Numerics

224.0.0.10 The multicast IPv4 address used by EIGRP routers to form neighbor adjacencies.

224.0.0.5 The All OSPF Routers multicast IPv4 address, listened for by all OSPF routers.

224.0.0.6 The All OSPF DR Routers multicast IPv4 address, listened to by DR and BDR routers.

A

AAA A framework that provides authentication, authorization, and accounting when securing the management plane.

access control list (ACL) A list that contains entries configured on a router or switch that can be used to identify traffic that will have a particular action applied to it, based on the service or feature that is using the list.

address family (named EIGRP/OSPFv3/MP-BGP) A method of configuring IPv4 and IPv6 routing services under the same routing process. IPv4 address families are used for IPv4 routing, and IPv6 address families are used for IPv6 routing.

Address Resolution Protocol (ARP) Defined in RFC 826, a protocol used on an Ethernet LAN by devices to determine the Layer 2 MAC address of a known Layer 3 IP address.

adjacency table A table used by CEF that stores the Layer 2 addressing for all FIB entries of next-hop devices.

administrative distance (AD) In Cisco routers, a means for one router to choose between multiple routes to reach the same subnet when those routes are learned by different routing protocols. The lower the administrative distance, the more preferred the source of the routing information.

ADVERTISE message The unicast message DHCPv6 servers send to respond to SOLICIT messages to offer addressing information to the DHCPv6 client.

APIPA See Automatic Private IP Addressing.

area border router (ABR) A router that connects an OSPF area to Area 0 (that is, the backbone area).

ARP See Address Resolution Protocol (ARP).

ARP cache A table that Ethernet-enabled devices use to maintain the IPv4-to-MAC address mappings.

AS_Path ACL An ACL based on regex for identifying BGP routes based on the AS_Path and used for direct filtering or conditional matching in a route map.

AS_Path A BGP attribute used to track the autonomous systems a network has been advertised through as a loop prevention mechanism.

ASBR See autonomous system boundary router (ASBR).

ASBR summary LSA A Type 4 LSA that allows routers to locate an ASBR that is in a different OSPF area.

atomic aggregate A BGP path attribute that indicates that a prefix has been summarized and the path information from component routes was not all included in the aggregate.

Authentication Header (AH) A protocol that uses a digital signature similar to a checksum to ensure that the original data packet (before encapsulation/encryption) has not been modified.

Automatic Private IP Addressing (APIPA) An IPv4 addressing method used by DHCPv4 clients when the DHCPv4 server is not available. The clients automatically assign themselves an IPv4 address in the 169.254.0.0/16 network.

autonomous system (AS) A set of routers running the same routing protocol under a single realm of control and authority.

autonomous system boundary router (ASBR) A router that redistributes external routes into an OSPF routing domain.

autonomous system number (ASN) A number between 1 and 64,511 (public) and 64,512 and 65,535 (private) assigned to an autonomous system for the purpose of proper BGP operation.

autonomous system path In BGP, a path through all the autonomous systems taken to reach a network on the Internet.

autosummarization A routing protocol feature in which a router that connects to more than one classful network advertises summarized routes for each entire classful network when sending updates out interfaces connected to other classful networks.

B

backbone area The OSPF Area 0 that connects to all other OSPF areas. The backbone area is the only area that should provide connectivity between all other OSPF areas.

backup designated router (BDR) A backup pseudonode that maintains the network segment’s state to replace the DR in the event of its failure.

BGP See Border Gateway Protocol (BGP).

Bidirectional Forwarding Detection (BFD) A detection protocol that works with all media types, routing protocols, topologies, and encapsulations. It is used to quickly detect reachability failures between two routers in the same Layer 3 network so that network issues can be identified as soon as possible and so convergence can occur at a far faster rate.

Border Gateway Protocol (BGP) An exterior routing protocol designed to exchange prefix information between different autonomous systems. The information includes a rich set of characteristics called path attributes, which allows for great flexibility regarding routing choices.

BGP community A well-known BGP attribute that allows for identification of routes for later actions such as identification of source or route filtering/modification.

BGP confederation A grouping of ASs that appear as a larger AS. A BGP confederation allows for scalability in an iBGP deployment.

