Question 1

What is the difference between a RIB and a FIB? A. The FIB is populated based on RIB content. B. The RIB maintains a mirror image of the FIB. C. The RIB is used to make IP source prefix-based switching decisions. D. The FIB is where all IP routing information is stored.

Accepted Answer

A. The FIB is populated based on RIB content. "Traditionally routers run routing protocols and the routing protocols (along with static configuration information) populate the Routing Information Base (RIB) of the router. The RIB is managed by the RIB manager and the RIB manager provides a northbound interface to its clients, i.e., the routing protocols, to insert routes into the RIB. The RIB manager consults the RIB and decides how to program the Forwarding Information Base (FIB) of the hardware by interfacing with the FIB manager."

Question 2

How does the RIB differ from the FIB? A. The FIB maintains network topologies and routing tables. The RIB is a list of routes to particular network destinations. B. The FIB includes many routes to a single destination. The RIB is the best route to a single destination. C. The RIB is used to create network topologies and routing tables. The FIB is a list of routes to particular network destinations. D. The RIB includes many routes to the same destination prefix. The FIB contains only the best route.

Accepted Answer

D. The RIB includes many routes to the same destination prefix. The FIB contains only the best route. RIB: is a data structure that contains all routes learned by a router, including multiple routes to the same destination prefix. These routes can come from various sources such as routing protocols, static routes, and directly connected networks. FIB: is a subset of the RIB and contains only the best route for each destination prefix as determined by the routing protocol's decision process.

Question 3

Which statement about Cisco Express Forwarding is true? A. The CPU of a router becomes directly involved with packet-switching decisions. B. It uses a fast cache that is maintained in a router data plane. C. It maintains two tables in the data plane: the FIB and adjacency table. D. It makes forwarding decisions by a process that is scheduled through the IOS scheduler.

Accepted Answer

C. It maintains two tables in the data plane: the FIB and adjacency table.

Question 4

1.6 Architecture CEF, CAM, TCAM, FIB, RIB, and adjacency tables What is the difference between CEF and process switching? A. CEF processes packets that are too complex for process switching to manage. B. Process switching is faster than CEF. C. CEF uses the FIB and the adjacency table to make forwarding decisions, whereas process switching punts each packet. D. CEF is more CPU-intensive than process switching.

Accepted Answer

C. CEF uses the FIB and the adjacency table to make forwarding decisions, whereas process switching punts each packet. "Punt" is often used to describe the action of moving a packet from the fast path (CEF) to the route processor for handling. 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. Process switching is the slowest switching methods (compared to fast switching and Cisco Express Forwarding) because it must find a destination in the routing table. Process switching must also construct a new Layer 2 frame header for every packet. With process switching, when a packet comes in, the scheduler calls a process that examines the routing table, determines which interface the packet should be switched to and then switches the packet. The problem is, this happens for the every packet.

Question 5

1.6 Architecture CEF, CAM, TCAM, FIB, RIB, and adjacency tables What are two differences between the RIB and the FIB? (Choose two.) A. FIB is a database of routing prefixes, and the RIB is the information used to choose the egress interface for each packet. B. The FIB is derived from the data plane, and the RIB is derived from the FIB. C. The RIB is a database of routing prefixes, and the FIB is the information used to choose the egress interface for each packet. D. The RIB is derived from the control plane, and the FIB is derived from the RIB. E. The FIB is derived from the control plane, and the RIB is derived from the FIB.

Accepted Answer

C. The RIB is a database of routing prefixes, and the FIB is the information used to choose the egress interface for each packet. D. The RIB is derived from the control plane, and the FIB is derived from the RIB. Both the RIB and FIB contain network prefixes with egress interface listed. If you execute an “show ip cef” command to display the contents of the CEF table (FIB), you will see the CEF entries listing the egress interface. Additionally, if CEF is enabled, the FIB is used to make forwarding decisions – RIB table is not referenced at all to determine the egress interface.

