Undersized packets on Cisco 3750 upstream and very high rate of STP topology changes on Meraki MS220-8P downstream

[rmiller]

I noticed a warning condition on the uplink port of my Meraki MS220-8P indicating a very high rate of STP topology changes. This ports are set to Autonegotiate on both devices. Consequently, I reviewed the port statistic on the C3750 that is upstream from the Meraki switch and it seems to have a high number of undersized packets on port Gi0/2. The Receive Statistics on the 3750 show this:

Receive Statistics
Interface	Port Description	Unicast	Multicast	Broadcast	Discarded	Alignment Errors	FCS Errors	Collision Fragments	Undersized	Oversized
Fa1/0/1	Voice and Data	0	0	0	0	0	0	0	0	0
Fa1/0/2	Voice and Data	710584723	2629671	185642	0	0	0	0	143110	0
Fa1/0/3	Voice and Data	394408	887667	16470	0	0	0	0	72538	0
Fa1/0/4	bs-host-server	1016540131	1904869	1203749	0	0	0	0	0	0
Fa1/0/5	bs-rec-prn-1	22944694	8589	11561	0	0	0	0	0	0
Fa1/0/6	Voice and Data	0	0	0	0	0	0	0	0	0
Fa1/0/7	Voice and Data	454235	887555	16470	0	0	0	0	36355	0
Fa1/0/8	unifi controller	786553217	12776671	11083220	0	0	0	0	0	0
Fa1/0/9	Voice and Data	126769276	2080574	3523657	0	0	0	0	155972	0
Fa1/0/10	Voice and Data	0	0	0	0	0	0	0	0	0
Fa1/0/11	bs-cc-mach	1632	87	944	0	0	0	0	14	0
Fa1/0/12	Voice and Data	138128361	1778301	2832672	0	0	0	0	112236	0
Fa1/0/13	Voice and Data	156880973	2438887	1003599	0	0	0	0	175970	0
Fa1/0/14	Voice and Data	0	0	0	0	0	0	0	0	0
Fa1/0/15	Voice and Data	51324416	15191	11214	0	0	0	0	0	0
Fa1/0/16	Voice and Data	0	0	0	0	0	0	0	0	0
Fa1/0/17	Voice and Data	0	0	0	0	0	0	0	0	0
Fa1/0/18		9653243	1692461	244376	0	0	0	0	59338	0
Fa1/0/19	Voice and Data	131361772	2474746	168234	0	0	0	0	447506	0
Fa1/0/20	Voice and Data	0	0	0	0	0	0	0	0	0
Fa1/0/21	Voice and Data	82496355	1105862	221769	0	0	0	0	44225	0
Fa1/0/22	Voice and Data	56913	1477	1331	0	0	0	0	0	0
Fa1/0/23	Voice and Data	0	0	0	0	0	0	0	0	0
Fa1/0/24	Voice and Data	1045848	484532	6787785	0	0	0	0	0	0
Gi1/0/1	Uplimk	2058243015	980731341	1086412230	0	0	0	0	0	0
Gi1/0/2	Uplimk	338061381	128337514	7889033	0	0	0	0	167514	0

These two switches are connected via fiberoptic cable. The MTU on the Cisco is 1500. The Meraki should be the same as that's the default and I didn't change it. What could be causing this problem and how do I troubleshoot?

[Georg Pauwen]

Hello,

the undersized frames are most likely coming from the Meraki. Have you tried another port and cable to connect the Meraki to the 3750 ? Also, which IOS version are you running on the 3750 ?

[rmiller]

I tried another SPF port on the Meraki but not another cable since that would require stringing a new fiber cable from another building. The 3750 is running 12.2(53).

[Georg Pauwen]

Hello,

try and set both ports to something other than auto negotiate...

[rmiller]

---

@Georg Pauwen wrote:

Hello,

try and set both ports to something other than auto negotiate...

---

I tried setting the port on the Meraki to 1Gb forced (the only options I had were 1Gb and 1Gb forced). As soon as I did the switch stopped communicating. The only way I could reconnect was to change the SFP to the other port. The web interface for the C3750 crashes the applet when I try to access the port settings and when it opens there is no option available other than auto. I'm a little hesitant to change it via CLI because I don't want to cause another disconnection. It didn't make sense to me that forcing the 1Gb connection would cause a disconnect...

The techs at Meraki thought it might be because the Meraki supports RSTP and the Spanning Tree Mode from the STP settings on the C3750 appears to be rapid-pvst. Could that be the issue? The Protocol under the Bridge Parameters tab does indicate RSTP.