From: Richard Gallagher (rgallagh@cisco.com)
Date: Thu Jun 16 2005 - 01:46:40 GMT-3
Check the following outputs:
- By default the protocol MTU (ip, mpls etc) is set the interface MTU:
FRswitch#
FRswitch#sh ip int s1 | i MTU
MTU is 1500 bytes
FRswitch#sh int s1 | i MTU
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
FRswitch#sh run int s1
Building configuration...
Current configuration : 101 bytes
!
interface Serial1
ip address 10.0.0.1 255.255.255.0
encapsulation frame-relay
no keepalive
end
- Now if we change the interface MTU, the protocol MTU should reflect
the same:
FRswitch#conf t
Enter configuration commands, one per line. End with CNTL/Z.
FRswitch(config)#int s1
FRswitch(config-if)#mtu 1400
FRswitch(config-if)#
FRswitch#sh run int s1
Building configuration...
3d22h: %SYS-5-CONFIG_I: Configured from console by console
Current configuration : 111 bytes
!
interface Serial1
mtu 1400
ip address 10.0.0.1 255.255.255.0
encapsulation frame-relay
no keepalive
end
FRswitch#sh int s1 | i MTU
MTU 1400 bytes, BW 1544 Kbit, DLY 20000 usec,
FRswitch#sh ip int s1 | i MTU
MTU is 1400 bytes
- But it's also possible to set the MTU for IP to a smaller size than
the interface MTU. Therefore all IP packets would be fragmented to 1300,
but IPX or MPLS packets would still use 1400 as thier max MTU
FRswitch#conf t
Enter configuration commands, one per line. End with CNTL/Z.
FRswitch(config)#int s1
FRswitch(config-if)#ip mtu 1300
FRswitch(config-if)#
FRswitch#sh int s1 | i MTU
MTU 1400 bytes, BW 1544 Kbit, DLY 20000 usec,
FRswitch#sh ip int s1 | i MTU
MTU is 1300 bytes
FRswitch#
- This make some more sense???
HTH, Rich
* Rich Gallagher - Cisco Systems TAC
* CCIE #7211 - R&S, C&S
John Matus wrote:
> ok, i'm going to have to plead ignorance on this. you say there is
> interface mtu and protocol mtu<?>. i've configured mtu on an
> interface but withe the "ip mtu" command.........so it sounds like
> that would be protocol mtu.......but then what would be the command
> for interface mtu???
>
>
> Regards,
>
> John D. Matus
> MCSE, CCNP
> Office: 818-782-2061
> Cell: 818-430-8372
> jmatus@pacbell.net
> ----- Original Message ----- From: "Richard Gallagher"
> <rgallagh@cisco.com>
> To: "ccie2be" <ccie2be@nyc.rr.com>; "Group Study"
> <ccielab@groupstudy.com>
> Sent: Wednesday, June 15, 2005 6:19 AM
> Subject: Re: system mtu
>
>
>> Cisco routers don't have a configurable "system mtu", this option
>> appears to be avaible only on certain switches eg 2950, 35xx
>> etc....on these platforms interfaces are generally of the same type
>> and therefore thereshould be no need for different mtu, added to that
>> you generally can't configure the interface mtu on these platforms.
>> Try this on a 3550 and you will see.
>>
>> Most routers just have a configurable interface mtu and then protocol
>> mtu (ip, mlps, ipx etc....), so if in your example a 3k byte packet
>> came in on an interface with a 10k and havd to be forwarded out of
>> interface with 1.5k then the router will fragment the packet.
>>
>> You'll should be able to see this if you enable "debug ip packet
>> detail" on a lab router - of course unless the DF bit is set :)
>>
>> HTH, Rich
>>
>> ccie2be wrote:
>>
>>> John,
>>>
>>> Suppose a router had a bunch of serial interfaces with an MTU of
>>> 10,000 and
>>> an ethernet interface with an MTU of 1500.
>>>
>>> And, let's say the system MTU was set to 10,000 and the eth
>>> interface MTU
>>> was set to 1500.
>>>
>>> Now, the router wants to send a packet of size 3000 out the ethernet
>>> interface.
>>>
>>> Based on the system MTU, no fragmentation is needed. What does the
>>> router
>>> do?
>>>
>>> Since it's not possible to send a 3000 byte packet out an ethernet
>>> interface
>>> with a max MTU of 1500, I would say the router will fragment that
>>> packet.
>>>
>>> HTH, Tim
>>>
>>> -----Original Message-----
>>> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
>>> Of John
>>> Matus
>>> Sent: Wednesday, June 15, 2005 12:32 AM
>>> To: ccielab@groupstudy.com
>>> Subject: system mtu
>>>
>>> question regarding mtu on an interface and globally and how the 2
>>> interact with each other...
>>> i think i know part of it but i don't know which direction it works..
>>>
>>> is it - if the interface exceeds it's own mtu, then it will
>>> fragment packets in accordance with the system mtu? or is it the
>>> other way around....
>>> or....
>>>
>>> is the global mtu the default for all interfaces and the interface
>>> mtu just overrides the global setting?
>>>
>>> TIA
>>>
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>>
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