Hi,   Hmac-SHA2 mac algorithm was introduced in Cisco IOS 15.5(2)S Release.   Here is the link for your reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/security/d1/sec-d1-cr-book/sec-cr-i3.html#wp6958353100   Cheers! Wasim

Generally, when a blocked port transitions into forwarding; for instance due to a unidirectional link, a TCN will be received from that particular port on the switch instead of the root port.  You can use the "show spanning tree vlan <id> detail" com...