BGP multihoming The method of providing redundancy and optimal routing by adding multiple links to external autonomous systems.

BGP multipathing The presentation of multiple paths to the RIB so that traffic can be load balanced.

boundary router A router that sits at the boundary of the routing domains and performs redistribution.

C

CE router The customer’s router, connected to the PE router of the MPLS domain.

Cisco DNA Center Assurance A component of Cisco DNA Center that enables you to predict problems faster through proactive monitoring and receive insights from network devices, network applications, network services, and clients.

Cisco Express Forwarding (CEF) An optimized Layer 3 forwarding path through a router or switch. CEF optimizes routing table lookups 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 the cached adjacency information is called the adjacency table.

classful 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.

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

class map A construct used with CoPP and QoS to define a traffic class.

Client 360 A dashboard within Cisco DNA Center Assurance that displays details about client devices.

C-network The customers network connected to an MPLS domain.

control plane The plane of operation that encompasses protocols used between routers and switches. These protocols include, for example, routing protocols and Spanning Tree Protocol (STP). Also, a router’s or switch’s processor and memory reside in the control plane.

Control Plane Policing (CoPP) A policy applied to traffic destined to or sourced by the router’s control plane CPU to limit known traffic to a given rate while protecting the CPU from unexpected extreme rates of traffic that could impact the stability of the router.

D

data availability The available state of a network that allows for the secure transport of data.

data confidentiality Data being viewable only by authorized users.

data integrity Data being modified only by authorized users.

data plane In IP routing, a set of processes that forward packets through a router or a multilayer switch.

dead interval The amount of time required for a hello packet to be received for the neighbor to be deemed healthy. Upon receipt, the value resets and decrements toward zero.

designated router (DR) A pseudonode to manage the adjacency state with other routers on the broadcast network segment.

destination protocol A routing protocol and process that receives network prefixes from the routing protocol sending the network prefixes.

Device 360 A dashboard within Cisco DNA Center Assurance that displays details about network devices.

DHCP See Dynamic Host Configuration Protocol (DHCP).

DHCPACK A DHCPv4 unicast message used by a DHCPv4 server to acknowledge that the addressing information is reserved for the client.

DHCPDISCOVER A DHCPv4 broadcast message used by a client to locate a DHCPv4 server.

DHCPOFFER A DHCPv4 unicast message used by a DHCPv4 server to provide a client with addressing information.

DHCPREQUEST A DHCPv4 broadcast message used by a client to request the addressing information that was provided in the offer.

DHCPv4 relay agent A device such as a router or multilayer switch that is able to relay DHCPv4 DISCOVER messages to a DHCPv4 server in a different IPv4 network.

DHCPv6 Guard A security feature designed to ensure that rogue DHCPv6 servers are not able to hand out addresses to clients, redirect client traffic, or starve out the DHCPv6 server and cause a DoS attack.

DHCPv6 relay agent A device such as a router or multilayer switch that is able to relay DHCPv6 SOLICIT messages to a DHCPv6 server in a different IPv6 network.

Dijkstra’s shortest path first (SPF) algorithm The algorithm used by link-state routing protocols.

discontiguous network In IPv4, an internetwork design in which packets forwarded between two subnets of a single classful network must pass through the subnets of another classful network. In OSPF, a network where Area 0 is not contiguous, which generally results in routes not being advertised pervasively through the OSPF routing domain.

distribute list A list used for filtering routes with an ACL for a specific BGP neighbor.

DMVPN Phase 1 A DMVPN topology in which the spokes only establish tunnels with the DMVPN hubs.

DMVPN Phase 3 A DMVPN topology in which the spokes can establish dynamic spoke-to-spoke tunnels between sites as needed.

DORA The DHCP process a client and server use to determine the appropriate IPv4 addressing information the client needs. Stands for Discover, Offer, Request, Ack.

Dynamic Host Configuration Protocol (DHCP) A standard (RFC 2131) protocol by which a host can dynamically broadcast a request for a server to assign to it an IP address, along with other configuration settings, including a subnet mask and default gateway IP address.

Dynamic Multipoint VPN (DMVPN) A VPN architecture that combines multipoint GRE tunnels, IPsec, and NHRP for dynamic VPN tunnel creation and registration.

