Explanation
6to4 tunnel is a technique which relies on reserved address space 2002::/16 (you must
remember this range). These tunnels determine the appropriate destination address by combining
the IPv6 prefix with the globally unique destination 6to4 border router's IPv4 address, beginning
with the 2002::/16 prefix, in this format:
2002:border-router-IPv4-address::/48
For example, if the border-router-IPv4-address is 64.101.64.1, the tunnel interface will have an
IPv6 prefix of 2002:4065:4001:1::/64, where 4065:4001 is the hexadecimal equivalent of
64.101.64.1. This technique allows IPv6 sites to communicate with each other over the IPv4
network without explicit tunnel setup but we have to implement it on all routers on the path.

Explanation
Cisco Express Forwarding (CEF) provides the ability to switch packets through a device in a very quick and efficient way while also keeping the load on the router's processor low. CEF is made up of two different main components: the Forwarding Information Base (FIB) and the Adjacency Table. These are automatically updated at the same time as the routing table.
The Forwarding Information Base (FIB) contains destination reachability information as well as next hop information. This information is then used by the router to make forwarding decisions.
The FIB allows for very efficient and easy lookups. Below is an example of the FIB table:

The adjacency table is tasked with maintaining the layer 2 next-hop information for the FIB. An example of the adjacency table is shown below:

It uses a fast cache that is maintained in a router data plane' fast cache is only used when fast switching is enabled while CEF is disabled.

Explanation
Broadcast and Non-Broadcast networks elect DR/BDR while Point-topoint/
multipoint do not elect DR/BDR. Therefore we have to set the two Gi0/0
interfaces to point-to-point or point-to-multipoint network to ensure that a DR/BDR election does not occur.

Explanation
The "aaa authentication login default local group tacacs+" command is broken down as follows:
+ The 'aaa authentication' part is simply saying we want to configure authentication settings.
+ The 'login' is stating that we want to prompt for a username/password when a connection is made to the device.
+ The 'default' means we want to apply for all login connections (such as tty, vty, console and aux). If we use this keyword, we don't need to configure anything else under tty, vty and aux lines. If we don't use this keyword then we have to specify which line(s) we want to apply the authentication feature.
+ The 'local group tacacs+" means all users are authenticated using router's local database (the first method). If the credentials are not found on the local database, then the TACACS+ server is used (the second method).