What Are the Components of CEF

Last Updated on Sun, 04 Feb 2024 | Mpls Network

CEF has two main data structures: the Forwarding Information Base (FIB) and the adjacency table. The FIB is also referred to as just the CEF table. Look at Figure 6-2 to see an overview of CEF and its components.

What Are the Components of CEF? 151

Figure 6-2 Overview of CEF

CEF Table (FIB)

Adjacency Table

CEF Table (FIB)

Adjacency Table

The Adjacency Table

The adjacency table is the CEF component that is responsible for the MAC or Layer 2 rewrite. When routers and hosts are adjacent, they learn about each other by some means. They can discover each other dynamically or by means of configuration. If routers are adjacent across a point-to-point connection, they discover each other trivially. However, on a multiaccess medium such as Ethernet, it is necessary for the routers to use a dynamic mechanism to discover each other. This mechanism is ARP, which maps Layer 2 (for example, Ethernet MAC) addresses to IP addresses.

Because routers usually run routing protocols among each other, the ARP table is built with the MAC addresses of the connecting routers on the Ethernet interfaces. If the interface is Frame Relay or ATM, it can be point-to-point or multipoint. In the first case, only one adjacency exists per interface; in the second case, multiple adjacencies can exist per interface. The adjacency table holds one adjacency or Layer 2 rewrite structure per router that is connected to that multipoint interface. This adjacency can be built from information learned from Address Resolution Protocol (ARP), ATM, or Frame Relay map statements, and inverse ARP on ATM or Frame Relay. Although the FIB decides where to forward the packet, the Layer 2 rewrite of the frame is done with the information found in the adjacency table. The Layer 2 rewrite string contains the new Layer 2 header that is used on the forwarded frame. For Ethernet, this is the new destination and source MAC address and the Ethertype (the protocol number for the Layer 3 payload). For PPP, the Layer 2 header is the complete PPP header, including the Layer 3 protocol ID. Example 6-2

shows an adjacency table holding one adjacency for a point-to-point interface and one for a multipoint interface (Ethernet). The highlighted section shows the Layer 2 rewrite string and how it was learned.

Example 6-2 CEF Adjacency Table

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Readers' Questions

  • FLORIAN
    What are the two main components of cisco express forwarding (cef) (choose two.)?
    24 days ago
    • The two main components of Cisco Express Forwarding (CEF) are:
    1. Forwarding Information Base (FIB): It is a data structure that contains the routing table entries for all known routes. The FIB is used to determine the next-hop address or outgoing interface for forwarding packets.
    2. Adjacency Table: It is a data structure that maintains Layer 2 or Layer 3 information about directly connected neighbors. The adjacency table is used to quickly resolve the next-hop MAC or IP address for forwarding packets to directly connected devices.
    • kaarlo
      What are the two main components of cisco express forwarding (cef) (choose two.)?
      2 months ago
      • The two main components of Cisco Express Forwarding (CEF) are:
      1. Forwarding Information Base (FIB): It is an optimized version of the routing table and contains the forwarding information used for packet forwarding decisions. The FIB is built based on the routes learned by the routing protocol and is stored in the data plane.
      2. Adjacency Table: It maintains information about the next-hop devices or directly connected neighbors. The adjacency table is used to quickly determine the next-hop MAC address for forwarding packets. The adjacency table is stored in the data plane.
      • erica
        What are the two main components of cisco express forwarding (cef) (choose two.)?
        2 months ago
        • The two main components of Cisco Express Forwarding (CEF) are:
        1. Forwarding Information Base (FIB): The FIB is a data structure in CEF that contains the routing information used for packet forwarding. It stores the best path for each destination network and its associated next-hop information.
        2. Adjacency Table: The adjacency table is another data structure used in CEF. It maintains Layer 2 addressing information (such as the MAC addresses) for directly connected neighbors. It is used to quickly determine the next-hop forwarding information for a packet.
        • Leo Bastian
          What are the two main components of cisco express forwarding (cef) (choose two.)?
          4 months ago
          • The two main components of Cisco Express Forwarding (CEF) are:
          1. Forwarding Information Base (FIB): It contains the precomputed lookup information for all known destinations and serves as a routing table for CEF. It consists of a collection of Layer 3 information, including next-hop addresses, outgoing interfaces, and precomputed adjacency information.
          2. Adjacency table: It contains Layer 2 (MAC) addressing information for reachable next-hop addresses. The adjacency table is used to quickly switch packets to their next-hop destination. It provides layer 2 address resolution for CEF and allows CEF to rewrite the MAC header with the correct layer 2 information.
          • drew
            What are the two main components of cisco express forwarding (cef) (choose two.)?
            5 months ago
            • The two main components of Cisco Express Forwarding (CEF) are:
            1. Forwarding Information Base (FIB): It stores the next-hop address and interface for each known destination network. The FIB is populated by the routing table and used for forwarding packets to their destinations.
            2. Adjacency table: It maintains layer 2 information, such as MAC addresses, for directly connected neighbors. The adjacency table is used to determine the outgoing interface and encapsulation for forwarding packets.Note: Other components of CEF include the CEF table, which contains all known prefixes and their respective FIB pointers, and the CEF switching optimization techniques, such as fast switching and distributed fast switching.
            • ermias
              What are the two main components of cisco express forwarding (cef) (choose two.)?
              6 months ago
              • The two main components of Cisco Express Forwarding (CEF) are:
              1. Forwarding Information Base (FIB): It is a data structure that contains the routing table entries and their associated next-hop information. The FIB is used to determine the best path for packet forwarding.
              2. Adjacency Table: It maintains information about directly connected neighbors. The adjacency table contains the MAC addresses and physical interfaces of the neighboring devices. It is used to quickly forward packets to directly connected devices without the need for route lookups.
              • CLARE
                What are the two main components of cisco express forwarding (cef) (choose two.)?
                7 months ago
                • The two main components of Cisco Express Forwarding (CEF) are:
                1. Forwarding Information Base (FIB): FIB is a data structure that stores the next-hop information for traffic forwarding in the network. It contains the precomputed lookup tables for efficient packet forwarding.
                2. Adjacency table/Database: The adjacency table contains the information about the MAC addresses and other Layer 2 details for directly connected neighbors or adjacent devices. It is used for fast switching of packets in Layer 2 forwarding.

