Quick summary of setup:
Xfinity Internet: NETGEAR CM2050V eMTA -> Zyxel XGS1250-12 -> RB4011 SPF+
I have the RB4011 configured as a router-on-a-stick. The eMTA comes into the Zyxel switch and gets assigned VLAN 100. VLAN 100 traffic goes to the RB4011 SPF+ and LAN traffic is assigned VLAN 99. LAN and WAN traffic is then sent back over the SPF+ to the Zyxel, which then sends it to the appropriate destination.
It's not fully optimized, but it works except for this occasional issue:
When the eMTA starts up, the RB4011 requests an IP via DHCP on the WAN interface (VLAN 100) and is assigned a local 192.168.100.x IP with a one minute lease. Every 30s, the RB4011 renews; it either gets the same 192.168.100.x IP if the eMTA is still initializing OR it gets an Xfinity public IP if the eMTA is online.
The issue is sometimes after the eMTA is online, it appears that the DHCP client on the RB4011 will not receive a response to the DHCP request. So, the local IP expires and no other IP lease is ever received. Restarting the eMTA and/or the switch and/or the RB4011 sometimes results in a success.
When all that fails, I have to actually connect the eMTA to the RB4011 on eth1 and update the config accordingly to use eth1 for WAN and the SFP+ only for LAN.
What could be the cause for this intermittent failure?
It seems like if the VLANs are configured accordingly (which I assume they are since it works most of the time), the issue would be at the eMTA?