It was a long battle, but I think I finally licked it. The two things that turned the tide for me were:
- Add a second IP address in 10.24.35.0/24 on my workstation for management (really should have realized sooner I'd need to do that);
- When moving traffic from router A to router B, change router A's default gateway to router B's management address.
To expand on that:
As previously mentioned, my internal subnets are 10.24.32.0/24 (LAN, on eth0), 10.24.33.0/24 (WLAN1, on eth1), 10.24.34.0/24 (WLAN2, on eth2), and now 10.24.35.0/24 (MGMT, on eth3). WAN is on eth4. Right now, LAN, WLAN1, WLAN2, and WAN are plugged into the ER-X. Both routers always have their MGMT interface plugged in, the ER-X at 10.24.35.2, the ER-POE at 10.24.35.4. My workstation has 10.24.32.10 bound as its norrmal address, plus 10.24.35.10 for management purposes. The ER-X's gateway is set to WAN of course; the ER-POE's is set to 10.24.35.2, the ER-X; and of course all the LAN and WLAN hosts use the .1 address on their respective subnet.
If I need to switch to the other router for some reason, I:
- change the ER-POE's gateway address to WAN
- unplug LAN, WLAN1, WLAN2, and uplink from the ER-X and plug them into the ER-POE
- change the ER-X's gateway address to 10.24.35.4 (the ER-POE)
- And last but not least, reboot the modem, because the accursed thing never refreshes ARP.
.......In theory, anyway. 