From: ccie2be (ccie2be@nyc.rr.com)
Date: Thu Jun 16 2005 - 08:55:01 GMT-3
Hey John,
This thread reminded me of a problem I once came across.
If 2 IS-IS rtr's are adjacent via a serial link and you're forced to change
the mtu on the interface on ONE side ONLY, you can negate the problem that
this causes by using clns mtu #.
So, this is an example of how you can use the PROTOCOL MTU to solve a lab
problem.
Tim
-----Original Message-----
From: John Matus [mailto:jmatus@pacbell.net]
Sent: Thursday, June 16, 2005 1:07 AM
To: Richard Gallagher
Cc: ccie2be; Group Study
Subject: Re: system mtu
yes!!! thanks!!!
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: "John Matus" <jmatus@pacbell.net>
Cc: "ccie2be" <ccie2be@nyc.rr.com>; "Group Study" <ccielab@groupstudy.com>
Sent: Wednesday, June 15, 2005 9:46 PM
Subject: Re: system mtu
> 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
>>>>
>>>> _________________________________________________________________
>>>> Express yourself instantly with MSN Messenger! Download today - it's
>>>> FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/
>>>>
>>>> _______________________________________________________________________
>>>> Subscription information may be found at:
>>>> http://www.groupstudy.com/list/CCIELab.html
>>>>
>>>> _______________________________________________________________________
>>>> Subscription information may be found at:
>>>> http://www.groupstudy.com/list/CCIELab.html
>>>
>>>
>>> _______________________________________________________________________
>>> Subscription information may be found at:
>>> http://www.groupstudy.com/list/CCIELab.html
>>
>>
>> _______________________________________________________________________
>> Subscription information may be found at:
>> http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Wed Jul 06 2005 - 14:43:41 GMT-3