Practice Q's - MLS CEF Flashcards
Which two tables are used by Cisco Express Forwarding? (Choose two.)
A. Fib
B. ACL table
C. Routing table
D. MAC address table
E. Adjacency table
Answer: A,E
Explanation:
Cisco Express Forwarding (CEF) is a Layer 3 switching technology based on information contained in the forwarding information base (FIB) and the adjacency table.
The FIB is conceptually equivalent to a routing table in that it contains information used in the packet-forwarding decision. It is derived from the routing table and is optimized for maximum lookup throughput. The adjacency table contains information about the adjacent route processors. The adjacency table contains the MAC information for the next-hop addresses for all FIB entries. A device is considered adjacent if it is reachable over a single Layer 2 connection. It is stored in DRAM. The adjacency table is derived from the ARP table.
CEF is a topology-based Layer 3 switching technology that is enabled by default on the latest Cisco products. The FIB table stores IP destination prefixes from the most specific to the least specific entry in the Ternary Content Addressable Memory (TCAM). The content of the FIB table is similar to the routing table and contains the forwarding information similar to what is found in the IP routing table. A single FIB entry can point to up to six adjacencies. When changes are made to the IP routing table, the FIB table is also updated.
The Layer 3 processor engine builds the FIB and adjacency tables in software. That information is distributed from the control-plane hardware to the data-plane hardware Application Specific Integrated Circuits (ASICs) at the port or line card. This enhances the Layer 3 forwarding operation by moving it from the software-based engine to the ASICs. Of course, there are exception packets that are still software-processed, such as:
Packets sent to a destination address that the CEF-based switch does not yet have a valid MAC address for will be sent instead to the Layer 3 engine. The Layer 3 engine will then perform the ARP request. Packets that arrive requiring fragmentation are also first sent to the Layer 3 engine.
When the adjacency table is full, a CEF TCAM table entry points to the Layer 3 engine to redirect the adjacency. Therefore, it will be forwarded by routing.
With respect to CEF switching, there is no ACL table. Access Control Lists (ACL) are used to filter traffic, but are not contained in a table and are not involved directly in Cisco Express Forwarding.
The routing table is not directly involved by CEF, although the FIB is derived from the routing table.
Which of the following statements best describes the purpose of ARP with respect to CEF?
A. ARP is used to build the FIB.
B. ARP is used to reindex the routing table.
C. ARP is used to build the adjacency table.
D. ARP is used to decrease the amount of time spent searching for an entry within a routing table.
Answer: C
Explanation:
Address Resolution Protocol (ARP) is used by Cisco Express Forwarding (CEF) to build the adjacency table. CEF is the switching method used by Catalyst switches. Unlike traditional multilayer switching (MLS), which merely caches Layer 3 information received when traffic passes through a switch, CEF attempts to optimize the routing process by reindexing the routing table and then building an adjacency table based on the routing table information. The type of MLS performed by CEF is called topology-based switching; traditional MLS is known as route caching, demand-based switching, and flow-based switching.
The routing table is reindexed by using a binary search method. The reindexed routing table is called the forwarding information base (FIB). Reindexing the routing table reduces the amount of time spent searching for an entry within a routing table.
After the FIB is created, an adjacency table is created to map the appropriate Layer 2 next-hop address or addresses to each FIB entry. ARP is used to retrieve the Layer 2 address information. If multiple Layer 2 next-hop addresses are available for an entry in the FIB, then CEF can employ load balancing for packets headed to that destination.
The final result is a single database of routing information (FIB) is built for the switching hardware.
Two extremely useful commands for verifying CEF are:
Both commands are shown below with explanations.
- SwitchA# show ip cef 192.168.6.0
192. 168.6.0/24, version 302, cached adjacency 192.168.166.5, 0 packets, 0 bytes
Via 192.168.166.5, VLAN 185, 0 dependencies
Next-hop 192.168.166.5, VLAN 185
Valid cached adjacency
Above it can be determined that there is a valid CEF entry for the destination network 192.168.6.0 and that there is a valid cached adjacency to the 192.168.166.5 next hop IP address.
In the command output below, it can be determined that 005565946856 is the MAC address of the 192.168.166.5 next-hop address:
- SwitchA# show adjacency detail | begin 192.168.166.5
IP VLAN 185 192.168.166.5(6) 0 packets, 0 bytes 005565946856
What command is used to enable CEF on a Cisco switch?
A. ip cef
B. ip cef distributed
C. ip route-cache cef
D. ip cef enable
Answer: A
Explanation:
The command to enable Cisco Express Forwarding (CEF) on a Cisco switch is ip cef. This enables CEF support on the entire switch. All interfaces that are configured to use CEF will be able to. The no form of this command will disable CEF support, including support on interfaces that have CEF configured on them.
