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New Member

MTU 1500


I keep reading in this forum that in an MPLS network if the MTU is 1500 bytes (default), a lot of packets would get dropped as the labels added to the original packet make it bigger than 1500 bytes(4 bytes per lable).

So i did a test today, I built a small network of three routers. I connected one of the routers to the internet and connected my laptop of the 3rd router on the far end. Had the router connected to the internet advertise a default route in a vrf to the router on the far end. I made sure that my traffic was being tag switched inside the core before going to the internet.

All interfaces inside my core had only 1500 mtu set.

to my surprise, i was able to go to yahoo, msn, hotmail, cnn and many other sites without any problem.

Does anyone know of a site that marks the packet as df=1 so that it is not fragmented inside the core?

I used the extended ping test with mtu of 1504 and df=1 and it failed as suspected.

does anyone know of an other application I can use to prove that mtu of 1500 would not work in an MPLS network rather than using a ping?

How will i know that packets were dropped because the mtu was more than the interface that support?


New Member

Re: MTU 1500

I've had troubles with telnet sessions running on Alpha Server CompaqTru64 5.1b. this implementation of telnet behaves this way.

Also you can decrease the mtu and then try again some web pages. Try mtu 1400 for example.

New Member

Re: MTU 1500

PMTUD probably adjusted the tcp mss value to allow packets to pass through. As your network was symetrical you would have no issues with mtu.

Try these pings through your label switch path with the df bit in the packet set. If you have your default mtu all at 1500 then you will not get the mtu 1500 through and only the 1496 assuming only 1 label is applied.

MTU 1500

Windows: ping x.x.x.x -l 1472 -f

Linux: ping x.x.x.x -s 1472 -M do

MTU 1496

ping x.x.x.x -l 1468 -f

ping x.x.x.x -s 1468 -M do

New Member

Re: MTU 1500

Set system to MTU size 1546. Reset router to take affect. It works very well for MPLS and you don't have to deal with different MTU size per interface.


Re: MTU 1500

What I think it is hardly to find a site w/ this setting and you can setup a web site to test it.