mtu mis-match in IF config in two switches

Answered Question
Feb 22nd, 2010

This is a very very simple and basic question,

just can not find a quick ref.

I have two switches, with trunk interface connected, one interface (sw1 IF 1) configured as MTU 9000, other (sw2, IF 2) is using default MTU 1500.

Q1: should both interface know that the max MTU is 1500?

Q2: if SW1 sends a big pkt, such as 4K bytes, can IF segment it, then send or just send this big one? what action will be taken by sw2 IF 2 if receives the pkt size > 1500, drop the pkt? any statistic counter show this?

(sw1 is 6500, sw2 is 3560G in this case).

Thank you very much for your time and help.

gy

I have this problem too.
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Correct Answer by Giuseppe Larosa about 6 years 9 months ago

Hello Gongyuan,

>> should both interface know that the max MTU is 1500?

generally speaking yes it would be better

>> Q2: if SW1 sends a big pkt, such as 4K bytes, can IF segment it, then send or just send this big one? what action will be taken by sw2 IF 2 if receives the pkt size > 1500, drop the pkt? any statistic counter show this?

the device with the highest MTU doesnìt know the other side doesn't agree on MTU size. fragmentation could happen only at the IP level not below.

so the frame will be sent out as a single frame and will be dropped on low MTU side, dropped packets will be classified ad giants among input error counters in show interface.

To be noted that if the link is a L2 trunk allowing some Vlans the user traffic size will depend on the mtu value on SVI interfaces associated to permitted vlans.

so even setting mtu 9000 does not imply that so big packets will ever sent out the port.

For C3560G the global command system mtu value can be used to increase the mtu.

see

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_44_se/configuration/guide/swint.html#wp1154596

for L2 only you can use global command

sytem mtu jumbo

be aware that these commands may lead to shorter tx/rx buffers on ports.

typically using a so big MTU provides benefits only within a L2 domain for maximizing data transfer between servers.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Mon, 02/22/2010 - 10:02

Hello Gongyuan,

>> should both interface know that the max MTU is 1500?

generally speaking yes it would be better

>> Q2: if SW1 sends a big pkt, such as 4K bytes, can IF segment it, then send or just send this big one? what action will be taken by sw2 IF 2 if receives the pkt size > 1500, drop the pkt? any statistic counter show this?

the device with the highest MTU doesnìt know the other side doesn't agree on MTU size. fragmentation could happen only at the IP level not below.

so the frame will be sent out as a single frame and will be dropped on low MTU side, dropped packets will be classified ad giants among input error counters in show interface.

To be noted that if the link is a L2 trunk allowing some Vlans the user traffic size will depend on the mtu value on SVI interfaces associated to permitted vlans.

so even setting mtu 9000 does not imply that so big packets will ever sent out the port.

For C3560G the global command system mtu value can be used to increase the mtu.

see

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_44_se/configuration/guide/swint.html#wp1154596

for L2 only you can use global command

sytem mtu jumbo

be aware that these commands may lead to shorter tx/rx buffers on ports.

typically using a so big MTU provides benefits only within a L2 domain for maximizing data transfer between servers.

Hope to help

Giuseppe

Gongyuan Yao Wed, 02/24/2010 - 12:37

Thanks a lot for your great help.

I have one more question for MTU.

When we create VLAN, we can specify mtu size too. If we leave it as default, it would be 1500.

Here are three MTU configured:

VLAN mtu: 1500,

Vlan Interface mtu: 8500

Hw Interface mtu:9000

If we sent pkt size such as 3000 within vlan, not problem to receive (since not go to L3 inter-vlan switching).

If we sent pkt size such as 3000 inter-vlan (router is 6500 FWSM), pkt lost, not received by switch (3750G--SPAN monitoring). From NAM capture, we saw the pkt as 3000 and went to the port that is connected to 3750G switch.

We are wondering where this pkt went to?

What rule is that VLAN mtu is playing in this set up?

If you have any comments, that would be great help.

Thx.

Giuseppe Larosa Wed, 02/24/2010 - 13:35

Hello Gongyuan,

as also reported in another recent thread:

>>Frames sizes that can be received by the switch  CPU are limited to 1998 bytes, no matter what value was entered with the  system  mtu or system mtu jumbo commands. Although frames  that are forwarded or routed are typically not received by the CPU, in  some cases packets are sent to the CPU, such as traffic sent to control  traffic, SNMP, Telnet, or routing protocols.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_50_se/configuration/guide/swint.html#wp2028976

>>

this means that you cannot expect the 3560G to be able to answer to an icmp packet of size 3000 bytes it can forward it to another system but it cannot process it with its main cpu.

Well, I see that  you are doing a span session on the C3560G but probably the limit of 1998 might apply also to captured packets

>> Routed packets are subjected to MTU checks on the  output ports. The MTU value used for routed ports is derived from the  applied system mtu value (not the system  mtu jumbo value). That is, the routed MTU is never greater than the  system MTU for any VLAN. The routing protocols use the system MTU value  when negotiating adjacencies and the MTU of the link. For example, the  Open Shortest Path First (OSPF) protocol uses this MTU value before  setting up an adjacency with a peer router. To view the MTU value for  routed packets for a specific VLAN, use the show  platform port-asic mvid privileged EXEC command.

You should use a server connected downstream to the C3560G

The interaction of the variuos mtu options including the mtu you can set on the L2 object is not very clear

with hw port set to 9000 all other settings are possible, but if port has mtu 1500 and L2 object has mtu 4000 we cannot expect a frame so big able to go out that port.

Hope to  help

Giuseppe

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