To my understanding, it works like this:
Say you take an IP packet coming into ether1, and it’s full MTU; 1500 bytes. Now, you want to bridge ether1 to an EoIP tunnel. EoI is GRE, and there’s a 28 byte overhead for the GRE encapsulation. Now you have a 1528 byte packet. Unless every device between that router and the EoIP endpoint has layer2 MTUs of at least 1528 bytes, you’re going to transmit two packets to move that one original packet. One packet will have something like 1472 bytes of the original packet, plus GRE overhead for 1500, and one will have the remaining 28 bytes of the original packet, plus 28 GRE overhead, so, something like 56 bytes. This introduces the obvious slowdowns, as well as not so obvious ones, like maybe you have a device in the middle that’s not so good at PPS. Or that queues up small packets into one big air frame, and therefore you’re waiting for reassembly on the far end. Now, if you’re going from a 1500 byte LAN across a 9000 byte fiber connection, you’ll not notice this. If you’re going to a satellite office behind DSL with PPPoE, or a cable modem, or whatever, you’re going to notice. From: Af [mailto:[email protected]] On Behalf Of Kade Sullivan via Af Sent: Wednesday, December 10, 2014 5:17 PM To: [email protected] Subject: Re: [AFMUG] EoIP over fiber - high latency? Could you elaborate on this? We have a couple EOIP links across "other" networks and have never adjusted the MTU anywhere. I just pulled up the EOIP interfaces on each router and they are all set for 1500. Should we be increasing this number as a best practice when building EOIP Tunnels? On Wed, Dec 10, 2014 at 3:52 PM, Shayne Lebrun via Af <[email protected]> wrote: Bear in mind that unless you’ve increased your MTU from end to end, or dropped the MTU on your two devices that the EoIP are bridging, you’re going to get packet fragmentation. Otherwise, what RouterOS version? From: Af [mailto:[email protected]] On Behalf Of Erich Kaiser via Af Sent: Wednesday, December 10, 2014 4:25 PM To: [email protected] Subject: [AFMUG] EoIP over fiber - high latency? So I have an EoIP tunnel setup over two fiber connections for a customer, I am seeing high latency over the tunnel any idea? MTU Issue? Using RB1100AHx2 on both ends.