Question 6

1.6 Architecture CEF, CAM, TCAM, FIB, RIB, and adjacency tables What is the difference between the MAC address table and TCAM? A. TCAM is used to make L2 forwarding decisions. CAM is used to build routing tables. B. Router prefix lookups happen in CAM. MAC address table lookups happen in TCAM. C. The MAC address table supports partial matches. TCAM requires an exact match. D. The MAC address table is contained in CAM. ACL and QoS information is stored in TCAM.

Accepted Answer

D. The MAC address table is contained in CAM. ACL and QoS information is stored in TCAM. CAM requires an exact match to return a result. This means that the value that is being searched for (in binary) must contain either 0s or 1s. TCAM does not require an exact match, and is queried using 0s, 1s, and Xs, where X essentially means “anything” (that is, either 0 or 1). It's kind of like a wildcard state.

Question 7

RIB and FIB How do the RIB and the FIB differ? A. RIB is derived from the control plane, and the FIB is derived from the RIB. B. FIB is derived from the control plane, and the RIB is derived from the data plane. C. RIB contains the interface for a destination, and the FIB contains the next hop information. D. FIB contains routes learned through a dynamic routing protocol and the RIB contains routes that are static or directly connected.

Accepted Answer

A. RIB is derived from the control plane, and the FIB is derived from the RIB. RIB is derived from the control plane, it is not used for forwarding. Every protocol such as OSPF, EIGRP, BGP has its own RIB and select their best candidates to try to install to global RIB so that it can then be selected for forwarding. FIB is used for forwarding, the information is derived from the RIB and from adjacency tables so that the packet can be rewritten with the correct encapsulation.

Question 8

What are two valid modes that Cisco Express Forwarding can operate in? (Choose two.) A. Central CEF mode B. Dense CEF mode C. Sparse CEF mode D. Distributed CEF mode E. Routed CEF mode

Accepted Answer

A. Central CEF mode D. Distributed CEF mode CEF can be enabled in one of two modes: Central CEF mode - When CEF mode is enabled, the CEF FIB and adjacency tables reside on the route processor, and the route processor performs the express forwarding. You can use CEF mode when line cards are not available for CEF switching, or when you need to use features not compatible with distributed CEF switching. Distributed CEF (dCEF) mode - When dCEF is enabled, line cards maintain identical copies of the FIB and adjacency tables. The line cards can perform the express forwarding by themselves, and this relieves the main processor - Gigabit Route Processor (GRP) - of involvement in the switching operation. This is the only switching method available on the Cisco 12000 Series Router.

Question 9

Which two conditions occur when the primary route processor fails on a switch that is using dual route processors with stateful switchover? (Choose two.) A. Data forwarding can continue along known paths until routing protocol information is restored. B. Data forwarding is stopped until the routing protocols reconverge after the switchover. C. The standby route processor is fully initialized and state information is maintained. D. User sessions are immediately recreated on the new active route processor. E. The standby route processor initialization is started when the primary router processor fails.

Accepted Answer

A. Data forwarding can continue along known paths until routing protocol information is restored C. The standby route processor is fully initialized and state information is maintained. Community vote distribution AC (74%) BC (21%) 5% A and C from the 2nd edition OCG under SSO and NFS section, "During a switchover, the standby RP takes over as the new active RP. The new active RP uses the SSO learnt checkpoint information and keeps the interfaces on the router from flapping and the router and/or line cards from reloading, however, SSO doesn’t checkpoint any Layer 3 control plane information about any neighbor router, and due to this, the existing routing protocol adjacencies go down and begin to reestablish. During this time, since the FIB was checkpointed by NSF, the data plane is not affected by the Layer 3 control plane going down and traffic continues to be forwarded while the routing protocol adjacencies reestablish. After routing convergence is complete, the FIB is updated with new routing or topology information from the RIB if necessary."

