set jumbo on the glabal level or on a VE.

A solution might be to create a VLAN with the ports that you need for jumbo.  Assign a VE and then set VE with 'ip mtu' command.

Hope this helps.

Thanks

Michael.

New Contributor
Posts: 4
Registered: ‎08-25-2010

Re: enable jumbo frame support per port

Thanks Michael -- I am running full layer 3 firmware.

==========================================================

SW: Version 04.1.00aT3e3 Copyright (c) 1996-2008 Foundry Networks, Inc.
      Compiled on May 05 2008 at 19:59:11 labeled as SXR04100a
      (3932417 bytes) from Primary sxr04100a.bin
      BootROM: Version 04.0.00T3e5 (FEv2)
  HW: Chassis FastIron SX 800-PREM (PROM-TYPE SX-FIL3U)

==========================================================

I can indeed adjust the MTU on ve's, as you describe. I am not clear as to what this would achieve, however. How does setting the MTU on a ve affect the MTU on the ports associated with the related vlan?

I guess my understanding of MTU isn't complete. The "Foundry Fastiron Configuration Guide" isn't very helpful in this regard, as their examples for adjusting MTU are clearly using physical ports, not ve's, and makes no mention of limitations that may exist in setting mtu per port...

thanks again

Gordon

New Contributor
Posts: 4
Registered: ‎08-25-2010

Re: enable jumbo frame support per port

OK, so I got to thinking a little on this, and took a port out of switching mode, by assigning it an ip address. Voila, ip mtu can now be set. So, it seems that when a port is layer 2 (part of a vlan), it cannot have its mtu set directly, (but indirectly via a VE associated w/ the vlan its in?). When a port is a layer 3 port (not part of a vlan), it can have its mtu set directly.

In other words, I guess the per-port mtu setting only works at layer 3.

Gordon

Super Contributor
Posts: 1,087
Registered: ‎12-13-2009

Re: enable jumbo frame support per port

Hi Gordon,

     Just to sum up for everybody. They do support jumbo in layer 2 as well.  But as you stated per port is layer 3 only.

from the manual in the Configuring basic layer 2 features section

Jumbo frame support

Ethernet traffic moves in units called frames. The maximum size of frames is called the Maximum

Transmission Unit (MTU). When a network device receives a frame larger than its MTU, the data is

either fragmented or dropped. Historically, Ethernet has a maximum frame size of 1500 bytes, so

most devices use 1500 as their default MTU.

Jumbo frames are Ethernet frames with more than 1,500 bytes MTU. Conventionally, jumbo

frames can carry up to 9,000 bytes MTU. Brocade FastIron devices support Layer 2 jumbo frames

on 10/100, 100/100/1000, and 10GbE ports. Brocade devices also support jumbo frames per

VLAN.

---------------------

one note though, lokos like layer 3 jump support a higher frame size.

Changing the MTU on an individual port

By default, the maximum Ethernet MTU sizes are as follows:
      • 1500 bytes – The maximum for Ethernet II encapsulation
      • 1492 bytes – The maximum for SNAP encapsulation

When jumbo mode is enabled, the maximum Ethernet MTU sizes are as follows:
      • 10,240 bytes– The maximum for Ethernet II encapsulation
      • 10,240 bytes – The maximum for SNAP encapsulation

NOTE
If you set the MTU of a port to a value lower than the global MTU and from 576 – 1499, the port fragments the packets. However, if the port MTU is exactly 1500 and this is larger than the global MTU, the port drops the packets.

NOTE
The ip mtu command is not supported on FGS, FLS, and FWS devices.

NOTE
You must save the configuration change and then reload the software to enable jumbo support.

New Contributor
Posts: 4
Registered: ‎05-26-2011

Re: enable jumbo frame support per port

Hi,

I'm keeping this simple for now and running L2 code on my SX800.

Based on your posts and the manual, it seems as though I can only enable jumbo frames for the entire switch via the command;

jumbo

Then reloading once saved.

I am only needing a few hosts to run at jumbo frames but I assume any port is now capable of allowing jumbo frames?

Do I need to set jumbo MTUs on my switch or can I assume that it will pass frame sizes up to 9,000 depending on my hosts config?

And I take it resources will not be taxed on the enitre switch, but for only those ports passing jumbo frames?

It does not sound like I can run ip mtu <size> with L2 code, is this correct?

Thanks in advance.

New Contributor
Posts: 4
Registered: ‎08-25-2010

Re: enable jumbo frame support per port

yes, you can only enable jumbo frames globally in your case, with L2 code.

Once you turn it on, test by sending large ICMP packets using ping,  to/from your jumbo requiring endpoints.

Super Contributor
Posts: 1,087
Registered: ‎12-13-2009

Re: enable jumbo frame support per port

jumbo Then reloading once saved. - correct

I am only needing a few hosts to run at jumbo frames but I assume any port is now capable of allowing jumbo frames? -correct

Do I need to set jumbo MTUs on my switch or can I assume that it will pass frame sizes up to 9,000 depending on my hosts config? - no you do not need to set the MTU

And I take it resources will not be taxed on the enitre switch, but for only those ports passing jumbo frames? - no more resurces will be consumed for all ports - though it will not like to will see what.

It does not sound like I can run ip mtu <size> with L2 code, is this correct? - correct

Now remeber the rule for jumbo is if you enable it you should enabled it everywhere (including servers).

e.g. if a server (with jumbo's) sends a packet to another server without jumbo's enabled then the packet will get dropped or fragmented. fraging eats CPU on the switch.

Thanks

Michael.

Occasional Contributor
Posts: 7
Registered: ‎04-22-2011

Re: enable jumbo frame support per port

Dear all,

I was looking at this posting; the manual doesn't make this at all clear....

I'm running Layer 3 code - version 7.2.0.2 SXR07202a.bin - and there seems to be an 'oddness' here - lets call it a feature...

To gain access to Jumbo Frames - you need to type: Jumbo - to enable Jumbo packets on the entire switch.

You cannot just ' enable it on the VE's ' - if you try to adjust the MTU's on just the VE's, it tells you to get lost (i.e. ip mtu 9000) will tell you that the maximum size is 1500....

So - I have a VERY SPECIFIC QUESTION:

If I enable Jumbo Frames on the entire switch (which it seems I have to do to increase the MTUs on VEs), how does that effect the rest of the switch which has different VLANs and different VE's?  I ONLY NEED this ONE VE (and associated physical ports) to have large MTUs; the rest should stay on 1500MTU.

I would also like to report that the example in Page 1064 does not work:

To change the MTU for interface 1/5 to 1000, enter the following commands.
FastIron(config)# interface ethernet 1/5
FastIron(config-if-1/5)# ip mtu 1000
FastIron(config-if-1/5)# write memory
FastIron(config-if-1/5)# end
FastIron# reload

This is junk; my version of firmware does not offer ip mtu out of the back of int e x/x

Thanks

Max

New Contributor
Posts: 4
Registered: ‎05-26-2011

Re: enable jumbo frame support per port

Thanks for the info.

I was wondering that if I had mixed nodes on layer 2, some at jumbo MTU while others at standard MTU, is there a chance of either high switch CPU load or corrupt packets?

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