MTU 9000

[Bab L]

Hi.

We have 4 switches in a stack and some interfaces MTU is 9000.

The switch global command is not configured for 9000 MTU neither are the interfaces (which I think you can;t anyway)

So why do some interfaces show MTU 9000?

Switches are 3750.

Thank you

[InayathUlla Sharieff]

Bab,

Someone might have changed it on the switch. By default on the Cat 3750 the MTU on all the interfaces are/will be set to 1500.

Here is the thing I can think of:-

Someone might have globally enabled the command: system mtu jumbo 9000 which in turn has changed the interfaces to be 9000 mtu size.

Example:-

SW5#show int g1/0/9
GigabitEthernet1/0/9 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 442b.03ce.1b09 (bia 442b.03ce.1b09)
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

SW5(config)#system mtu jumbo 9000  << changed it manually.
Changes to the system jumbo MTU will not take effect until the next reload is do ne
SW5(config)#end
SW5#wr

Now you can see all my interfaces will be mtu 9000:-

SW5#show int g1/0/1 | in MTU
MTU 9000 bytes, BW 10000 Kbit, DLY 1000 usec,  <<< MTU is 9000.
SW5#

HTH

Regards

Inayath

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[Bab L]

Hi and thanks for the reply.

The global command is not configured and not all interface are 9000. Most of the interface are normal.

switch 1 provision ws-c3750g-12s
switch 2 provision ws-c3750g-12s
switch 3 provision ws-c3750g-24t
switch 4 provision ws-c3750g-24t
switch 5 provision ws-c3750-24ts
system mtu routing 1500

See below. Please note that Gig interface are 9000 and FA are 1500. How is this configured?

GigabitEthernet4/0/24 is down, line protocol is down (notconnect)
Hardware is Gigabit Ethernet, address is *******
Description: Trunk - ******Gi1/0/1
MTU 9000 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto-speed, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 1y10w, output 1y10w, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/0 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
315294 packets input, 72519753 bytes, 0 no buffer
Received 167915 broadcasts (166588 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 166588 multicast, 0 pause input
0 input packets with dribble condition detected
101432233 packets output, 10040182491 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out

FastEthernet5/0/1 is administratively down, line protocol is down (disabled)
Hardware is Fast Ethernet, address is ****************************
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, Auto-speed, media type is 10/100BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/0 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 1 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out

Thank you

[Bab L]

Ok. I found this:

show run | i system
System MTU size is 1500 bytes
System Jumbo MTU size is 9000 bytes
Routing MTU size is 1500 bytes

what consitutes whats 9000 and what 1500 though? Is it becasue its GB and FA?

[InayathUlla Sharieff · Accepted solution]

The Maximum Transmission Unit (MTU) is the maximum length of data that can be transmitted by a protocol in one instance. For example, the MTU of Ethernet (by default 1500) is the largest number of bytes that can be carried by an Ethernet frame (excluding the header and trailer). MTUs are found at various layers of the OSI model, and can often be tweaked to more efficiently transport large volumes of data.

Ethernet

The default Ethernet MTU is 1500 bytes, not including the header or trailer. Sometimes a slightly higher MTU is preferable to accommodate Q-in-Q tunneling or other encapsulation. The MTU can be raised on Cisco IOS with the system mtu command under global configuration:

Switch(config)# system mtu ?
  <1500-1998>  MTU size in bytes
  jumbo        Set Jumbo MTU value for GigabitEthernet or TenGigabitEthernet
           interfaces

The maximum MTU is dependent on the hardware platform, but the IEEE 802.3 standards require a minimum MTU of 1500 bytes. Additionally, a jumbo MTU for 1 Gbps and 10 Gbps interfaces can be allowed up to 9000 bytes. Changing either of these values will require a device power cycle.

Switch(config)# system mtu 1508
Changes to the system MTU will not take effect until the next reload is done
Switch(config)# system mtu jumbo 9000
Changes to the system jumbo MTU will not take effect until the next reload is done
Switch# show system mtu

System MTU size is 1500 bytes
On next reload, System MTU will be 1508 bytes

System Jumbo MTU size is 1500 bytes
On next reload, System Jumbo MTU will be 9000 bytes

IP

As with Ethernet frames, the MTU can be adjusted for IP packets. However, the IP MTU is configured per interface rather than system-wide, with the ip mtu command:

Router(config)# interface f0/0
Router(config-if)# ip mtu ?
  <68-1500>  MTU (bytes)

Notice that the maximum IP MTU is capped at the Ethernet MTU, because it is being applied to an Ethernet interface. The configured IP MTU determines how large a packet to be transmitted out the interface may be. IP packets larger than the MTU are discarded, and may prompt the router to send a Fragmentation Needed ICMP packet back to the source to facilitate path MTU discovery.

It's also worth noting that while the Ethernet and IP MTUs effectively refer to the same section of an IP/Ethernet packet, they can be configured independently. For example, assume we want to shrink the IP MTU of an interface to 1200 bytes:

Router(config)# interface f0/0
Router(config-if)# ip mtu 1200

The IP MTU has been modified from its default of 1500:

Router# show ip interface f0/0
FastEthernet0/0 is up, line protocol is up
  Internet address is 10.0.0.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1200 bytes
...

However, the interface's Ethernet MTU remains unchanged:

Router# show interface f0/0
FastEthernet0/0 is up, line protocol is up 
  Hardware is Gt96k FE, address is c200.5867.0000 (bia c200.5867.0000)
  Internet address is 10.0.0.1/24
  MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, 
 reliability 255/255, txload 1/255, rxload 1/255

TCP

There are two contexts in which the TCP Maximum Segment Size (MSS) can be configured: transient traffic and terminating traffic.

Transient Traffic

When a TCP client initiates a connection to a server, it includes its MSS as an option in the first (SYN) packet. On an Ethernet interface, this value is typically 1460 (1500 byte Ethernet MTU - 20 byte IP header - 20 byte TCP header).

HTH

Regards

Inayath

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