Network Glossary I

I/G bit The most significant bit in the most significant byte of an Ethernet MAC address, its value implies that the address is a unicast MAC address (binary 0) or not (binary 1).

iBGP See Internal BGP.

IEEE 802.1X An IEEE standard that, when used with EAP, provides user authentication before their connected switch port allows the device to fully use the LAN.

IGMP See Internet Group Management Protocol.

IGMP snooping A method for optimizing the flow of multicast IP packets passing through a LAN switch. The switch using IGMP snooping examines IGMP messages to determine which ports need to receive traffic for each multicast group.

IGMPv1 Host Membership Query A message sent by the multicast router, by default every 60 seconds, on each of its LAN interfaces to determine whether any host wants to receive multicast traffic for any group.

IGMPv1 Host Membership Report A message that each host sends, either in response to a router Query message or on its own, to all multicast groups for which it would like to receive multicast traffic.

IGMPv2 Group-Specific Query A message sent by a router, after receiving a Leave message from a host, to determine whether there are still any active members of the group. The router uses the group address as the destination address.

IGMPv2 Host Membership Query A message sent by a multicast router, by default every 125 seconds, on each of its LAN interfaces to determine whether any host wants to receive multicast traffic for any group.

IGMPv2 Host Membership Report A message sent by each host, either in response to a router Query or on its own, to all multicast groups for which it would like to receive multicast traffic.

IGMPv2 Leave A message sent by a host when it wants to leave a group, addressed to the All Multicast Routers address 224.0.0.2.

IGMPv3 Host Membership Query A message sent by a multicast router, by default every 125 seconds, on each of its LAN interfaces to determine whether any host wants to receive multicast traffic for any group.

IGMPv3 Host Membership Report A message sent by each host, either in response to a router query or on its own, to all multicast groups for which it would like to receive multicast traffic. The destination address on the Report is 224.0.0.22, and a host can specify the source address(es) from which it would like to receive the group traffic.

InARP See Inverse ARP.

Inform In the context of SNMP, the Inform command is sent by an SNMP manager to communicate a set of variables, and their values, to another SNMP manager. The main purpose is to allow multiple managers to exchange MIB information, and work together, without requiring each manager to individually use Get commands to gather the data.

infrastructure mode A wireless LAN that includes the use of access points. Infrastructure mode connects wireless users to a wired network and allows wireless users to roam throughout a facility between different access points. All 802.11 data frames in an infrastructure wireless LAN travel through the access point.

ingress PE An E-LSR in an MPLS VPN network whose role in a particular discussion is to receive unlabeled packets over customer links and then forward the packets as labeled packets into the MPLS network.

inner label An MPLS term referring to the MPLS label just before the IP header. Also called the VPN label when implementing MPLS VPNs.

input event Any occurrence that could change a router’s EIGRP topology table, including a received Update or Query, a failed interface, or the loss of a neighbor.

Inside Global address A NAT term describing an IP address representing a host that resides inside the enterprise network, with the address being used in packets outside the enterprise network.

Inside Local address A NAT term describing an IP address representing a host that resides inside the enterprise network, with the address being used in packets inside the enterprise network.

inspection rule A set of parameters for CBAC to perform in its traffic inspection process.

interface ID 64 bits at the end of an IPv6 global address, used to uniquely identify each host in a subnet.

Inter-Switch Link Cisco-proprietary VLAN trunking protocol.

internal BGP Refers to how a router views a BGP peer relationship, in which the peer is in the same AS.

internal DSCP A term used with Cisco LAN switches, referring to a DSCP value used when making QoS decisions about a frame. This value may not be the actual DSCP value in the IP header encapsulated inside the frame.

internal router (OSPF) A router that is not an ABR or ASBR in that all of its interfaces connect to only a single OSPF area.

Internet Group Management Protocol A communication protocol between hosts and a multicast router by which routers learn of which multicast groups’ packets need to be forwarded onto a LAN.

Invalid timer With RIP, a per-route timer that increases until the router receives a routing update that confirms the route is still valid, upon which the timer is reset to 0. If the updates cease, the Invalid timer will grow, until reaching the timer setting (default 180 seconds), after which the route is considered invalid.

Inverse ARP Defined in RFC 1293, this protocol allows a Frame Relay–attached device to react to a received LMI “PVC up” message by announcing its Layer 3 addresses to the device on the other end of the PVC.

IPCP See IP Control Protocol.

IP Control Protocol The portion of PPP focused on negotiating IP features—for example, TCP or RTP header compression.

IP forwarding The process of forwarding packets through a router. Also call IP routing.

IP PBX A component that interfaces with a phone using IP and provides connections to the Public Switched Telephone Network (PSTN).

IP Precedence A 3-bit field in the first 3 bits of the ToS byte in the IP header, used for QoS marking.

IP prefix list See prefix list.

IP routing The process of forwarding packets through a router. Also called IP forwarding.

IP Source Guard A switch feature that examines incoming frames, comparing the source IP and MAC addresses to the DHCP snooping binding database, filtering frames whose addresses are not listed in the database for the incoming interface.

IPv4 Version 4 of the IP protocol, which is the generally deployed version worldwide (at publication), and uses 32-bit IP addresses.

IPv6 Version 6 of the IP protocol, which uses 128-bit IP addresses.

ISATAP An IPv6/IPv4 tunneling method that is designed for transporting IPv6 packets within a site where a native IPv6 infrastructures is not available.

ISL See Inter-Switch Link.

isolated VLAN With private VLANs, a secondary VLAN in which the ports can send and receive frames only with promiscuous ports in the primary VLAN.