                new-york#show adjacency

                Protocol Interface

                Address

                IP Ethernet3/3

                10.200.203.1(12)

                IP Serial4/0

                point2point(11)

                new-york#show adjacency detail

                Protocol Interface

                Address

                IP Ethernet3/3

                10.200.203.1(12)

                48 packets, 3673 bytes

                epoch 0

                sourced in sev-epoch 6

                Encap length 14

                00604700881F00036CC898570800

                ARP

                IP Serial4/0

                point2point(11)

                41 packets, 2637 bytes

                epoch 0

                sourced in sev-epoch 6

                Encap length 4

                0F000800

                The CEF table or FIB is the CEF component that is responsible for the Layer 3 forwarding decision that is made. The CEF table looks similar to the IP routing table on the router. In fact, each prefix in the routing table has the same prefix in the CEF table. The CEF table holds the essential information—taken from the routing table—to be able to make a forwarding decision for a received IP packet. This information is the IP prefix, the recursively evaluated next hop, and the outgoing interface. Information such as distance and metric of the protocol that put the prefix in the routing table are used to decide which paths are put into the routing table. However, they are not essential to forward a packet, so they are omitted from the CEF table. Example 6-3 shows the FIB on a router.

                Example 6-3 CEF Table or FIB

                new-york#show ip cef Prefix Next Hop

                0.0.0.0/32 receive

                10.200.200.0/24 10.200.203.1

                10.200.201.0/24 10.200.203.1

                10.200.203.0/24 attached

                Interface

                Ethernet3/3 Ethernet3/3 Ethernet3/3

                >-3 CEF Table or FIB

                (Continued)

                10.200

                203.0/32

                receive

                10.200

                203.1/32

                attached

                Ethernet3/3

                10.200

                203.2/32

                receive

                10.200

                203.255/32

                receive

                10.200

                254.1/32

                10.200.203.1

                Ethernet3/3

                ©

                254.2/32

                10.200.203.1

                Ethernet3/3

                ©

                254.3/32

                receive

                224.0.«

                1.0/4

                drop

                224.0.«

                1.0/24

                receive

                255.255.255.255/32

                receive

                An important aspect of the CEF table is that recursive prefixes are immediately resolved. If, for instance, a Border Gateway Protocol (BGP) prefix is in the routing table, and it points to a BGP next hop—which is learned via an Interior Gateway Protocol (IGP)—the BGP prefix is inserted into the CEF table with a next hop that is learned from recursing to the BGP next hop. In Example 6-4, a BGP prefix 10.99.1.1/32 with a next hop of 10.200.254.4 recurses to the IGP prefix 10.200.254.4. Therefore, the prefix 10.99.1.1/32 inherits the next-hop 10.200.200.2 from the IGP prefix 10.200.254.4. The CEF table immediately shows that the next hop of 10.99.1.1/32 is 10.200.200.2. This recursion is not done in the routing table. The next hop of the BGP prefix 10.99.1.1/32 is 10.200.254.4 in the routing table.

                Example 6-4 Example of Recursion london#show ip bgp 10.99.1.1

                BGP routing table entry for 10.99.1.1/32, version 13

                Paths: (1 available, best #1, table Default-IP-Routing-Table) Not advertised to any peer Local

                10.200.254.4 (metric 85) from 10.200.254.4 (10.200.254.4) Origin IGP, metric 0, localpref 100, valid, internal, best london#show ip cef 10.99.1.1

                nexthop 10.200.200.2 Ethernet0/0/0 label 23

                london#show ip cef 10.200.254.4

                10.200.254.4/32

                nexthop 10.200.200.2 Ethernet0/0/0 label 23

                Continue reading here: Load Balancing in CEF

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