Cisco
Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational digital communications technology conglomerate corporation headquartered in San Jose, California. Cisco develops, manufactures, and sells networking hardware, ...
Express Forwarding (CEF) is an advanced
layer 3
In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers.
Functions
The network layer provides the means of transfe ...
switching technology used mainly in large
core network
A backbone or core network is a part of a computer network which interconnects networks, providing a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same buildin ...
s or the
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
to enhance the overall network performance. Although CEF is a Cisco
proprietary protocol
In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.
Intellectual property rights and enforcement
Ownership by a single organization gives the owner the ability to place restricti ...
other vendors of multi-layer switches or high-capacity routers offer a similar functionality where layer-3 switching or
routing
Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netw ...
is done in hardware (in an ASIC) instead of by software and the (central) CPU.
Function
CEF is mainly used to increase
packet switching
In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...
speed by reducing the overhead and delays introduced by other routing techniques. CEF consists of two key components: The Forwarding Information Base (FIB) and adjacencies.
The FIB is similar to the routing table generated by multiple
routing protocols
A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packets ...
, maintaining only the next-hop address for a particular IP-route.
The adjacency table maintains layer 2 or switching information linked to a particular FIB entry, avoiding the need for an
Address Resolution Protocol
The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given internet layer address, typically an IPv4 address. This mapping is a critical function ...
(ARP) request for each table lookup. There are several types of adjacencies. Some are listed below:
* Cache adjacency: This type of entry contains the correct outbound interface and the correct
Medium Access Control
In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublay ...
(MAC) address for its FIB entry. The
MAC address
A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking techno ...
is the IP address's MAC address if the destination's subnet is directly connected to the router, or is the MAC address of the router that the packet needs to be sent to if the destination's subnet is not directly connected to the router currently processing the packet.
* Receive adjacency: This type of entry handles packets whose final destinations include the router itself. This includes packets whose IP addresses are assigned to the router itself, broadcast packets, and multicasts that have set up the router itself as one of the destinations.
* Null adjacency: Handles packets destined to a NULL interface. Packets with FIB entries pointing to NULL adjacencies will normally be dropped.
* Punt adjacency: Deals with packets that require special handling or that cannot be switched by CEF. Such packets are forwarded to the next switching layer (generally fast switching) where they can be processed and hopefully forwarded correctly.
* Glean adjacency: This adjacency is created when the router knows that either the destination IP's subnet is directly connected to the router itself and it does not know that destination device's MAC address, or the router knows the IP address of the router to forward a packet to for a destination, but it does not know that router's MAC address. Packets that trigger this entry will generate an ARP request.
* Discard adjacency: FIB entries pointing to this type of adjacency will be discarded.
* Drop adjacency: Packets pointing to this entry are dropped, but the prefix will be checked.
In order to take full advantage of CEF, it is recommended to use distributed CEF (dCEF), where there is a FIB table on each of the line cards. This avoids the need for querying the main processor or routing table in order to get the next-hop information. Instead, fast switching will be performed on the line card itself.
CEF currently supports
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
,
Frame Relay
Frame Relay is a standardized wide area network (WAN) technology that specifies the physical and data link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across Integrated Se ...
FDDI
Fiber Distributed Data Interface (FDDI) is a standard for data transmission in a local area network.
It uses optical fiber as its standard underlying physical medium, although it was also later specified to use copper cable, in which case it m ...
,
tunnels
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A Pipeline transport, pipeline is not a tunnel, though some recent tunnels have used ...
, and
Cisco HDLC
Cisco HDLC (cHDLC) is an extension to the High-Level Data Link Control (HDLC) network protocol, and was created by Cisco Systems, Inc. HDLC is a bit-oriented synchronous data link layer protocol that was originally developed by the International ...