Best practice with pppoe+MTU

marieL · September 22, 2008, 3:01pm

I must learn how to manage a network of CPE, Point-MultiPoint, Point-Point, Subscriver server, all with Mikrotik and RouterOS.
Point-Multipoint create an EoIP tunnel with subscriver.
All the interfaces of all the network have a MTU=1500.
All the CPE’s create a pppoe tunnel with subscriver.

In this case, which is the best MaxMTU and MaxMRU in PPPPOE servers to avoid any problem ?

parrini · September 22, 2008, 7:31pm

Well, I have asked this before here, without success. I use 1480 on the PPPoE server profile. Apparently everything is working fine.

marieL · September 23, 2008, 7:19am

Thanks parrini.
1480 for both MTU and MRU ?

M.

mrz · September 23, 2008, 8:16am

Default MTU MRU values (1480) are optimal values that will suite in most of cases.

marieL · September 23, 2008, 11:23am

I suspect somewhere ICMP packet receive a deny.
How can I test the ruter of the network if they correctly notify the sender that a too large packet must be fragmented ?
Wath type of ICMP packet I must allow ?
Now are allower: 0, 3, 4, 8, 11, 12.

Thanks

parrini · September 23, 2008, 12:01pm

1480 MTU and MRU. For ICMP you have to choose what you want to pass and take into consideration the direction of the packet. I think your selection is ok, Type 3 Code 4 should take care of fragmentation. Maybe you should also consider Type 30 (Traceroute).

marieL · September 23, 2008, 12:04pm

thanks parrini.
I will take a look

fatonk · September 23, 2008, 12:37pm

You can calculate the MTU, it depends on technology that you use, if you are using only PPPoE you should adjust the MTU at 1480, but if you have after PPPoE some IPIP tunnel or EoIP that will give more overhead, because of new headers etc etc.

Regards.

Faton

marieL · September 23, 2008, 1:58pm

Hmmm…
I have pppoe connections inside a EoIP tunnel.
EoIP have MTU=1500.

Eoip attach its header to the pppoe frames (24bytes?); pppoe attach 20bytes to IP frame.

If this is correct, I must:

in pppoe servers set MAXMTU and MAXMRU to 1500-24-20 = 1456
change the MTU of the public interface to 1456 to publish the correct MTU size

Where am I wrong ?

thanks

rafa_lore · September 23, 2008, 2:24pm

Hello
The best MTU you have to find for your networks, Look this page
http://help.expedient.com/broadband/mtu_ping_test.shtml
or
http://www.dslreports.com/faq/5793

Bye
Rafael Lore

dibatech · September 24, 2008, 9:54am

I agree. MTU setting of 1456 as well as MRU works well. especially if this network is also supplied connectivity from some pppoe-session / vpn from ISP.

macgaiver · September 24, 2008, 11:24am

I use MRRU=1520 option and this way was able to avoid any MTU problems without change-mss mangle rules

in windows make sure that - in “networking” tab → “settings” button → “Negotiate multi-link over single-link connections” is enabled.

marieL · September 24, 2008, 12:37pm

macgaiver,
what is your full MTU configuration ?
I have the real interfaces (eth and wlan) with MTU= 1500, the eoip tunnel with MTU=1500.
Have you the same configuration ?
Which values do you use for MAX MATU and MAX MRU for pppoe server ?

thanks

Nuke · September 24, 2008, 3:05pm

I played around a bit with wireshark to check on a few things.

Seems like the header on an EoIP tunnel is 42(that includes the 20 byte IP header) bytes, and PPPoE is normally 8 bytes. So its seems like the Eoip MTU should be 1458 and the PPPoE mtu 1450. Though it works fine to use 1500 mtu on the EoIP and 1492 for the PPPoE, it just fragments alot of the packets, putting a little more strain on the router.

amt · June 7, 2016, 9:14am

can use auto mtu for ppoe server and eoip tunnel ?

simeitg · July 29, 2017, 10:27pm

Why every CCR show 1480 MTU if you create a pppoe-server? Why not 1492 or 1500?

amt · July 31, 2017, 8:18am

yep Im thinking same thing.

simeitg · July 31, 2017, 7:49pm

A friend said that is 1480 to avoid fragmentation.