Mikael Abrahamsson wrote:
I have many times ping:ed with 10000 byte packets on a device that has "ip mtu 9000" configured on it, so it sends out two fragments, one being 9000, the other one around 1100 bytes, only to get back a stream of fragments, none of them larger than 1500 bytes.
here's some data on INEX from a server interface with 9000 mtu. fping has 40 bytes overhead:
# wc -l ixp-router-addresses.txt 85 ixp-router-addresses.txt # fping -b 1460 < ixp-router-addresses.txt | grep -c unreachable 0 # fping -b 1500 < ixp-router-addresses.txt | grep -c unreachable 10 # fping -b 5000 < ixp-router-addresses.txt | grep -c unreachable 11 # fping -b 8960 < ixp-router-addresses.txt | grep -c unreachable 12
Out of interest, there were 5 different vendors in the output, according to the MAC addresses returned.Some of this may be caused by inappropriate icmp filtering on the routers, but the point is that it would be unwise to depend on routers doing the right thing here. If you're going to have a jumbo mtu vlan at an IXP, the VLAN needs to be a hard specification, not an aspiration with any variance. Nick