3560CX jumbo frames

[kevingrss]

I need to enable jumbo frames on some 3560CX-12PD-S compact switches running 15.2(4)E6. The documented procedure is:

system mtu jumbo 9198

write

reload

show system mtu

System MTU size is 1500 bytes
System Jumbo MTU size is 9198 bytes
System Alternate MTU size is 1500 bytes
Routing MTU size is 1500 bytes

That looks good but it isn't actually working because the jumbo frames aren't getting through the switch and it is admitting so:

sh int gi 1/0/10 controller

...

Transmit GigabitEthernet1/0/10 Receive
107749004 Bytes 2435282491 Bytes
36257 Unicast frames 32800 Unicast frames
44461 Multicast frames 12779539 Multicast frames
5912 Broadcast frames 12 Broadcast frames
0 Too old frames 30865976 Unicast bytes
0 Deferred frames 2404415747 Multicast bytes
0 MTU exceeded frames 768 Broadcast bytes
0 1 collision frames 0 Alignment errors
0 2 collision frames 0 FCS errors
0 3 collision frames 0 Oversize frames
0 4 collision frames 0 Undersize frames
0 5 collision frames 0 Collision fragments
0 6 collision frames
0 7 collision frames 131041 Minimum size frames
0 8 collision frames 9877 65 to 127 byte frames
0 9 collision frames 2295 128 to 255 byte frames
0 10 collision frames 472 256 to 511 byte frames
0 11 collision frames 542 512 to 1023 byte frames
0 12 collision frames 19729 1024 to 1518 byte frames
0 13 collision frames 0 Overrun frames
0 14 collision frames 0 Pause frames
0 15 collision frames
0 Excessive collisions 0 Symbol error frames
0 Late collisions 0 Invalid frames, too large
0 VLAN discard frames 12648395 Valid frames, too large
0 Excess defer frames 0 Invalid frames, too small
5392 64 byte frames 0 Valid frames, too small
45886 127 byte frames
18141 255 byte frames 0 Too old frames
1579 511 byte frames 0 Valid oversize frames
2620 1023 byte frames 0 System FCS error frames
14 1518 byte frames 0 RxPortFifoFull drop frame
12998 Too large frames
0 Good (1 coll) frames
0 Good (>1 coll) frames

I know the software is a bit old but so is the switch model and I don't see anything MTU related in the release notes. I will definitely try a software update if there aren't any better ideas.

[balaji.bandi]

May be i am wrong, your system MTU still 1500, how your jumbo frames can up to 9 K

you need to set up MTU also same right?

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960cx_3650cx/software/release/15-2_4_e/configurationguide/b_1524e_consolidated_3560cx_2960cx_cg/b_1524e_consolidated_3560cx_2960cx_cg_chapter_0110.html

[kevingrss]

The CLI and Cisco documentation indicates that "System Jumbo MTU" is what applies to Gigabit and TenGigabit ports which is what I'm dealing with on the 3560CX-12PD-S. It's not perfectly clear but, by elimination, I assume "System MTU" applies to 10/100 ports. In any case, the range of allowable values for system MTU is restricted and won't go as big as I need.

Switch(config)#sys mtu ?

<1500-1998> MTU size in bytes
jumbo Set Jumbo MTU value for GigabitEthernet or TenGigabitEthernet interfaces
routing Set the Routing MTU for the system

This switch is configured for Layer-2 bridging so routing MTU should not come into play.

[Reza Sharifi]

"system mtu jumbo 9198" is the correct command to use.

see this post:

https://community.cisco.com/t5/switching/quot-too-large-frames-quot-what-does-this-mean-for-performance/td-p/1515600

HTH

[kevingrss]

"system mtu jumbo 9198" is the configuration I'm using and it is not letting my jumbo frames through - marking them all as too large.

The post you reference applies to 3560/3750 which is significantly different than the 3560CX I'm working with.

[Reza Sharifi]

For almost all the small switches (3560s, 3650s, 3750s, 3850, etc..) the configuration for the jumbo frame is done globally using that same command. You can configure it per interface on larger switches (4500, 6500, 6800, etc..).

HTH

[kevingrss]

Yes, and remember to reload after changing the configuration.

Usually "system mtu <size>" applies to all ports but the 3560CX is a bit more complicated. I'm still looking for a solution.

[kevingrss]

dup

[kevingrss · Accepted solution]

Hi all. I've resolved this issue.

The switch does support jumbo frames and "system mtu jumbo 9198" is the correct statement to enable jumbo frame support.

What the "Valid frames, too large" counter is trying to tell you is that the frame is too large to be accounted for in any of the normal size categories; It is too large for any of those. But they are valid frames so this is basically a count of forwarded jumbo frames. I assume unforwarded jumbo frames would be counted separately as "Too large frames".

One other thing to be aware of is that the "5 minute output rate" and "5 minute input rate" reports depend on this size classification to work accurately. Since packets larger than 1500 octets are all lumped together as "Too large frames", when jumbo frames are present, the "5 minute output rate" and "5 minute input rate" can be reported as much as 6 times lower than actual.