On 6/5/12, Owen DeLong <owen@delong.com> wrote:
This is a horrible misconfiguration of the devices on that link. If your MTU setting on your interface is larger than the smallest MTU of any L2 forwarder on the link, then, you have badly misconfigured
Not really; The network layer and L2 protocols should both be designed to handle this, it is a design error in the protocol that it doesn't. You say it's "misconfiguration", but if IP handled the situation reasonably, it shouldn't be necessary to configure anything in the first place. Whether the neighbors are LAN or cross-tunnel, the issues are similar. It's only a misconfiguration because of flaws in the protocol. Just like you expect to plug devices in a typical LAN and it's not a configuration error to fail to manually find every switch in the LAN and enter MAC addresses into a forwarding table by hand; likewise, you shouldn't expect to key a MTU into every device by hand. IP should be designed so that devices on the link that _can_ handle the large transmission unit, which provides efficiency gains, should be allowed to fully utilize those capabilities, without breakage of connectivity to devices on the same link that have more limited capabilities and can only receive the Minimum required frame size (smaller MTU), and without separating the subnet or installing dividing Proxy ARP servers to send ICMP TooBig packets.
Adding probing to compensate for this misconfiguration merely serves to perpetuate such errant configurations.
Just like adding MAC address learning to Ethernet switches to compensate for the misconfiguration of failing to manually enter hardware addreses into your switches, serves to perpetuate such errant configurations, where the state of the forwarding tables are unreliably left in a non-deterministic state.
You've got an issue if there are 100ms between two peers on your LAN. You're right, you don't need to probe for possible MTUs below 1280. LAN, sure. However, consider that there are intercontinental L2 links.
Intercontinental multi-access L2 links, perhaps, are a horrible misconfiguration.
Owen -- -JH