Cisco Express Forwarding
Encyclopedia
Cisco
Express Forwarding (CEF) is an advanced layer 3 switching technology used mainly in large core networks or the Internet
to enhance the overall network performance.
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, maintaining only the next-hop address for a particular IP-route.
The adjacency maintains layer 2 or switching information linked to a particular FIB entry, avoiding the need for an ARP
request for each table lookup. There are five types of adjacencies:
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
, Frame Relay
, ATM
, PPP
, FDDI
, tunnels
, and Cisco HDLC
.
Cisco Systems
Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
Express Forwarding (CEF) is an advanced layer 3 switching technology used mainly in large core networks or the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
to enhance the overall network performance.
Function
CEF is mainly used to increase packet switchingPacket switching
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
speed by reducing the overhead and delays introduced by other routing techniques. CEF consists of two key components: The Forwarding Information Base (FIB)
Forwarding Information Base
A forwarding information base , also known as a forwarding table, is most commonly used in network bridging, routing, and similar functions to find the proper interface to which the input interface should send a packet to be transmitted by the router.In contrast to routing information bases , also...
and adjacencies.
The FIB is similar to the routing table generated by multiple routing protocols, maintaining only the next-hop address for a particular IP-route.
The adjacency maintains layer 2 or switching information linked to a particular FIB entry, avoiding the need for an ARP
Address Resolution Protocol
Address Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
request for each table lookup. There are five types of adjacencies:
- 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 can not be switched by CEF, like packets sent to the router itself. Such packets are forwarded to the next switching layer (generally fast switching) where they can be forwarded correctly.
- Glean adjacency: Handles packets destined for currently attached hosts, but without layer 2 information.
- 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
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, Frame Relay
Frame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
, ATM
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
, PPP
Point-to-Point Protocol
In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
, FDDI
Fiber Distributed Data Interface
Fiber Distributed Data Interface provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to . Although FDDI logical topology is a ring-based token network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its...
, tunnels
IP tunnel
An IP tunnel is an Internet Protocol network communications channel between two networks. It is used to transport another network protocol by encapsulation of its packets....
, and Cisco HDLC
Cisco HDLC
Cisco HDLC is an extension to the High-Level Data Link Control network protocol created by Cisco Systems, Inc. HDLC is a bit-oriented synchronous data link layer protocol that was originally developed by the International Organization for Standardization...
.