Cisco 6500 MTU on FastEthernet port

Answered Question
Jun 28th, 2010

I had a question, why am I able to configure the MTU on a FastEthernet between 9216-9216?  Is the MTU parameter under the copper interface meaning something else?  Usually its between 1480 and 1524.

show run int fast 4/31
switchport
speed 100
duplex full
end

int fast 4/31
mtu ?
<9216-9216>  MTU size in bytes

show int fast 4/31 status
Port    Name               Status       Vlan       Duplex  Speed Type
Fa4/31                     notconnect   1             full    100 10/100BaseTX

-Manny

I have this problem too.
0 votes
Correct Answer by taali about 6 years 6 months ago

Hi Manny

It appears that the mtu size command is in reference to egress traffic on a FastEthernet port.

See the config guide for 12.2(SXH)

Configuring a nondefault MTU size on a 10, 10/100,  or 100 Mbps Ethernet port limits ingress packets to the global LAN port  MTU size and permits egress traffic of any size larger than 64 bytes.

http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst6500/ios/12.2SX//configuration/guide/intrface.html#wp1059156

HTH

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Manny,

according to http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#c1

the MTU describes the           largest physical packet size, measured in bytes, that a network can transmit.           Any messages larger than the MTU are divided into smaller packets before           transmission.

Smaller frames (while flowing through a network device) usually create more CPU interrupts and more processing overhead. In other words a higher MTU size (especially on GigE interfaces) means (generally spoken) a higher throughput.

The important thing is to have the higher MTU size configured through the whole network or fragmentation will take place.

--

Uwe


taali Thu, 07/01/2010 - 15:18

Manny

Certain line cards on the 6500 support jumbo frames.  From the cli output, you have one of those line cards and that's why you are seeing 9216 as an option for MTU size.

HTH

fibernet570 Fri, 07/02/2010 - 10:56

Thanks for the link however, the linecard WS-X6548-RJ-45 allows MTU of only 9216 and not "1500-9216".  Can you really push over 9K MTU over FastEthernet?  If I'm misunderstanding the reasoning behind allowing jumbo MTU on FastE's please elaborate.

-Manny

Correct Answer
taali Fri, 07/02/2010 - 13:32

Hi Manny

It appears that the mtu size command is in reference to egress traffic on a FastEthernet port.

See the config guide for 12.2(SXH)

Configuring a nondefault MTU size on a 10, 10/100,  or 100 Mbps Ethernet port limits ingress packets to the global LAN port  MTU size and permits egress traffic of any size larger than 64 bytes.

http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst6500/ios/12.2SX//configuration/guide/intrface.html#wp1059156

HTH

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