I'm encountering a puzzling issue with OSPF adjacencies in a mixed network environment and would greatly appreciate any insights or suggestions.
Network Setup: Devices Involved:
- A Cisco 2900 router.
- A physical Layer 3 switch running FRR.
- A PC running GNS3 with one virtual Layer 3 switch (also running FRR).
Topology: The virtual Layer 3 switch is connected to the physical network via a hub. Both the Cisco router and the physical Layer 3 switch are also connected to the same hub.
The Issue:
- The Cisco 2900 router successfully establishes OSPF adjacencies with both the physical Layer 3 switch and the virtual Layer 3 switch using the default broadcast (multicast) hello packets.
- However, the physical and virtual Layer 3 switches do not form an OSPF adjacency with each other—even though they can ping each other successfully.
- Packet captures show that the physical switch is not receiving the multicast OSPF hello packets (sent to 224.0.0.5) from the virtual switch.
- When I reconfigure the virtual switch to use non-broadcast (unicast) hello packets, the OSPF adjacency is successfully formed. However, I prefer to use the default broadcast configuration across all devices.
Additional Notes: I have verified that there are no issues with common OSPF parameters:
- Authentication: Not a factor.
- Router Priorities: Properly configured.
- MTU Settings: Consistent across the devices. The Cisco router forming adjacencies with both switches confirms that basic connectivity and multicast forwarding through the hub are functioning correctly.
My Question: Has anyone experienced a similar issue or have any insights into why the physical and virtual Layer 3 switches fail to form an OSPF adjacency using multicast hello packets, despite all devices being able to communicate and the Cisco router forming adjacencies without issues? I'm particularly interested in understanding any potential subtleties in how FRR handles multicast on different platforms or other configuration nuances that might cause this behavior.
Thank you in advance for your help!
