Supported Cards with MTU >1500

RouterOS General
1

Hi there.

Urgently requesting confirmation of any Mikrotik-supported ether cards with MTU greater than 1500. In particular 1520 or 1540 would be fantastic.

Intent is keeping real MTU at 1500 all the way to the client, even when IPIP or PPTP tunnels are employed. Atheros interface supports 1600, which is fantastic, 'cause we have 30 Atheros 5212 cards deployed, so we just need to swap out our EE100 and RTL cards for ones supporting larger packet size.

Please advise ASAP. We’ve got some PMTUD black-holes that we’re dealing with, and re-writing TCP MSS via mangle only solves the problem going one way.

2

wireless cards? other network-cards?

atheros should work with MTU=1600.

with standard ethernet, afaik it’s not possible to have MTU > 1500, same with PPP-derived protocols.

but we didn’t find a difference using EoIP tunnel with MTU=1500 over wireless links with MTU=1500 or MTU=1600. i am not quite sure what is happening inside an EoIP tunnel, but it works as expected.

another point: most clients will use MTU path discovery.

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3

It is indeed possible for Ethernet to have MTU > 1500. Read here for more info: http://www.psc.edu/~mathis/MTU/index.html

I am just looking for confirmation that someone has used a card with MTU of >1500 (i.e. 1536) with Mikrotik, and if so, which card. In the case of 802.1q VLANs it should be common to have MTU of >1500.

Unfortunately there are some routers that block ICMP, thus creating what is known as a PMTUD “black hole”, which prevents Path MTU Discovery from working.

4

hello

thanks for the link, but these are only drafts or suggestions… (and we still don’t talk about jumbo frames on GigEthetnet?!)

we haven’t. instead we are using EoIP tunnels sometimes.
802.1q is using only a otherwise unused field of 18 bytes from the Ethernet II 1500byte MTU, as IEEE 802.3 uses only 1492. the mtu remains 1500 bytes.

with wireless cards MTU<=1600 is possible but i haven’t seen a difference in throughput.

5

Here is a link to hardware that currently supports MTU >1500.

http://www.uoregon.edu/~joe/jumbo-clean-gear.html

Now my question is, does Mikrotik support any of the cards listed at >1500 MTU?

6

mag: Indeed IEEE802.1q VLAN tagging puts 4 additional bytes to a standard Ethernet header. That is why some cards (or drivers) which do not support frames larger than 1500 fail to transfer VLAN-tagged frames larger than 1496 bytes.

Using EoIP leeds to further (rather significant) fragmentation, which is not good on wireless links. That is why I would advise using MTU=1600 on wireless links which are EoIP endpoints (note: EoIP endpoints, as otherwise there will be no difference). I posted similar thought on IPsec topic some weeks ago (remember the wireless link performance halved when transmitting IPsec packets as they are larger than original full-MTU packets and thus, fragmented; lower MTUs worked flawlessly without impact on performance whether they are IPsec-encrypted or not)