E

external BGP (eBGP) A term referring to how a router views a BGP peer relationship, in which the peer is in another AS.

eBGP session A BGP session maintained with BGP peers from a different autonomous system.

edge LSR A router that sits at the edge of the MPLS domain and adds labels to packets that are entering the MPLS domain (known as an ingress LSR), removes labels from packets that will be leaving the MPLS domain (known as an egress LSR), and forwards packets as needed based on labels or the lack of labels.

Exterior Gateway Protocol (EGP) A routing protocol that was designed to exchange routing information between different autonomous systems. EGP has been replaced by BGP and is no longer supported in Cisco IOS.

egress LSR A router at the edge of the MPLS domain that removes labels from packets that are leaving the MPLS domain.

EIGPR stub-site router An EIGRP router that advertises to all other upstream neighbors that it is isolated but provides a mechanism to prevents transit routing through its WAN interfaces while still allowing connectivity to local downstream neighbors.

EIGRP classic configuration An EIGRP configuration mode in which most of the configuration resides under the EIGRP process, but some settings are configured under the interface configuration submode.

EIGRP named mode configuration An EIGRP configuration mode that provides a hierarchical configuration and stores settings in three subsections: address family, interface, and topology.

EIGRP stub router An EIGRP feature that advertises to all other neighbors that it is isolated and should not be queried when routes go active.

encapsulating interface The interface that receives tunneled traffic from the underlay network and removes the outer IP headers, or the interface that receives traffic and adds outer IP headers for the underlay network.

Encapsulating Security Payload (ESP) A protocol that ensures that the original payload (before encapsulation) maintains data confidentiality by encrypting the payload and adding a new set of headers during transport across a public network.

EUI-64 A specification for the 64-bit interface ID in an IPv6 address, composed of the first half of a MAC address (with the seventh bit flipped), the added hex values FFFE, and by the last half of the MAC address.

extended ACL An ACL that is able to match packets based on multiple criteria, such as source and destination IP address, source and destination port numbers, protocols, and QoS parameters.

external LSA A Type 5 LSA that advertises an external route into a routing domain and indicates the router acting as the ASBR for that route.

external OSPF route A route that is injected into the OSPF routing domain that is learned from outside the native OSPF process.

F

feasibility condition The condition which says that for a route to be considered a backup route, the reported distance received for that route must be less than the feasible distance calculated locally. This logic guarantees a loop-free path.

feasible distance The metric value for the lowest-metric path to reach a destination.

feasible successor A route that satisfies the feasibility condition that is maintained as a backup route.

FF02::A The multicast IPv6 address that EIGRP routers use to form neighbor adjacencies.

FIB See Forwarding Information Base (FIB).

Flexible NetFlow A version of NetFlow that allows you to customize traffic analysis parameters for your specific requirements.

flow cache A temporary storage location for captured flows.

flow exporter A component of Flexible NetFlow that identifies where captured flows will be exported to.

flow monitor A component of Flexible NetFlow that is applied to an interface to identify the applied flow record and flow exporter.

flow record A record that defines what will be captured when using Flexible NetFlow. Cisco IOS supports predefined records as well as user-defined records.

Forwarding Information Base (FIB) A CEF database that contains Layer 3 information, similar to the information found in an IP routing table. In addition, an FIB contains information about multicast routes and directly connected hosts.

front-door VRF A VRF that is used to isolate the encapsulating interface to prevent issues with recursive routing or identifying the outbound interface.

G

ge (prefix list) An indicator that the mask of a network must be greater than or equal to the specified value in order to be a match to the prefix list.

global unicast address A type of unicast IPv6 address that has been allocated from a range of public globally unique IP addresses, as registered through ICANN, its member agencies, and other registries or ISPs.

GRE tunnel A tunnel that supports a variety of protocols (IPv4, IPv6, DECnet, MPLS) over an IP-based network.

H

hello interval The frequency at which hello packets are advertised out an interface.

hello packet A packet that is sent out to detect neighbors for establishing adjacency and ensuring that neighbors are still available.

hello timer The amount of time between hello packets being advertised out an interface.

hold timer The amount of time required for a hello packet to be received for the neighbor to be deemed healthy. Upon receipt, the value resets and decrements towards zero.

