Network Glossary T

T1 A name used for DS1 lines inside the North American TDM hierarchy.

T3 A name used for DS3 lines inside the North American TDM hierarchy.

TACACS+ A Cisco-proprietary protocol that defines how to perform authentication between an authenticator (for example, a router) and an authentication server that holds a list of usernames and passwords.

Tag Distribution Protocol The original MPLS protocol used to advertise the binding (mapping) information about each particular IP prefix and associated label. It is slightly different from LDP, but functionally equivalent. See also LDP.

tail drop An event in which a new packet arrives, needing to be placed into a queue, and the queue is full—so the packet is discarded.

Tc See Time Interval.

TDP See Tag Distribution Protocol.

Time Interval (Tc) Variable name for the time interval used by shapers and by CAR.

TCP code bits Single-bit fields in the TCP header. For example, the TCP SYN and ACK code bits are used during connection establishment.

TCP flags The same thing as TCP code bits. See TCP code bits.

TCP header compression The process of taking the IP and TCP headers of a packet, compressing them, and then uncompressing them on the receiving router.

TCP intercept A Cisco router feature in which the router works to prevent SYN attacks either by monitoring TCP connections flowing through the router, or by actively terminating TCP connection until the TCP connection is established and then knitting the client-side connection with a server-side TCP connection.

TCP SYN flood An attack by which the attacker initiates many TCP connections to a server, but does not complete the TCP connections, by simply not sending the third segment normally used to establish the connection. The server may consume resources and reject new connection attempts as a result.

TDM See time-division multiplexing.

TDM hierarchy The structure inside telcos’ original digital circuit build-out in the mid-1900s, based upon using TDM to combine and disperse smaller DS levels into larger levels, and vice versa.

Temporal Key Integrity Protocol An enhanced version of WEP that is part of the 802.11i standard and has an automatic key-update mechanism that makes it much more secure than WEP.

TKIP is not as strong as AES in terms of data protection.

terminal history The feature in a Cisco IOS device by which a terminal session’s previously typed commands are remembered, allowing the user to recall the old commands to the command line through a simple key sequence (for example, the up-arrow key).

time-division multiplexing The process of combining multiple synchronized input signals over a single medium by giving each signal its own time slot, and then breaking out those signals.

Time to Live A field in the IP header that is decremented at each pass through a Layer 3 forwarding device.

TKIP See Temporal Key Integrity Protocol.

token bucket A conceptual model used by shapers and policers to represent their internal logic.

ToS byte See Type of Service byte.

totally NSSA area A type of OSPF NSSA area for which neither external (type 5) LSAs are introduced, nor type 3 summary LSAs; instead, the ABRs originate and inject default routes into the area. External routes can be injected into a totally NSSA area.

totally stubby area A type of OSPF stub area for which neither external (type 5) LSAs are introduced, nor type 3 summary LSAs; instead, the ABRs originate and inject default routes into the area. External routes cannot be injected into a totally stubby area.

traffic contract In shaping and policing, the definition of parameters that together imply the allowed rate and bursts.

transient multicast group Multicast addresses that are not assigned by IANA.

transit network (OSPF) A network/subnet over which two or more OSPF routers have become neighbors, thereby being able to forward packets from one router to another across that network.

transit router (OSPF) A router that is allowed to receive a packet from an OSPF router and then forward the packet to another OSPF router.

transmit power The signal strength of the RF signal at the output of the radio card or access point transmitter, before being fed into the antenna. Measured in milliwatts, watts, or dBm.

Trap In the context of SNMP, the Trap command is sent by an SNMP agent, to a manager, when the agent wants to send unsolicited information to the manager. Trap is not followed by a Response message from the receiving SNMP manager.

Triggered Extensions to RIP for On-Demand Circuits Defined in RFC 2091, the extensions define how RIP may send a full update once, and then send updates only when routes change, when an update is requested, or when a RIP interface changes state from down to up.

triggered updates A routing protocol feature for which the routing protocol sends routing updates immediately upon hearing about a changed route, even though it may normally only send updates on a regular update interval.

trunking Also called VLAN trunking, a method (using either the Cisco ISL protocol or the IEEE 802.1Q protocol) to support carrying traffic between switches for multiple VLANs that have members on more than one switch.

TTL See Time to Live.

TTL scoping Controls the distribution of multicast traffic by checking the TTL values configured on the interfaces. It forwards the multicast packet only on those interfaces whose configured TTL value is less than or equal to the TTL value of the multicast packet.

Type of Service byte A 1-byte field in the IP header, originally defined by RFC 791 for QoS marking purposes.