When configuring a MikroTik CCR2004-16G-2S+ router for load balancing with connections from two different ISPs—one static IP and one dynamic IP—you might encounter unusual behavior such as the dynamic IP connection showing the static IP of the other ISP. Even after physically disconnecting the static IP cable and configuring the dynamic IP on another MikroTik switch (CRS), the switch may incorrectly display the static IP. This issue can arise due to several network configuration challenges.
In this guide, we will explore the possible causes behind such IP conflicts and how to resolve them.
Problem Overview
In a dual ISP setup, one connection has a static IP (e.g., 27.100.4.1/24) and the other uses a dynamic IP. After configuring the second connection with DHCP on a CRS switch like Mikrotik CRS326-24G-2S+RM switch, you may notice that the dynamic connection incorrectly displays the static IP from the first ISP, even when the static ISP's cable is disconnected.
Possible Causes and Solutions
1. Misconfigured DHCP Client on the CRS Switch
If the DHCP client on the switch is misconfigured, it might pick up the static IP of the other ISP due to a stale lease or incorrect binding to the interface.
- Solution: Check the DHCP client settings to ensure it is correctly bound to the dynamic ISP's interface. Release and renew the DHCP lease using the following commands:/ip dhcp-client release [client interface]
/ip dhcp-client renew [client interface]
2. ARP Cache Issues or Conflicting IP Assignments
IP conflicts or ARP table confusion can cause the switch to display the wrong IP address, possibly due to old address mappings.
- Solution: Clear the ARP cache on both the CCR2004 router and the switch:/ip arp remove [all]
3. Incorrect IP Address Mapping or Routing Table Confusion
The routing table on either the router or the switch may still reference routes from the static IP ISP, even though the connection has been moved to the dynamic ISP.
- Solution: Check the routing tables on both devices using
/ip route printand remove any routes pointing to the static IP.
4. Switch Layer Misconfiguration (VLANs, Bridge)
If there are VLAN or bridge configurations on the switch that involve the static IP’s subnet, the switch may continue to receive the static IP through the network route.
- Solution: Verify that VLAN and bridge settings are correctly configured, ensuring the separation of static and dynamic ISP connections.
5. DHCP Server Confusion
In some cases, the switch may be mistakenly pulling an IP from the static ISP’s DHCP server on the CCR2004, leading to the wrong IP address being assigned.
- Solution: Ensure the switch is receiving its IP from the correct DHCP server and disable any unnecessary DHCP servers on the CCR2004.
6. Masquerading or NAT Issues
Active NAT or masquerading rules on the CCR2004 might still be applying the static IP to traffic meant for the dynamic connection.
- Solution: Review and adjust
/ip firewall natrules to make sure they aren’t affecting the dynamic ISP’s connection.
Conclusion
Resolving this IP address conflict requires checking for misconfigured DHCP clients, ARP table entries, routing issues, and NAT/firewall settings. By methodically troubleshooting each potential cause, you can ensure that the dynamic ISP connection is correctly configured and displays the right IP address.
By following these steps, you'll prevent any further issues with static and dynamic IP address overlaps in your MikroTik dual ISP setup.