I

iBGP session A BGP session maintained with BGP peers from the same autonomous system.

implicit deny An invisible entry at the end of an ACL, a prefix list, a route map, or a VACLs that automatically prevents all traffic or routes that do not match any of the earlier entries.

implicit permit An invisible permanent statement in an IPv6 ACL that comes before the implicit deny to allow ND traffic. The implicit permit statements are permit icmp any any nd-na and permit icmp any any nd-na.

ingress LSR A router at the edge of the MPLS domain that adds labels to packets that are entering the MPLS domain.

interarea route An OSPF route learned from an ABR from another area. Such routes are built based on Type 3 LSAs.

interface priority The reference value for an interface to deem preference for being elected as the designated router.

intermediate LSR A router that sits within the MPLS domain that primarily forwards packets using label information.

Internet service provider (ISP) An organization that provides Internet services to its customers.

intra-area route An OSPF route learned from routers within the same area. Such routes are built based on Type 1 and Type 2 LSAs.

IP SLA An IOS tool that you can use to test network connectivity and measure network performance.

IPv6 ACL An ACL that is used to identify IPv6 traffic based on multiple criteria, such as source and destination IP addresses, source and destination port numbers, protocols, and QoS parameters and either allow or prevent the traffic.

IPv6 neighbor discovery inspection/IPv6 snooping A security feature that learns and populates a binding table for stateless autoconfiguration addresses. It analyzes ND messages and places valid bindings in the binding table and drops all messages that do not have valid bindings. A valid ND message is one where the IPv6-to-MAC mapping can be verified.

K

K values A set of values that EIGRP uses to calculate the best path.

keychain A collection of one or more keys (that is, passwords) used for authentication, where each key has an associated key ID and key string.

key ID (keychain) The numeric value that identifies the key used for authentication.

key string (keychain) The alphanumeric string of characters that is being used for authentication. This is not to be confused with the name of the keychain.

L

label A 4-byte shim header added between the packet and frame headers that is used for forwarding the packet from router to router through the MPLS domain.

Label Distribution Protocol (LDP) A protocol used between MPLS-enabled routers to generate and exchange labels that will be used to forward packets in the MPLS domain.

Label Forwarding Information Base (LFIB) A data plane table that is used to forward labeled packets.

Label Information Base (LIB) A control plane table that stores label information.

label stack Two labels (VPN label and LDP label) added to a packet to forward the packet through the MPLS Layer 3 VPN.

label-switched path (LSP) The cumulative labeled path (sequence of routers) that a labeled packet takes through the MPLS domain.

label switching router (LSR) A router in an MPLS domain that forwards packets using label information.

LDP label See label.

le (prefix list) An indicator that the mask of a network must be less than or equal to the specified value in order to be a match to a prefix list.

Level 4 encryption On Cisco IOS devices, a type of encryption in which passwords are hashed using SHA 256.

Level 5 encryption On Cisco IOS devices, a type of encryption in which passwords are hashed using MD5.

Level 7 encryption On Cisco IOS devices, a type of encryption in which passwords are encrypted using a weak Type 7 encryption.

line A configuration mode that can be used to manage a Cisco IOS device (for example, a console line or a vty line).

link-local address A type of unicast IPv6 address that represents an interface on a single data link. Packets sent to a link-local address cross only that particular link and are never forwarded to other subnets by a router. Used for communications, such as neighbor discovery, that do not need to leave the local link.

link-state advertisement (LSA) A class of OSPF data structures that hold topology information. LSAs are held in memory in the LSDB and communicated over the network in LSU messages.

LOCAL_PREF A BGP path attribute that is communicated throughout a single AS to signify which route of multiple possible routes is the best route to be taken when leaving that AS. A larger value is considered to be better.

local AS community A BGP community that does not allow for network prefixes to be advertised to eBGP peers or another member AS.

Loc-RIB The main BGP table that contains all of the active BGP prefixes and path attributes that is used to select the best path and install routes into the RIB.

login A Cisco IOS command used on lines to define that authentication is required, using a line password, to access the line for management purposes.

login local A Cisco IOS command used on lines to define that authentication is required, using the local username and password database, to access the line for management purposes.

