Wired Intelligent Edge

We have Aruba 6300 switches in our environment, while the ISP is using Cisco devices. We observed that some site applications were not working. After the ISP changed the MTU setting to 1500, the applications started working.

Is there any Aruba documentation available that explains the recommended MTU configuration and behavior on Aruba 6300 switches, especially when connecting to Cisco devices?

There's no Aruba-versus-Cisco MTU trick here. Both platforms count interface MTU the same way, as the payload excluding the Ethernet header, so 1500 on the 6300 and 1500 on the Cisco side mean the same thing. The 6300 defaults to 1500 and does jumbo up to 9198 via the interface mtu command, with a separate ip mtu for routed interfaces. The mtu page in the 6300/6400 CLI guide documents exactly the behavior you hit: frames bigger than the egress MTU are dropped and an ICMP fragmentation-needed goes back to the sender.

Your symptom, some applications dead until the ISP set 1500, is the classic path MTU blackhole. Large packets (TLS handshakes, transfers) were dying where the transport was effectively smaller than 1500, while pings and small flows worked fine. The rule that keeps this from recurring: match MTU end to end, and if you ever turn on jumbo, turn it on for the whole path or not at all. A ping sweep with the do-not-fragment bit set finds your real path MTU in about a minute.

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Dustin Burns

@Worldcom Exchange, Inc.


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