This can be due to MTU mismatch. By default Cisco tunnel mode is GRE which add extra 24 Byte header into packet. When you change tunnel mode from GRE to IPIP; it adds 20 Byte header into packet. Tweaking MTU settings on your edge router can have positive impact.. here is some guidelines..
Set the MTU on the Client's network interface to 1476 bytes, forcing the SMSS to be smaller, so packets will not have to be fragmented when they reach R2. However, if you change the MTU for the Client, you should also change the MTU for all devices that share the network with this Client. On an Ethernet segment, this could be a large number of devices. If the GRE tunnel runs over links that can have an MTU greater than 1500 bytes plus the tunnel header, then another solution is to increase the MTU to 1524 (1500 plus 24 for the GRE overhead) on all interfaces and links between the GRE endpoint routers. From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of ZEESHAN SANAULLAH Sent: Friday, June 20, 2008 7:20 PM To: [email protected] Subject: [OSL | CCIE_RS] GRE and IPIP tunnels hello !!! im having this strange problem. I have two routers connected by DSL connection .... its a layer 3 path between routers i.e they are not layer 2 neighbors . The ISP routes the IPs. when i use a gre tunnel ... the tunnel interface comes up but I cannot ping the other end. when I enter this command tunnel mode ipip then i can ping the other router. The DSL modems are by Aztech and I have faced similar problem with them before at another customer. The most strangest thing is that when I issue a tunnel protection ipsec profile command the DSL modem ethernet port goes down and i cannot even ping default Gateway and I have to reset the modems to bring the ethernet back _____ Introducing Live Search cashback . It's search that pays you back! Try it Now <http://search.live.com/cashback/?&pkw=form=MIJAAF/publ=HMTGL/crea=introsrch cashback>
