CEF & Fast Switching
The command “show ip cef” is used to display the CEF Forwarding Information Base (FIB) table. There are some entries we want to explain:
+ If the “Next Hop” field of a network prefix is set to receive, the entry represents an IP address on one of the router’s interfaces. In this case, 192.168.201.2 and 192.168.201.31 are IP addresses assigned to interfaces on the local router.
+ If the “Next Hop” field of a network prefix is set to attached, the entry represents a network to which the router is directly attached. In this case the prefix 192.168.201.0/27 is a network directly attached to router R2’s Fa0/0 interface.
But there are some special cases:
+ The all-0s host addresses (for example, 192.168.201.0/32) and the all-1s host addresses (not have in the output above but for example, 192.168.201.255/32) also show as receive entries.
+ 255.255.255.255/32 is the local broadcast address for a subnet
+ 0.0.0.0/32: maybe it is a reserved link-local address
+ 0.0.0.0/0: This is the default route that matching all other addresses (also known as “gateway of last resort”). In this case it points to 192.168.201.1 -> Answer C is correct.
Reference: CCNP Routing and Switching ROUTE 300-101 Official Cert Guide
The “show adjacency” command is used to display information about the Cisco Express Forwarding adjacency table or the hardware Layer 3-switching adjacency table.
There are two known reasons for an incomplete adjacency:
+ The router cannot use ARP successfully for the next-hop interface.
+ After a clear ip arp or a clear adjacency command, the router marks the adjacency as incomplete. Then it fails to clear the entry.
Note: Two nodes in the network are considered adjacent if they can reach each other using only one hop.
The “show ip cache” command displays the contents of a router’s fast cache. An example of the output of this command is shown below:
Note: If CEF is disabled and fast switching is enabled, the router begins to populate its fast cache.