The first thing I would look at is the functionality of the firewall. Is it possible that it is routing between the VLANs without substituting its own MAC address - a sort of "transparent" mode? Have a look at the switch where the firewall is attached, and see if all these spurious entries converge there.
What sort of firewall is it?
The other thought is that if it isn't the firewall, then someone has taken a switch and used it to bridge the VLANs to each other. Again, the technique is to follow the spurious entries back to their source, and you will probably find they all converge on the offending element.
Acle, I don't know the Clavister firewall, but I'm speculating about how it behaves from the evidence of your MAC forwarding table.
I looks to me like the firewall is receiving a packet on VLAN 3, realising it has to forward it on VLAN 18, and forwards it transparently with its orginal MAC source address. (Most routers would substitute their own MAC address as source when they forward, so you would only see the router's MAC address in the forwarding table of the remote VLAN.)
The only way you will find out for sure is with a sniffer.
We are pleased to announce availability of Beta software for 16.6.3.
16.6.3 will be the second rebuild on the 16.6 release train targeted
towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are
looking for early feedback from customers befor...
Introduction Featured Speakers Luis Espejel is the Telecommunications
Manager of IENova, an Oil & Gas company. Currently he works with Cisco
IOS® and Cisco IOS XE platforms, and NX to some extent. He has also
worked as a Senior Engineer with the Routing P...
In this session you can learn more about Layer 3 multicast and the best
practices to identify possible threats and take security measures. It
provides an overview of basic multicast, the best security practices for
use of this technology, and recommendati...