CEF & Fast Switching

CEF Quick summary: 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 adjacency table is tasked with maintaining the layer 2 next-hop information for the FIB. Adjacency Types That Require Special Handling: Glean adjacency – in short when the router is directly connected to hosts the FIB table on the router will maintain a prefix for the subnet rather than for the individual host prefix. This subnet prefix points to a GLEAN adjacency. A glean adjacency entry indicates that a particular next hop should be directly connected, but there is no MAC header rewrite information available. When the device needs to forward packets to a specific host on a subnet, Cisco Express Forwarding requests an ARP entry for the specific prefix, ARP sends the MAC address, and the adjacency entry for the host is built.  Punt adjacency – When packets to a destination prefix can’t be CEF Switched, or the feature is not supported in the CEF Switching path, the router will then use the next slower switching mechanism configured on the router. Null adjacency: Packets destined for a Null0 interface are dropped. Null adjacency can be used as an effective form of access filtering. Other special adjacency types are Discard adjacency and Drop adjacency but they are not mentioned here.

Question 1

Explanation

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

Question 2

Explanation

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.

Reference: http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/17812-cef-incomp.html

Question 3

Explanation

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.

Question 4

Question 5

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 adjacency table is tasked with maintaining the layer 2 next-hop information for the FIB.

Question 6

Explanation

The explanation of this question is too lengthy so we recommend to read this article: http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/26083-trouble-cef.html

Question 7

Explanation

Glean adjacency – in short when the router is directly connected to hosts the FIB table on the router will maintain a prefix for the subnet rather than for the individual host prefix. This subnet prefix points to a GLEAN adjacency.
Punt adjacency – When packets to a destination prefix can’t be CEF Switched, or the feature is not supported in the CEF Switching path, the router will then use the next slower switching mechanism configured on the router.

Question 8

Explanation

Nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer. In addition to the FIB, CEF uses adjacency tables to prepend Layer 2 addressing information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.

Reference: https://www.cisco.com/c/en/us/td/docs/ios/12_2/switch/configuration/guide/fswtch_c/xcfcef.html

Question 9