Question 10

How does Cisco Express Forwarding switching differ from process switching on Cisco devices? A. Cisco Express Forwarding switching saves memory by storing adjacency tables in dedicated memory on the line cards, and process switching stores all tables in the main memory. B. Cisco Express Forwarding switching uses adjacency tables built by the CDP protocol, and process switching uses the routing table. C. Cisco Express Forwarding switching uses dedicated hardware processors, and process switching uses the main processor. D. Cisco Express Forwarding switching uses a proprietary protocol based on IS-IS for MAC address lookup, and process switching uses the MAC address table.

Accepted Answer

C. Cisco Express Forwarding switching uses dedicated hardware processors, and process switching uses the main processor. Cisco Express Forwarding (CEF) is a switching technology used in Cisco devices to improve packet forwarding efficiency. It is a Layer 3 IP switching technology that uses dedicated hardware processors (such as ASICs - Application-Specific Integrated Circuits) to make forwarding decisions based on information in the Forwarding Information Base (FIB) and Adjacency Table. CEF offloads packet switching from the main CPU, which allows for faster and more efficient packet forwarding. In contrast, process switching involves the main processor (CPU) of the router or switch making forwarding decisions for each packet. This method is less efficient and can result in higher CPU utilization, especially in high-traffic environments. Process switching doesn't rely on dedicated hardware processors for forwarding decisions. Option C accurately describes the difference between Cisco Express Forwarding and process switching, as CEF uses dedicated hardware processors, while process switching relies on the main CPU. The other options do not accurately describe the key differences between the two switching methods.

Question 11

What is one difference between the RIB and the FIB? A. The RIB keeps all routing information received from peers, and the FIB keeps the minimum information necessary to make a forwarding decision. B. The RIB works at the data plane, and the FIB works at the control plane. C. The FIB contains routing prefixes, and the RIB contains the Layer 2 and Layer 3 information necessary to make a forwarding decision. D. The RIB is known as the CEF table, and the FIB is known as the routing table.

Accepted Answer

A. The RIB keeps all routing information received from peers, and the FIB keeps the minimum information necessary to make a forwarding decision. Option A is correct. The RIB keeps all routing information received from peers, while the FIB keeps the minimum information necessary to make a forwarding decision. In more detail, the RIB, also known as the routing table, contains all the routes learned from various routing protocols, connected networks, static routes etc. It’s used by the router to make the best path decisions. On the other hand, the FIB, also known as the CEF (Cisco Express Forwarding) table in Cisco routers, contains a subset of the RIB routes that are used to make actual forwarding decisions in the data plane. It’s optimized for quick lookup, which is crucial for efficient packet forwarding.

Question 12

In which two ways does TCAM differ from CAM? (Choose two.) A. CAM is used to make Layer 2 forwarding decisions, and TCAM is used for Layer 3 address lookups. B. CAM is used by routers for IP address lookups, and TCAM is used to make Layer 2 forwarding decisions. C. CAM is used for software switching mechanisms, and TCAM is used for hardware switching mechanisms. D. The MAC address table is contained in TCAM, and ACL and QoS information is stored in CAM. E. The MAC address table is contained in CAM, and ACL and QoS information is stored in TCAM.

Accepted Answer

A. CAM is used to make Layer 2 forwarding decisions, and TCAM is used for Layer 3 address lookups. E. The MAC address table is contained in CAM, and ACL and QoS information is stored in TCAM. "The CAM table is the primary table used to make Layer 2 forwarding decisions." A- Correct, therefore B incorrect As frames arrive on switch ports, the source MAC addresses are learned and recorded in the CAM table. TCAM provides three results: 0, 1, and "don't care." TCAM is most useful for building tables for searching on longest matches such as IP routing tables organized by IP prefixes. The TCAM table stores ACL, QoS and other information generally associated with upper-layer processing. E- Correct, therefore D incorrect Correct answers are A and E

Done CEF, CAM, TCAM, FIB, RIB, and adjacency tables Flashcards

Chapter 1 OCG.

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Done CEF, CAM, TCAM, FIB, RIB, and adjacency tables Flashcards

Chapter 1 OCG.

Brainscape's Knowledge Genome^TM