Network Glossary C

CB Marking See Class-Based Marking.

CBAC See Context-Based Access Control.

CBWFQ See class-based weighted fair queuing.

CDP Control Protocol The portion of PPP focused on supporting the CDP protocol.

CDPCP See CDP Control Protocol.

CE See customer edge.

CEF See Cisco Express Forwarding.

Cell Loss Priority A bit in the ATM cell header that, when set to 1, means that if a device needs to discard frames, it should discard the frames with DE 1 first.

CGMP See Cisco Group Management Protocol.

Challenge Handshake Authentication Protocol An Internet standard authentication protocol that uses secure hashes and a three-way handshake to perform authentication over a PPP link.

CHAP See Challenge Handshake Authentication Protocol.

CIDR See classless interdomain routing.

CIR See committed information rate.

Cisco Express Forwarding An optimized Layer 3 forwarding path through a router or switch. CEF optimizes routing table lookup by creating a special, easily searched tree structure based on the contents of the IP routing table. The forwarding information is called the Forwarding Information Base (FIB), and by caching adjacency information is called the adjacency table.

Cisco Group Management Protocol A Cisco-proprietary feature. After a Cisco multicast router receives IGMP Join or Leave messages from hosts, it communicates to the connected Cisco switches, telling them which hosts (based on their unicast MAC addresses) have joined or left each multicast group. Switches examine their CAM tables and determine on which ports these hosts are connected and either forward multicast traffic or stop forwarding on those ports only.

Class-Based Marking An MQC-based feature of IOS that is used to classify and mark packets
for QoS purposes.

class-based weighted fair queuing A Cisco IOS queuing tool that uses MQC configuration commands and reserves a minimum bandwidth for each queue.

class map A term referring to the MQC class-map command and its related subcommands, which are used for classifying packets.

Class of Service A 3-bit field in an ISL header used for marking frames. Also, used generically to refer to either the ISL CoS field or the 802.1Q User Priority field.

Class Selector A DiffServ PHB that defines eight values that provide backward compatibility with IP Precedence.

classful IP addressing A type of logic for how a router uses a default route. A convention for discussing and thinking about IP addresses by which class A, B, and C default network prefixes (of 8, 16, and 24 bits, respectively) are considered.

classful routing A type of logic for how a router uses a default route. When a default route exists, and the class A, B, or C network for the destination IP address does not exist in the routing table, the default route is used. If any part of that classful network exists in the routing table, but the packet does not match any existing subnet of that classful network, the packet does not match the default route and thus is discarded.

classless IP addressing A convention for IP addresses in which class A, B, and C default network prefixes (of 8, 16, and 24 bits, respectively) are ignored.

classless interdomain routing Defined in RFCs 1517–1520, a scheme to help reduce Internet routing table sizes by administratively allocating large blocks of consecutive classful IP network numbers to ISPs for use in different global geographies. CIDR results in large blocks of networks that can be summarized, or aggregated, into single routes.

classless routing A type of logic for how a router uses a default route. When a default route exists, and no more specific match is made between the destination of the packet and the routing table, the default route is used.

Clear To Send On a serial cable, the pin lead set by the DCE to tell the DTE that the DTE is allowed send data.

client tracking Records client authentication and roaming events, which are sent to the CiscoWorks Wireless LAN Solution Engine (WLSE) to monitor client associations to specific access points.

CLP See Cell Loss Priority.

CLUSTER_LIST An optional nontransitive BGP path attribute that lists the route reflector cluster IDs through which a route has been advertised, as part of a loop-prevention process similar to the AS_PATH attribute.

collision domain A set of all devices for which any frame sent by one of the devices would collide with any frames transmitted at the same time by any of the other devices in the set.

Committed Burst With shaping, the number of bits allowed to be sent every Tc. Also defines the size of the token bucket when Be = 0.

committed information rate In shaping and policing, commonly used to refer to the shaping or policing rate. For WAN services, a common reference to the bit rate defined in the WAN service business contract for each VC.

Common Spanning Tree A single instance of STP that is applied to multiple VLANs, typically when using the 802.1Q trunking standard.

COMMUNITY An optional transitive BGP path attribute used to store 32-bit decimal values. Used for flexible grouping of routes by assigning the group the same COMMUNITY value. Other routers can apply routing policies based on the COMMUNITY value. Used in a large number of BGP applications.

community VLAN With private VLANs, a secondary VLAN in which the ports can send and receive frames with each other, but not with ports in other secondary VLANS.

component route A term used in this book to refer to a route that is included in a larger summary route.

confederation A BGP feature that overcomes the requirement of a full mesh of iBGP peers inside a single AS by separating the AS into multiple sub-autonomous systems.

confederation ASN The ASN assigned to a confederation sub-AS.

confederation eBGP peer A BGP peer connection between two routers inside the same ASN, but in different confederation sub-autonomous systems.

confederation identifier In an IOS confederation configuration, the actual ASN as seen by eBGP peers.

configuration register A 16-bit number set with a router config-register command. It is used to set several low-level features related mainly to accessing the router and what the router does when powered on.

conform A category used by a policer to classify packets relative to the traffic contract. The bit rate implied by all conforming packets is within the traffic contract.

Congestion Avoidance A method for how a TCP sender grows its calculated CWND variable, thereby growing the allowed window for the connection. Congestion Avoidance grows CWND linearly.

congestion window A mechanism used by TCP senders to limit the dynamic window for a TCP connection, to reduce the sending rate when packet loss occurs. The sender considers both the advertised window size and CWND, using the smaller of the two.

Context-Based Access Control Part of the Cisco IOS Firewall feature set, CBAC inspects traffic using information in the higher-layer protocols being carried to decide whether to open the firewall to specific inbound traffic. CBAC supports both UDP and TCP and multiple higher-layer protocols and can be applied inbound or outbound on an interface.

control plane In IP routing, a term referring to the building of IP routing tables by IP routing protocols.

CoS See Class of Service.

counting to infinity A type of routing protocol convergence event in which the metric for a route increases slightly over time because of the advertisement of an invalid route.

CQ See custom queuing

cross-over cable Copper cable with RJ-45 connectors in which a twisted pair at pins 1,2 on the first end of the cable is connected to pins 3,6 on the other end, with a second pair connected to pins 3,6 on the first end and pins 1,2 on the other end.

CS See Class Selector.

CSMA/CD Carrier sense multiple access with collision detection. A media-access mechanism where devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit simultaneously, a collision occurs and is detected by all colliding devices. This collision subsequently causes each device to delay retransmissions of the collided frame for some random length of time.

CST See Common Spanning Tree.

CTS See Clear To Send.

Custom queuing A Cisco IOS queuing tool most notable for its reservation of a minimum bandwidth for each queue.

customer edge An MPLS VPN term referring to a router at a customer site that does not implement MPLS.

CWND See congestion window.