Forwarding adjacency. RSVP LSP tunnel through which one or more other RSVP LSPs can be tunneled.

fabric schedulers

Identify a packet as high or low priority based on its forwarding class, and associate schedulers with the fabric priorities.


Process by which a standby or secondary system component automatically takes over the functions of an active or primary component when the primary component fails or is temporarily shut down or removed for servicing. During failover, the system continues to perform normal operations with little or no interruption in service.

far-end alarm and control


Fast Ethernet

Term encompassing a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbps, versus the original Ethernet speed of 10 Mbps. See also Gigabit Ethernet.

fast port

Fast Ethernet port on a J4300 Services Router, and either a Fast Ethernet port or DS3 port on a J6300 Services Router. The number of ports applies only to enabled ports. A two-port Fast Ethernet PIM with one enabled port counts as one fast port. The same PIM with both ports enabled counts as two fast ports.

fast reroute

Mechanism for automatically rerouting traffic on an LSP if a node or link in an LSP fails, thus reducing the loss of packets traveling over the LSP.


Filter-based forwarding. A filter that classifies packets to determine their forwarding path within a router. FBF is used to redirect traffic for analysis.


Frame check sequence. A calculation that is added to a frame for error control purposes. It is used in HDLC, Frame Relay, and other data-link layer protocols.


Fiber Distributed Data Interface. A set of ANSI protocols for sending digital data over fiber-optic cable. FDDI networks are token-passing networks, and support data rates of up to 100 Mbps (100 million bits). FDDI networks are typically used as backbones for wide area networks.


Far-end alarm and control. A T3 signal used to send alarm or status information from the far-end terminal back to the near-end terminal, and to initiate T3 loopbacks at the far-end terminal from the near-end terminal.


Forwarding Engine Board. In M5 and M10 routers, provides route lookup, filtering, and switching to the destination port.


Forwarding equivalence class. Criteria used to forward a set of packets in an equivalent fashion. Forwarding equivalence classes are defined in the base LDP specification and may be extended through the use of additional parameters. FECs are also represented in other label distribution protocols.


Forward explicit congestion notification. In a Frame Relay network, a header bit transmitted by the source device requesting that the destination device slow down its requests for data. FECN and BECN minimize the possibility that packets will be discarded when more packets arrive than can be handled. See also BECN.

Federal Information Processing Standards


Fiber Distributed Data Interface


field-replaceable unit

See FRU.


First in, first out. Queuing and buffering method where the first data packet stored in the queue is the first data packet removed from the queue. All JUNOS software interface queues operate in this mode by default.

File Transfer Protocol

See FTP.


Process or device that screens packets based on certain characteristics, such as source address, destination address, or protocol, and forwards or discards packets that match the filter. Filters are used to control data packets or local packets. See also packet.

filter-based forwarding

See FBF.


Federal Information Processing Standards. Defines, among other things, security levels for computer and networking equipment. FIPS is usually applied to military environments.


Security gateway positioned between two different networks, usually between a trusted network and the Internet. A firewall ensures that all traffic that crosses it conforms to the organization's security policy. Firewalls track and control communications, deciding whether to pass, reject, discard, encrypt, or log them. Firewalls also can be used to secure sensitive portions of a local network.

first in, first out


flap damping

See damping.


See route flapping.

flash drive

Nonvolatile memory card in Juniper Networks M-series and T-series routing platforms used for storing a copy of the JUNOS software and the current and most recent router configurations. It also typically acts as the primary boot device.

Flexible PIC Concentrator

See FPC.

floating static route

Route that should be used only when all dynamically learned versions of that same route are no longer in the routing table.

flood and prune

Method of forwarding multicast data packets in a dense-mode network. This process repeats every 3 minutes.


Stream of routing information and packets that are handled by the Routing Engine and the Packet Forwarding Engine. The Routing Engine handles the flow of routing information between the routing protocols and the routing tables and between the routing tables and the forwarding tables, as well as the flow of local packets from the router physical interfaces to the Routing Engine. The Packet Forwarding Engine handles the flow of data packets into and out of the router physical interfaces.

flow collection interface

Interface that combines multiple cflowd records into a compressed ASCII data file and exports the file to an FTP server for storage and analysis, allowing users to manipulate the output from traffic monitoring operations.

flow control action

JUNOS software syntax used in a routing policy or firewall filter. It alters the default logical processing of the policy or filter when a set of match conditions is met.

flow monitoring

Application that monitors the flow of traffic and enables lawful interception of packets transiting between two routers. Traffic flows can be passively monitored by an offline router or actively monitored by a router participating in the network.

forward explicit congestion notification


forwarding adjacency

See FA.

forwarding classes

Affect the forwarding, scheduling, and marking policies applied to packets as they transit a routing platform. The forwarding class plus the loss priority define the per-hop behavior. Also known as ordered aggregates in the IETF Differentiated Services architecture.

Forwarding Engine Board

See FEB.

forwarding equivalence class

See FEC.

forwarding information base

See forwarding table.

forwarding table

JUNOS software forwarding information base. The JUNOS routing protocol process installs active routes from its routing tables into the Routing Engine forwarding table. The kernel copies this forwarding table into the Packet Forwarding Engine, which determines which interface transmits the packets.


Flexible PIC Concentrator. An interface concentrator on which PICs are mounted. An FPC inserts into a slot in a Juniper Networks router. See also PIC.


In TCP/IP, fragmentation refers to the process of breaking packets into the smallest maximum size packet data unit (PDU) supported by any of the underlying networks. In the Open Systems Interconnection (OSI) reference model, this process is known as segmentation. For JUNOS applications, split Layer 3 packets can then be encapsulated in MLFR or MLPPP for transport.

frame check sequence

See FCS.

Frame Relay

An efficient replacement for the older X.25 protocol because it does not require explicit acknowledgment of each frame of data. Frame Relay allows private networks to reduce costs by using shared facilities between the end-point switches of a network managed by a Frame Relay service provider. Individual data-link connection identifiers (DLCIs) are assigned to ensure that each customer receives only their own traffic.

frequency-division multiplexed channel

Signal carried at different frequencies and transmitted over a single wire or wireless medium.


Frame Relay Forum. A technical committee that promotes Frame Relay by negotiating agreements and developing standards.


End-to-end Frame Relay Implementation Agreement. An implementation of MLFR using multiple virtual connections to aggregate logical bandwidth for end-to-end Frame Relay. Released by the Frame Relay Forum.


Multilink Frame Relay Implementation Agreement. An implementation of MLFR in which a single logical connection is provided by multiplexing multiple physical interfaces for user-to-network interface and network-to-network interface (UNI/NNI) connections. Released by the Frame Relay Forum.


Field-replaceable unit. A router component that customers can replace onsite.


File Transfer Protocol. Application protocol that is part of the TCP/IP protocol stack. Used for transferring files between network nodes. FTP is defined in RFC 959.


OSPF adjacency state that represents a fully functional neighbor relationship.


JUNOS software permanent interface used for out-of-band network access to the router.


JUNOS software permanent interface used for communications between the Routing Engine and the Packet Forwarding Engine. This interface is not present in all routers.


JUNOS software permanent interface used for communications between the Routing Engine and the Packet Forwarding Engine. This interface is not present in all routers.