LSP See label-switched path (LSP).

LSR See label switching router (LSR).

M

maximum paths The number of paths that a router can use to load balance traffic.

MULTI_EXIT_DISC (MED) A BGP path attribute that allows routers in one autonomous system to set a value and advertise it into a neighboring AS, impacting the decision process in that neighboring autonomous system. A smaller value is considered better. Also called the BGP metric or multi-exit discriminator (MED).

method list In AAA authentication, a list of methods, such as a RADIUS server, the type of authentication, the local database, and the line passwords, that can be used to successfully authenticate. Typically listed in the sequence in which they will be performed.

metric With routing protocols, a measurement of favorability that determines which entry will be installed in a routing table if more than one router is advertising that exact network and mask with one routing protocol.

MPLS Layer 3 VPN A VPN that provides peer-to-peer connectivity between private customer sites across a shared network such as an ISP.

Multiprotocol BGP (MP-BGP) An updated version of BGPv4 that includes components supporting the routing of both IPv4 and IPv6 networks.

Multiprotocol Label Switching (MPLS) A switching method that uses labels to forward packets instead of the packets’ destination IP addresses.

multipoint redistribution A process in which redistribution occurs at multiple points between two different routing protocols.

mutual redistribution A process in which two routing protocols redistribute into each other in both directions on the same router.

N

named ACL An access list that identifies the various statements/entries in the ACL based on a name rather than a number.

named EIGRP An EIGRP configuration approach that allows you to configure all EIGRP commands under a single hierarchical configuration.

Neighbor Discovery (ND) A protocol used in IPv6 for many functions, including address autoconfiguration; duplicate address detection; router, neighbor, and prefix discovery; neighbor address resolution; and parameter discovery.

NetFlow A Cisco IOS feature that collects detailed information about traffic flows on routers and high-end switches. Collected information can optionally be sent to a NetFlow collector (flow exporter), which can produce reports about the traffic flows.

network command A command used to enable the RIPv2, EIGRP for IPv4, and OSPFv2 routing processes on an interface.

network LSA A Type 2 LSA that advertises the routers connected to the DR pseudonode. Type 2 LSA remains within the OSPF area of origination.

Next Hop Resolution Protocol (NHRP) A protocol that provides address resolution for hosts on nonbroadcast multi-access (NBMA) networks.

next-hop server (NHS) A server that is responsible for registering addresses and responding to any queries.

NHRP redirect An NHRP message that is sent toward the source spoke upon detecting the hairpinning network traffic out of the DMVPN tunnel interface.

NHRP shortcut The method of installing an NHRP learned route into the router’s global RIB.

No_Advertise A BGP community that does not allow for the network prefix to be advertised to any BGP peer (eBGP or iBGP).

No_Export community A BGP community that does not allow for the network prefix to be advertised to an eBGP peer. Prefix advertisement to another Member_AS is acceptable.

NSSA See OSPF not-so-stubby area (NSSA).

NSSA external LSA A Type 7 LSA that allows for an external route to exist in an OSPF totally NSSA or NSSA.

O

Object Tracking An IOS feature in which IOS repeatedly checks the current state of some item so that other items can then react to a change in that state. For example, Object Tracking can track the state of IP SLA operations, with static routes and policy routes reacting to a change in the Object Tracking feature.

offset list A list used for increasing the delay for received or advertised EIGRP routes.

optional non-transitive BGP path attributes that might be recognized by a BGP implementation and are not advertised between autonomous systems.

optional transitive BGP path attributes that might be recognized by a BGP implementation and are advertised between autonomous systems.

origin authentication Authentication that is accomplished through Pre-Shared Key (static) or certificate-based authentication (dynamic).

OSPF ABR See area border router (ABR).

OSPF area A group of routers and links, identified by a 32-bit area number, whose detailed topology information OSPF shares among all routers in the group. Routers inside an area learn full detailed topology information about the area; this detailed information is not advertised outside the area.

OSPF ASBR See autonomous system boundary router (ASBR).

OSPF interface table See interface table.

OSPF neighbor table See neighbor table.

OSPF link-state database See link-state database.

