I'm experiencing problems with fragmentation due to Cisco GRE tunnel overhead: the way I understand it, the MTU if a GRE tunnel will always be less than the MTU of the underlying IP cloud (in our case 1500 bytes) due to the IP encapsulation overhead. So 1500 byte packets attempting to traverse the tunnel will be fragmented, or dropped if the DF bit is set, in which case an ICMP message is send back to the originating host We're trying to use GRE tunnels extensivly in some fancy added-value Internet services, and it seems that there is a small but significant amount of application traffic out there that has problems when traversing a GRE tunnel with MTU < 1500. We've seen two problems: - 1500 byte packets with DF set. This is either application traffic, or MTU path discovery is broken, because the same packets get sent repeatedly - 1500 byte packets get fragmented, but the destination host cannot cope with the fragmentation (firewall issues?) We see this on a variety of platforms (from 2500, 7507) and a variety of IOS releases (11.1(18)CC, 11.1(2), 11.2(5). Talking to another provider indicates that the same problem exists with other vendors, and is having the same severe impact. Thinking about it, this is a problem is to be expected with IP tunnels of all types, but I am surprised at the extent it's influence on our customer's applications (such as large emails). I do not want to overstate the proportion of traffic we see with this problem - but it does seem to be enough to render GRE tunnels very problematic - to say the least. But I know lots of people are using GRE for this or similar applications...so what am I missing here. thanks in advance for help/tips Phil ______________________________________________________________ Philip Bridge ++41 31 688 8262 bridge@ip-plus.net www.ip-plus.ch PGP: DE78 06B7 ACDB CB56 CE88 6165 A73F B703