With IPHost Network Monitor you can run simple snmp requests against a Cisco device in your network.
cIpv6InterfacePhysicalAddress
Cisco Ipv6 Interface Physical Address
1.3.6.1.4.1.9.10.86.1.2.3.1.6
The interface's physical address. For example, for an IPv6 interface attached to an 802.x link, this object normally contains a MAC address. Note that in some cases this address may differ from the address of the interface's protocol sub-layer. The interface's media-specific MIB must define the bit and byte ordering and the format of the value of this object. For interfaces which do not have such an address (e.g., a serial line), this object should contain an octet string of zero length.
Back to CISCO-IETF-IP-MIB MIB page.
IPHost Network monitor allows you to monitor cIpv6InterfacePhysicalAddress on Cisco device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Cisco firewalls right now.