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ISIS MTU problem

NAVIN PARWAL
Level 2
Level 2

Folks,

Does ISIS check for MTU when forming adjencies? I remember reading somewhere that ISIS pads the hello packets to full MTU to overcome this problem. Please can someone confirm.

Thanks

3 Replies 3

mheusing
Cisco Employee
Cisco Employee

Confirmed. :-)

Also be aware that ISIS does not recreate a LSP on every router but rather floods the LSP and there is no fragmentation. Thus make sure the MTU is consistent on all interfaces within an ISIS level. You can configure the setting on an interface to match a neighbor MTU with:

interface Serial1

mtu 2000

clns mtu 1500 ! this one affects ISIS as CLNS is used to send ISIS packets

Hope this helps! Please use the rating system.

Regards, Martin

Martin,

I have one question. If ISIS fixes the problem using Padding of hello massages then we do people run into MTU mismatch problems. My understanding was that ISIS pads the first couple of packets to detect MTU mismatch faster or does it pad every hello packet every sent?

Hi Parwal,

every ISIS Hello is padded by default. This is the reason why an ISIS adjacency will not come up, if the MTU does not match. Have a look at

http://www.cisco.com/en/US/docs/ios/12_2/iproute/command/reference/1rfisis.html#wp1044432

explaining it and how to turn it off.

Hope this helps! Please use the rating system.

Regards, Martin

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