Cisco Express Forwarding allows a Layer 3 switch to determine the next-hop destination MAC address of the first frame in a transmission made of many frames, and then utilizes the much faster switching process for all the remaining frames. This requires that routing be enabled on the switch, since the route to the initial frame must be determined.
The output of the show ip interface vlan id command can be used to determine whether IP routing is enabled. Partial output of the show ip interface vlan id command for two switches is shown below. The first (Switch A) has IP routing enabled and the second (Switch B) does NOT have IP routing enabled. The second switch is missing the section about CEF, since CEF cannot be enabled unless IP routing is enabled.
The command ip cef distributed is used to enable distributed CEF (dCEF), not the CEF mentioned in the scenario.
The command ip route-cache cef is a valid command to enable CEF on an individual interface, but the command is only valid in interface configuration mode.
The command ip cef enable is an invalid command due to incorrect syntax.
The company has just completed an implementation that uses Cisco Express Forwarding (CEF) as a Layer 3 IP switching technology for optimized network performance and scalability. The following is the network infrastructure of the company. (Click the Exhibit(s) button.)
You are creating the verification plan for this implementation. This includes verifying the routes known to the routers.
Which component of the CEF switching technology contains routes to the 10.1.0.0/24 network along with the routes to the 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24 networks?
A. FIB
B. Adjacency table
C. Routing table
D. Topology table
Answer: A
Explanation:
The forwarding information base (FIB) lookup table contains routes to 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24. CEF switching technology is an example of a topology-based switching mechanism that uses the FIB. The FIB contains the routing or forwarding information that the network prefix can reference. Thus, the FIB is the component that CEF based switching uses to store a route to 10.1.0.0/24 along with the routes to 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24. In the FIB, these specific entries would be ordered with the longest match followed by less specific subnets. When the switch receives a packet, it can easily examine the destination address and find the longest match entry in the FIB.
The adjacency table does not contain routes to 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24. The adjacency table is used by CEF to prepend Layer 2 addressing information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries. It stores the information for the nodes that are adjacent. Nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer.
The routing table does not contain routes to 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24. The router stores routing information, but CEF does not use the routing table for the purpose of making IP destination prefix-based switching decisions.
The topology table does not contain routes to 10.1.1.0/24, 10.1.2.0/24, or 10.1.3.0/24. The topology table is not a component of CEF switching technology. It is a component of EIGRP and stores the details of all the destinations along with the list of neighbors that advertise the destination. For each of these entries, the metrics of the neighbor advertising the destination are also stored.
During a CEF packet rewrite, which of the following changes are NOT made to the packet?
A. The source MAC address is changed to the MAC address of the outbound Layer 3 switch interface.
B. The destination MAC address is changed to the MAC address of the next-hop router’s MAC address.
C. Layer 3 TTL is decremented by one.
D. Layer 2 TTL is decremented by one.
Answer: D Explanation:
There is no Layer 2 TTL in the packet, so the Layer 2 time to live (TTL) cannot be decremented by one. All other options are correct. The following changes will be made when the Cisco Express Forwarding (CEF) packet rewrite process occurs:
(end)
Which protocol is used to maintain the contents of the Cisco Express Forwarding (CEF) adjacency table?
A. ARP
B. RARP
C. PING
D. INARP
Answer: A
Explanation:
The CEF adjacency table is maintained as each adjacent node is discovered. Link header entries are created and stored in the adjacency table as the information is learned through the ARP protocol.
Cisco Express Forwarding (CEF) is a Layer 3 switching technology based on information contained in the Forwarding Information Base (FIB) and the adjacency table.
The FIB is conceptually equivalent to a routing table in that it contains information used in the packet forwarding decision. The adjacency table contains information about the adjacent route processors. The adjacency table contains the MAC information for the next-hop addresses for all FIB entries. A device is considered adjacent if it is reachable over a single Layer 2 connection. It is stored in DRAM.
The Layer 3 processor engine builds the FIB and adjacency tables in software. That information is distributed from the control-plane hardware to the data-plane hardware Application Specific Integrated Circuits (ASICs) at the port or line card. This enhances the Layer 3 forwarding operation by moving it from the software-based engine to the ASICs. Of course, there are exception packets that are still software-processed, such as non-conforming protocols and datalink encapsulations.
Reverse ARP (RARP) is used an obsolete networking protocol used by a host computer to obtain its Internet Protocol (IPv4) address when it has available its link-layer address, such as an Ethernet address. It has been replaced with DHCP. It is not used maintain the contents of the Cisco Express Forwarding (CEF) adjacency table
INverse ARP (INARP) is used by Frame relay connection to dynamically learn the DLCI associated with a connection. It is not used maintain the contents of the Cisco Express Forwarding (CEF) adjacency table
PING is a diagnostic tool used to test connectivity. It is not used maintain the contents of the Cisco Express Forwarding (CEF) adjacency table.
