Layer 2 MTU.

Cisco Freak · ‎02-01-2015

Hi Guys,

I have a query regarding Layer 2 MTU in switch.

SW-3750#sh int fa1/0/1
FastEthernet1/0/1 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0015.6264.3303 (bia 0015.6264.3303)
MTU 1504 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255

SW-3750#sh ip int fa1/0/1
FastEthernet1/0/1 is up, line protocol is up
Inbound access list is not set
SW-3750#

This interface has MTU value 1504. My query is, does this value include the L2 header as well.

CF

Jon Marshall · ‎02-01-2015

No it doesn't.

The MTU you see under the interface configuration does not include either the frame header or the trailer.

So what that is saying is that there are 1504 bytes to carry the payload of the packet plus the IP TCP/UDP and IP headers.

Jon

Cisco Freak · ‎02-01-2015

I am trying to ping across 2 switches with different MTU values:

vacl-2#p 1.1.1.1 si 1500

Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/6/9 ms
vacl-2#p 1.1.1.1 si 1501

Type escape sequence to abort.
Sending 5, 1501-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

vacl-2#show system mtu

System MTU size is 1600 bytes
System Jumbo MTU size is 1504 bytes
System Alternate MTU size is 1600 bytes
Routing MTU size is 1504 bytes
vacl-2#

vacl-2#sh int vlan 1 | i MTU
MTU 1600 bytes, BW 1000000 Kbit, DLY 10 usec,
vacl-2#

Destination:

vacl-1#sh int vlan 1 | i MTU|add
Hardware is EtherSVI, address is 0015.6264.3340 (bia 0015.6264.3340)
Internet address is 1.1.1.1/8
MTU 1998 bytes, BW 1000000 Kbit, DLY 10 usec,
vacl-1#

It is pinging when size is 1500 and when the packet size is increased to 1501, its not pinging. Can you please tell me why this is so?

CF

Jon Marshall · ‎02-01-2015

If that was the real question then why not post it originally :-)

Can you provide "sh int <physical interface>" for both the ports used to connected the switches together.

Note the frame header and CRC add another 18 bytes to the frame. So you know from your ping test that the MTU reported in your first post must be the payload only because if it included the frame header and CRC then it would be -

1504 - 18 = 1486

so your ping with a size of 1500 would never have worked.

Jon

bernardo81 · ‎10-06-2015

Hi John,

you seem to have very good knowledge about this confusing topic.

Can you please advise a document that explains this MTU (Layer 2 and layer 3 MTU ) from scratch in detailed and easy way ? (with drawing if possible)

Like how to test layer2 MTU , layer 3 MTU ...

Thank you in advance

David_Che · ‎02-02-2015

fa1/0/1 can accept packet size from vlan interface 1 is 1504 : 4 bytes 802.1Q + 1500 byes (IP header + IP payload)

so you can ping 1500 bytes, however when you try to ping 1501, it will be 1505 bytes to f1/0/1 to handle, it is obvious 1505 > 1504, this packet need to e fragmented, as you know, switch did not have capability to fragment and reassemble, it will discard the packet.

Although vlan interface 1's MTU = 1600, it is just logical mtu value, it still depends on f1/0/1 to handle.

Hope it make sense.

Cisco Freak · ‎02-02-2015

Are you sure that the 4 bytes 802.1Q is also considered as part of mtu value. I think its not.

CF

David_Che · ‎02-02-2015

I am sure what I say.

Cisco Freak · ‎02-03-2015

Hi David,

I have one more question. I have connected two switches using an access port and trying to ping each other. Since this is access port, there is no VLAN tagging involved.

sw8#show system mtu

System MTU size is 1600 bytes
System Jumbo MTU size is 9000 bytes
System Alternate MTU size is 1600 bytes
Routing MTU size is 1500 bytes
sw8#

sw8#sh int fa1/0/43 | i MTU
MTU 1600 bytes, BW 100000 Kbit, DLY 100 usec,
sw8#

SW3#sh system mtu

System MTU size is 1600 bytes
System Jumbo MTU size is 1504 bytes
System Alternate MTU size is 1600 bytes
Routing MTU size is 1600 bytes
SW3#

SW3#sh int fa0/43 | i MTU
MTU 1600 bytes, BW 100000 Kbit, DLY 100 usec,
SW3#

sw8#ping 1.1.1.3 si 1500

Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 1.1.1.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/6/9 ms
sw8#

sw8#ping 1.1.1.3 si 1501

Type escape sequence to abort.
Sending 5, 1501-byte ICMP Echos to 1.1.1.3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

Still its not pinging with size 1501. And the interesting thing is the sending switch is trying fragment the packet:

*Mar 1 10:55:20.097: IP: s=1.1.1.8 (local), d=1.1.1.3 (Vlan1), len 1501, sending
*Mar 1 10:55:20.097: ICMP type=8, code=0
*Mar 1 10:55:20.097: IP: s=1.1.1.8 (local), d=1.1.1.3 (Vlan1), len 1501, output feature
*Mar 1 10:55:20.097: ICMP type=8, code=0, Check hwidb(72), rtype 1, forus FALSE, sendself FALSE, mtu 0, fwdchk FALSE
*Mar 1 10:55:20.097: IP: s=1.1.1.8 (local), d=1.1.1.3 (Vlan1), len 1500, sending fragment
*Mar 1 10:55:20.097: ICMP type=8, code=0
*Mar 1 10:55:20.097: IP: s=1.1.1.8 (local), d=1.1.1.3 (Vlan1), len 21, sending last fragment
*Mar 1 10:55:20.097: IP: s=1.1.1.8 (local), d=1.1.1.3 (Vlan1), len 21, sending last fragment

CF

David_Che · ‎02-03-2015

Really interesting , Can you capture the ping packets both on sw8 and sw3?

Cisco Freak · ‎02-03-2015

I wish if I could do a packet capture by port spanning. But I am practicing this in a remote lab where I don't have access to do a packet capture.

Is there any other way we can do a packet capture?

I am totally confused with this topic.

CF

David_Che · ‎02-03-2015

Do you reload the switch to make the MTU change take effect?

3750(config)# system mtu 1600

3750(config)# exit

after 'wr memory'

3750# reload

Cisco Freak · ‎02-03-2015

I did reload the switch. otherwise, the 'sh system mtu' command itself will show you that its need a reboot to accept the 1600 mtu value.

Check this:

SW3(config)#system mtu 1700
Changes to the system MTU will not take effect until the next reload is done
SW3(config)#end
SW3#show sy
SW3#show system mtu
SW3#show system mtu

System MTU size is 1600 bytes
On next reload, System MTU will be 1700 bytes
System Jumbo MTU size is 1504 bytes
System Alternate MTU size is 1600 bytes
Routing MTU size is 1600 bytes

CF

Madhukrishnan Gopinathan Nair · ‎02-03-2015

Hi CF,

Why is the switch fragmenting them? What is the MTU for the vlan interfaces.

Also what model of switches are they?

Please provide sh int <> from bothe side and also sh int vlan <> from both side

Thanks,

M