[c-nsp] mtu issue

ann kok oiyankok at yahoo.ca
Fri Dec 5 09:39:42 EST 2008


I sent mail to ask about network issue yesterday and I got the reply from newsgroup. You are quite helpful

When I set mtu1500, it works now.

but I have problem. 

In cisco configuration. configure CPE as PPPoE client. It needs to set the mty 1492. and the LAN is 1452. Now how I can do it now?

Create and config the dialer int of the cisco router for PPPoE which a negotiated IP addres and a mty size of 1492

int Dialer0
ip address negotiated
ip mtu 1492
encapsulation ppp
dial pool 1

configure the ethernet 0.0 int of the router

int ethernet 0/0
ip nat insise
ip tcp miss-adjust 1452

Thank you

Yeah, this is probably a mtu problem.
The router needs to set the tcp mss smaller because the packets coming back
are too big but the DF bit is set, so it can't get forwarded.
I have this problem with GRE tunnels and such where there's extra headers
involved requiring a higher MTU if the payload stays the same.





I have problem to access this site eg: outsidewebsite.com
but this machine can work other website

I also tried to use other machine in different network to access the same
website (outsidewebsite.com). It works. but those machines are not acessing
outsidewebsite.com in the same network

I am sure there is no firewall and this website won't block any 80 from

I get "incorret" in the tcpdump.

Do you have any idea?

12:38:36.339248 IP (tos 0x8, ttl 64, id 54019, offset 0, flags [DF], proto
TCP (6), length 40) > outsidewebsite.com.http: ., cksum
0xe114 (correct), ack 304 win 6432

12:38:36.355895 IP (tos 0x8, ttl 64, id 54020, offset 0, flags [DF], proto
TCP (6), length 238) > outsidewebsite.com.http: P, cksum
0xb071 (incorrect (-> 0xf336), 206:404(198) ack 304 win 6432

Thank you

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