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Old 08-16-2004, 04:06 AM   #1
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Registered: Aug 2004
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gre tunnel problem


I set-up a gre tunnel between two linux boxes.
A short diagram is like this

lan1 -> router1 ----- router2 <- lan2

The tunnel works fine, i can ping from lan1 to lan2 but the problem is the speed between lan1 and lan2 is lower than it supposed to be
If i log in to router1 and transfer something from router2 or lan2 it is working at maximum speed, 1024kbits/s. The problem is if i log in to a computer in lan1 and try to transfer something either from router2 or lan2 is working very poor, with 256kbits or worse.
I also tried to add that line in iptables with --clamp-mss... same no luck
I ran out of ideas and this is driving me crazy. PLease help!

Last edited by sixone; 08-16-2004 at 04:08 AM.
Old 08-17-2004, 12:01 PM   #2
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What's the MTU on the GRE tunnel?

You may need to set the LAN MTU to match this to avoid fragmenting packets before they are sent..
Old 08-18-2004, 02:13 AM   #3
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MTU on tunnel is 1476 (its is GRE). I tried to set-up the MTU on network card 1476 and on a Windows client also.. Same, there is no difference.. Meanwhile i've installed freeswan which is setting the mtu somwhere around 16000.. The things are exactly the same, there is no change in good or bad with the speed.. And i've tried this on three linux distributions with three types of kernel.
Old 05-30-2005, 12:57 PM   #4
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I also have a problem with a GRE tunnel (my first one ) so I decided to post it here. Here's how things look:

networkA is with routerA that has eth0 (ip a.b.c.d) and br0 (ip
networkB is also (that's the tricky part ) with routerB that has eth0 (ip e.f.g.h) and eth1 (ip

After reading a while about it, I found out that I had to use proxy_arp to get things to work. So here's what i did:
ip tunnel add tun0 mode gre remote a.b.c.d local e.f.g.h ttl 255
ip link set tun0 up
ip addr add dev tun0
ip route add via dev tun0
and, on the other router, the same thing, but mirrored (switched remote with local). Pinging from routerA to routerB gives me only icmp echo requests on both end of the tunnel (tun0 monitored with tcpdump). My guess is that the problem is with that last route I added, but I can't figure it out. ip_forward is 1, proxy_arp is 1.

P.S.: does it matter what IP do I assign to tun0? My guess is no, but I want to make sure.


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