MTU problem in LAN

RouterOS General
1

Hello,

I have RB450G connected to the Internet through eth1 with public ip address and gateway, i mean it is not pppoe or another thing just IP Address and gateway and an invalid ip address on eth2 with DHCP server enabled, and it is src nated to that valid ip address,

192.168.0.10 - eth2(192.168.0.1)eth1(62.31.10.1)—>>>second mikrotik router - ISP

i cant open some web pages and some open with very low speed, at last i found with the code below i can open web pages
but i want to know what is the correct MTU and why i have MTU problem

chain=forward action=change-mss new-mss=1420 tcp-flags=syn protocol=tcp src-address=172.90.90.0/24 tcp-mss=1400-65535

and this is mturoute.exe report:

C:\Documents and Settings\DELL>mturoute -t www.google.com
mturoute to www.google.com, 30 hops max, variable sized packets
* ICMP Fragmentation is not permitted. *
* Speed optimization is enabled. *
* Maximum payload is 10000 bytes. *
 1  +-  host: 172.90.90.250  max: 1500 bytes
 2  +-  host: 217.219.177.241  max: 1500 bytes
 3  +-  host: 10.60.60.25  max: 1500 bytes
 4  ...-+++++++++.-++  host: 85.185.110.77  max: 1496 bytes
 5  +.-  host: 172.20.1.1  max: 1496 bytes
 6  +-  host: 217.219.176.237  max: 1496 bytes
 7  +.-  host: 78.38.245.146  max: 1496 bytes
 8  ++--+---+++-+++-  host: 217.218.158.42  max: 1500 bytes
 9  .-+++++++++.-++  host: 78.38.245.81  max: 1496 bytes
10  +.-  host: 10.10.53.26  max: 1496 bytes
11  ++--+---+++-+++-  host: 63.218.109.253  max: 1500 bytes
12  .-++++.-+++++.-  host: 63.216.0.38  max: 1410 bytes
13  ++--+---+++-.-+++  host: 74.125.49.77  max: 1496 bytes
No response from traceroute for this TTL.  Tried 3 times
14  .-++++++++.-++.-  host: 209.85.252.46  max: 1488 bytes
*14 (An additional device responded for 209.85.252.46)
*14 (An additional device responded for 209.85.252.46)
15  ++--+---+++-.-+++  host: 216.239.43.7  max: 1496 bytes
16  .-++++++++.-++.-  host: 72.14.233.63  max: 1488 bytes
17  ++--+---+++-.-+++  host: 209.85.255.74  max: 1496 bytes
18  .-++++++++.-++.-  host: 66.249.95.133  max: 1488 bytes
19  +.-  host: 72.14.233.166  max: 1488 bytes
20  +.-  host: 209.85.250.188  max: 1488 bytes
21  ++--+---+++-.-+++  host: 216.239.43.241  max: 1496 bytes
22  +.-  host: 72.14.236.167  max: 1496 bytes
23  ++--+---+++-+++-  host: 74.125.232.48  max: 1500 bytes
*23 (An additional device responded for 74.125.232.48)

if anybody know what’s happend to packets please let me know.

thanks

2

but i want to know what is the correct MTU

Procedure is experimental to find correct MTU, if you do not have precise information about ISP configuration.

why i have MTU problem

Ask your ISP for more information about it.

3

I asked them and they said “we didnt touch the MTU, it is your problem they said we have to use CISCO devices! not mikrotik”
thats why i want to solve it, by now i used mangle to solve my problem but i would like to know the problem occures from our routers or they routers.

the best i found:

 2   chain=forward action=change-mss new-mss=1442 tcp-flags=syn protocol=tcp src-address=172.90.90.0/24 tcp-mss=1400-65535 

 3   chain=forward action=change-mss new-mss=1442 tcp-flags=syn protocol=tcp dst-address=172.90.90.0/24 tcp-mss=1400-65535
4

Output from mturoute shows you where the problem is. Path to 85.185.110.77 can pass 1500bytes. Further from 85.185.110.77 MTU is less than 1500.

5

so if they didnt solve the problem, i have to use mangle, that ip addresss dosnt help us how to set MTU?