[c-nsp] MTU size difference and OSPF

[Jason Koh]

Hi Steinar

It doesn't work. ip mtu 1500 is a default command on the interface, which 
means after I set it, it doesn't appear.

#sh ip int g0/1.11
GigabitEthernet0/1.11 is up, line protocol is up
  Address determined by non-volatile memory
  MTU is 1500 bytes

#ping mpls pseudowire 192.168.38.19 500 source 192.168.38.35 size 1488
Sending 5, 1488-byte MPLS Echos to 192.168.38.19,
     timeout is 2 seconds, send interval is 0 msec:

Codes: '!' - success, 'Q' - request not transmitted,
       '.' - timeout, 'U' - unreachable,
       'R' - downstream router but not target,
       'M' - malformed request

Type escape sequence to abort.
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/5 ms
#ping mpls pseudowire 192.168.38.19 500 source 192.168.38.35 size 1500
Sending 5, 1500-byte MPLS Echos to 192.168.38.19,
     timeout is 2 seconds, send interval is 0 msec:

Codes: '!' - success, 'Q' - request not transmitted,
       '.' - timeout, 'U' - unreachable,
       'R' - downstream router but not target,
       'M' - malformed request

Type escape sequence to abort.
QQQQQ
Success rate is 0 percent (0/5)

----- Original Message ----- 
From: <sthaug at nethelp.no>
To: <kohtohshyr at pacific.net.sg>
Cc: <cisco-nsp at puck.nether.net>; <chooweikeong at staff.pacific.net.sg>
Sent: Monday, December 04, 2006 3:47 PM
Subject: Re: [c-nsp] MTU size difference and OSPF


>> The problem arises when I have to fit the GigaEthernet interface of the 
>> 7206 to accomodate packets with bigger than 1500 due to MPLS. On the main 
>> interface,
>> I have to use this command
>>
>> #interface Gi0/1
>> #mtu 1520
>>
>> Without this command, I cannot key in the following commands in Gi0/1.1
>>
>> #mpls mtu 1520
>>
>> This allows me to send packets with the MPLS labels as well as keep the 
>> normal IP MTU to 1500.
>>
>> However at this point, if the OSPF between the Cat6509 and the 7206 is 
>> broken for any reason, it will never reestablish. It will always loop 
>> between EXSTART/DOWN because of the MTU difference.
>
> Use "ip mtu 1500" on your Gi0/1.1 to match the IP MTUs on the respective
> routers.
>
> Steinar Haug, Nethelp consulting, sthaug at nethelp.no
>