[c-nsp] Ping test with DF bit and MTU / IP MTU value

[Thomason, Simon]

Hey Muhammad,

If you set the MTU size to 1520 and set the DF (do not fragment bit) then it will tell all device to never fragment the packets... So if a single hope in the chain can not support the max size of the packet you are sending it will drop the packet.

Sorry not certain if I missed something here but that seems like a simple enough question and answer?!?!

Cheers,

Simon

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Muhammad Atif Jauhar
Sent: Wednesday, 19 January 2011 12:21 PM
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] Ping test with DF bit and MTU / IP MTU value

Hi,

I have very basic question related to MTU / IP MTU value and ping test with
DF bit

*Scenario: *
I have one router connected to HUB router with Link leasing from SP
*            * 1. On spoke side, we have GigaEthernet Interface with MTU
value set to 1520
             2. On HUB side, that link is terminated on layer-2 switch and
from there to router sub-interface with IP MTU value set to 1520.
             3.  Configurations on both router
*                                 a.  SPOKE Side*
*                                             *interface GigabitEthernet0/1
                                             mtu 1520
                                             bandwidth 10000
                                             ip address 192.168.0.1
255.255.255.252
                                             ip ospf network point-to-point
                                             ip ospf cost 1000
                                             ip ospf hello-interval 1
                                             ip ospf dead-interval 3
                                             ip ospf mtu-ignore
                                             duplex auto
                                             speed auto
                                             mpls ip
                                   *b. HUB Side*
*
                                             interface
GigabitEthernet0/1.100
                                             bandwidth 10000
                                             encapsulation dot1Q 100
                                             ip address 192.168.0.2
255.255.255.252
                                             ip mtu 1520
                                             ip ospf network point-to-point
                                             ip ospf cost 1000
                                             ip ospf hello-interval 1
                                             ip ospf dead-interval 3
                                             ip ospf mtu-ignore

mpls ip


*
*Issue:*

I am testing the link and face issue.
                  1. while performing ping test without df bit, I am able to
ping with size upto 18024
                  2. while performing ping test with df bit, I am able to
ping with size 1520
                  3. while performing ping test with df bit, I am able to
ping with size more than 1520

*Kindly let me know, why I am not able to ping with DF bit with size more
than 1520 (MTU value). How can I troubleshoot the issue. *
*
*
*Any comment please*
--
Regards,

Muhammad Atif Jauhar
(+60-10-2155076)
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