Meraki MS355 dropping COX modem connection

[murphytb2]

I put a Meraki MS355 switch between my Firewall and the Cox modem and it will not connect.  Take a laptop using same cable and IP on firewall interface. I connect with no issues. Plug the cable back into switch port no connectivity to internet.  I've rebooted Cox modem. No change.  The thing I noticed is the Layer 2 port on Meraki switch is showing random  IP addresses. Like the switch is acquiring an IP address from Cox.  

[JGill]

Assuming you have the COX modem locked down Access port VLAN, then connecting to a firewall or MX Appliance.   We see lots of IP / broadcast ARP entries from some of the broadband modems as they can be a shared ethernet segment.    Is the COX modem set for a static IP or DHCP?

As a rule make sure the VLAN's are unique and set for Access ports, only the MX's Public WAN  and COX LAN (yellow port) plugged in. We have see some devices have issues with POE negotiations also, so I'd say turn off POE also and see if that makes a difference. 

[murphytb2]

Cox modem is setup for static IP.  Turned off POE. no Change  Firewall switch port and Cox modem are using same VLAN.  This was working for about a month and just stopped.   Connect laptop to same cable to modem no issues accessing internet.  Plug cable back into switch no Internet connectivity.  Cox changed out modem, say no issues with their side.  If I put and old switch between the Firewall and Meraki, the connection comes up.  No difference in port config.  auto/auto vlan xxx no poe access port

I configured a another on Meraki switch and that didn't make a difference either. 

[murphytb2]

Did PCAP. It shows traffic from firewall hitting port 47 and port 23 (modem connection), but receiving no traffic from Cox.  Cox states that they look at their modem, they don't see anything connected to it.   I know the cable is good, we connected a laptop to it and connected to the internet.  Tried another port on Meraki, no change. 

[PhilipDAth]

Does the Meraki Dashboard show any firmware upgrades about a month ago?

[murphytb2]

YEs