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Switching Mechanism Questions

March 16th, 2020 in ENCOR 350-401 Go to comments

Question 1

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:

show_ip_cef.jpg

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:

show_adjacency.jpg

Note: A fast cache is only used when fast switching is enabled while CEF is disabled.

Question 2

Explanation

Cisco IOS software basically supports two modes of CEF load balancing: On per-destination or per-packet basis.

For per destination load balancing a hash is computed out of the source and destination IP address (-> Answer E is correct). This hash points to exactly one of the adjacency entries in the adjacency table (-> Answer D is correct), providing that the same path is used for all packets with this source/destination address pair. If per packet load balancing is used the packets are distributed round robin over the available paths. In either case the information in the FIB and adjacency tables provide all the necessary forwarding information, just like for non-load balancing operation.

The number of paths used is limited by the number of entries the routing protocol puts in the routing table, the default in IOS is 4 entries for most IP routing protocols with the exception of BGP, where it is one entry. The maximum number that can be configured is 6 different paths -> Answer A is not correct.

Reference: https://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html

Question 3

Explanation

The Forwarding Information Base (FIB) table – CEF uses a FIB to make IP destination prefix-based switching decisions. The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and these changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table.

Reference: https://www.cisco.com/c/en/us/support/docs/routers/12000-series-routers/47321-ciscoef.html

Question 4

Explanation

CEF uses a Forwarding Information Base (FIB) to make IP destination prefix-based switching decisions. The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table. Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for route cache maintenance that is associated with earlier switching paths such as fast switching and optimum switching.

Note: In order to view the Routing information base (RIB) table, use the “show ip route” command. To view the Forwarding Information Base (FIB), use the “show ip cef” command. RIB is in Control plane while FIB is in Data plane.

Comments
  1. Anonymous
    April 30th, 2020

    Where are the questions??????????????

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