PMP450 CRC Errors, MTU and Cisco Switches

Hey All

I have been looking into an issue at one of our sites, it is cisco 3750 network switch hanging off a Cambium PMP450 SM.  The link to the SM is rock solid.  The SM Terminates to a PMP450 AP which then traverses into a Cisco 3850.

All switch interfaces that connect to cambium devices are running at 100Mb full duplex.

I am noticing large amounts of CRC errors occur on both the Cisco interfaces and also on the Cambium PMP450 "Ethernet Statistic" page. For the Satelitte site (3750 into PMP450 SM) CRC errors are only noticed during certain periods, and it is never a constitent issue. 

I am wondering if this may have anything to do with MTU size, Jumbo frame comes in from gigabit interface on the cisco switch, gets switched out to the 100Mb interface facing the PMP450 and the PMP450 drops it as it thinks it is a CRC error.

The only thing that makes me think that it is not a cable fault

  • Cable has been tested 2-3 times with Fluke DTX
  • CRC errors eb and flow.  They are not consistent, as you would expect from a faulty cable

Wondering if anyone else has expereinced anything like this?



Out of curiosity how large are the jumbo frames?

I think I have the same problem or similar but i have a Cisco 1801

Already changed router, firmware routers, firmware antenna and cable.

FE0/0 (Wan Cisco):
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
0 runts, 11253 giants, 1 throttles
11253 input errors, 0 CRC, 11253 frame, 0 overrun, 0 ignored

The problem I think is due to the fact that in the infrastructure, for example the switches where the AP are connected, packets up to 1700 bytes are accepted.

As alluded to, the 450 platform can support packets up to 1700 bytes.  If a packet attempts to ingress the radio that has MTU greater than this, it will be discarded.

Could true jumbo packets be attempting to traverse the link??  Can the router be configured to prevent this?

Also, prior to R14.1.2, there was an issue with Cisco CDP packets, which were not properly recognized and were discarded.  This was corrected in that software release, so should no longer be an issue.

1 Like