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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?



Re: MTU 1500

Hi mate do a google on Iperf. It seems to be gaining popularity within the UK anyway.


Community Member

Re: MTU 1500

well many of those sites dont use the max mtu anyway...

probably around 1434 etc...

Community Member

Re: MTU 1500

I tried over 50 websites, has anyone run into the MTU issues either with an application or website, I am trying to recreate the issue we had without effecting the live network.


Community Member

Re: MTU 1500

Dont forget about PMTUD.

Not sure what you are looking for. Sending packet with size more than MTU on a segment does not necessary mean any visibility on user level.

PMTUD protocol should handle the case when a packet with DF=1 is bigger than minimum MTU on a link. Please read appropriate RFC.

Community Member

Re: MTU 1500


Since your packets were label switched inside your small core, DF value has no significance, since IP headers were not visible to LSRs. Obviously in this environment PMTUD is not relevant because of the same reason.

When accessing web sites, you usually get the data in spikes and not in constant stream as for example with file transfers (FTP). With these spikes TCP never reaches full speed, so IP packet size stays below 1500bytes.

Try to do an FTP file transfer through your network and you will see it "hang" very quickly. This is an indication that you're running into MTU issues, which you will :-)


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