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New Member

ip path mtu discovery

on a cisco router how do i check if it's on?

an existing working bgp connection exists already on this router

if i do a show ip bgp neighbors <peer-address of neighbor> | i segment

i get a max data segment size of 1440 bytes

i can not find out why this value is 1440 (mtu of outgoing interface is 1500, and i would expect a 1460 value)

what command can i use or how can i check if ip path mtu discovery is turned on

also, how can i figure out why the max data segment size is 1440 instead of 1460?

thanks              

3 REPLIES
Super Bronze

Re: ip path mtu discovery

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Look for global configuration statement ip tcp path-mtu-discovery.

New Member

Re: ip path mtu discovery

if this command isn't in the running config of the router, how do i know if it's turned on?

i read it's turned on by default for bgp...?

max datasegment size of 1440 does this indicate anything? i read if it's 536 then path mtu discovery is turned off, correct?

Super Bronze

Re: ip path mtu discovery

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

if this command isn't in the running config of the router, how do i know if it's turned on?

Sorry, I'm unaware of any show command that would display its status.  (Well beyond show running all, incase it's the default.)

i read it's turned on by default for bgp...?

Not that I'm aware of.

max datasegment size of 1440 does this indicate anything? i read if it's 536 then path mtu discovery is turned off, correct?

536 should be the default for traffic to non-locally connected network.  Is your peer BGP address on a locally connected network?

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