Access Points

Contributor I

Power Injectors and AP-65s

We have a few dozen AP-65 APs fed using either the PowerDsine 3001 and 3006 power injectors connected to Cisco 2950 and 2960s using auto/auto on fastEthernet ports.

The switches they are connected to report what seems to me to be a high rate of errors consisting of type "Rcv-Err" and "Giants". The APs plugged into 2960, 3550 and 3560 POE switches report these errors at much lower rates, if any at all.

It wouldn't surprise me if the power injectors normally introduced some errors but the rate seems a bit excessive to me. Has anyone else notice this? Is this a common problem? If so what is an acceptable error rate for installations involving Power Injectors? Is there anything I can do to mitigate this problem?

Aruba Employee

Power Injectors and AP-65s

Giants are normal as this is a byproduct of the AP doing MTU discovery.
These oversized MTU discovery packets are sometimes clocked as giant
errors. You can enable jumbo frames on the switches or you can
specifically set the AP MTU size in the AP system-profile on the
controller. When the MTU is specifically set, then MTU discovery does
not occur.
Aruba Employee

Re: Power Injectors and AP-65s

I've seen this as well, and Brian's solution works. APs send 1590 byte packets for MTU discovery.

In the AP system-profile, set the SAP MTU to 1500 bytes and the errors will disappear.

As Brian said, you could also increase the size of your MTU on your switches, but not all switches support jumbo frames. In your case:

2950 - no support

2960/3560 - Supported, but globally set for fe and ge ports. Must reload after executing the commands.
FE ports:
(config)# system mtu

GE ports
(config)# system jumbo mtu

3550 - Depends on the model. The gigabit versions support up to 2000 bytes, the fastethernet versions up to 1546. So the 3550-12G and 12T can do 2000, the others are 1546, hence only the 12G and 12T could support Aruba's MTU discovery.

(config)# system mtu
Contributor I

Re: Power Injectors and AP-65s

Thanks Brian and Mike.

The errors were indeed being caused by the MTU issue. The 2950s seemed to be counting the oversized packets as both Giants and Rcv-Error while the 2960s, 3560s, etc. counted them as just Giants.

The inflated count was from the APs not being able to contact their controllers for a few hours as the power injectors were put in before the switch ports were properly configured.

Occasional Contributor II

Re: Power Injectors and AP-65s

Seeing this behavior also on a ProCurve that doesn't support jumbo frames (it counts giant frames as "errors" when viewing through show interfaces).

I notice a large burst of giants when the AP first boots and then a small trickle (5-10 an hour) following. Just more MTU traffic, I'm guessing?

Is this safe to ignore aside from the counter filling up? :eek:
Aruba Employee

Re: Power Injectors and AP-65s

Ether - Honestly, I would set the SAP MTU to 1500 in your AP system-profile.
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