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Large ping packets fail

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Bobo_The_Clown
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Registered: ‎05-01-2013

Large ping packets fail

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I have an older ActionTec Fios router.  If I ping anything device on the inside Ethernet interface with a ping packet great than 1470 bytes, it will fail, i.e, ping 192.168.1.1 -l 1500 (tac lowercase L).  Normal ethernet packets are 1500 bytes, so this probably finally explains the slowness on my setup.

 

I could set the MTU size of my PC down to 1470, but should have to do that.

 

Any ideas short of talking Verizon into replacing it or go with my own router. I know I still need the ActionTec for the guide and such.

 

 

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Smith6612
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Posts: 7,557
Registered: ‎12-15-2010

Re: Large ping packets fail

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I would consider just getting a Gigabit switch that supports Jumbo frames. The limit you're seeing is due to the 1500 MTU cap on standard Ethernet switches. With a Jumbo-frame supported switch, it saves on getting another router up and running if you're pleased with the ActionTec's performance otherwise. Keep in mind your MTU will cap to the lowest link when traveling through it.

 

Also, your effective MTU is affected by overhead. While the MTU may be set to 1500, you have to subtract overhead. On average, it's 1500 bytes - 28 bytes which sets you to 1472. Anything lower is often due to additional overhead caused by the protocol in use. Users such as I who are stuck on PPPoE wind up with 1492 as an MTU. It's still a 1500 byte MTU, however 8 bytes are taken up by an 8 byte PPPoE header. From there the MTU is further reduced by what I mentioned earlier. Hopefully that explains things.

 

Unless your MTU is so low to the point where you're processing packets inefficiently, a few bytes of MTU being "lost" from 1500 won't put a dent on your performance.

iggdawg
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Posts: 4
Registered: ‎06-05-2012

Re: Large ping packets fail

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MTU is normally 1500 bytes on Ethernet.  But when you specify 1500 bytes in the ping application, you're telling it how much data you want in the padding of the packet.  In addition to this 1500 bytes it needs to wrap IP and Ethernet headers before it transmits the actual frame out on the local network.  So the resulting frame is larger than 1500 bytes, and gets dropped by the first interface it hits that's set for 1500 bytes.

 

EDIT - Er yeah, what Smith said.

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