OSPF not-so-stubby area (NSSA) An OSPF area that does not allow external routes (Type 4 or Type 5 LSAs) in it. This area allows for routes to be redistributed into it.

OSPF stub area An OSPF area that does not allow for external routes (Type 4 or Type 5 LSAs) in it. A default route is advertised by the ABR in lieu of the blocked prefixes.

OSPF totally NSSA An OSPF area that does not allow for interarea or external routes (Type 3, Type 4, or Type 5 LSAs) in it. A default route is advertised by the ABR in lieu of the blocked prefixes. This area allows for routes to be redistributed into this area.

OSPF totally stubby area An OSPF area that does not allow for interarea or external routes (Type 3, Type 4, or Type 5 LSAs) in. A default route is advertised by the ABR in lieu of the block prefixes.

OSPFv3 The version of OSPF that supports IPv6 routing.

OSPFv3 ABR See area border router (ABR).

OSPFv3 area See OSPF area.

OSPFv3 ASBR See autonomous system boundary router (ASBR).

OSPFv3 interface table A table that lists all the interfaces participating in an OSPFv3 routing process.

OSPFv3 link-state database A table that lists all the LSAs that an OSPFv3 router is aware of.

OSPFv3 neighbor table A table that lists all the OSPFv3 neighbors that have been formed.

OSPFv3 address family See address family.

P

P router The provider’s routers inside an MPLS domain.

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

passive interface An interface that has been enabled with a routing protocol to advertise its associated interfaces into its RIB but that does not establish neighborship with other routers associated to that interface.

path vector routing protocol A routing protocol that selects the best path based on path attributes.

PE router A provider’s router connected to the CE router of the customer’s network.

peer group A feature that allows for the grouping of BGP peers based on similar BGP session information and outbound routing policy.

peer templates A feature that allows for the modular reuse of BGP settings between iBGP or eBGP peers.

periodic rekey The process of issuing new security keys between endpoints every specified time interval or within a specific volume of traffic.

Penultimate Hop Popping (PHP) An MPLS efficiency feature that allows the next-to-last router in the LSP to remove the label so the last router in the L.SP does not have to.

P-network The provider’s network in an MPLS domain.

Policy-based routing A method of forwarding packets down a different path, based on the characteristics of the traffic.

policy map A construct used with CoPP to associate the traffic class (as defined by the class map) with one or more policies resulting in a service policy.

port 22 The well-known port number used by SSH.

port 23 The well-known port number used by Telnet.

prefix list A list used to select routes based on binary patterns, specifically the high-order bit pattern, high-order bit count, and an optional prefix-length parameter.

proxy ARP A router feature used when a router sees an ARP request searching for an IP host’s MAC address when the router believes the IP host could not be on that LAN because the host is in another subnet. If the router has a route to reach the subnet where the ARP-determined host resides, the router replies to the ARP request with the router’s MAC address.

R

RA See router advertisement (RA).

RA Guard A feature that analyzes RAs and can filter out unwanted RAs from unauthorized devices.

RADIUS A standards-based protocol used to communicate with AAA servers.

recursive routing A routing loop for encapsulated interfaces in which the preferred path of the router is to take the tunnel rather than the underlay network. This then brings down the tunnel interface.

redistribution The process on a router of taking the routes from the IP routing table, as learned by one routing protocol, and injecting routes for those same subnets into another routing protocol.

regular expression (regex) A method of parsing and matching with search patterns using special key characters.

replay detection A method of sending sequence numbers to protect against hackers trying to capture and insert network traffic.

REPLY message A message that a DHCPv6 server sends to finalize the DHCPv6 addressing process.

reported distance The distance reported by a router to reach a prefix. The reported distance value is the feasible distance for the advertising router.

REQUEST message A message that a DHCPv6 client sends to a DHCPv6 server to confirm the addresses provided and any other parameters.

RIB See Routing Information Base (RIB).

route reflector A router that is configured to advertise routes learned from an iBGP peer to another iBGP peer.

route reflector client A router that receives routes that are advertised from a route reflector.

route tag A field in a route entry in a routing update that is used to associate a generic number with a route. It is used when passing routes between routing protocols, allowing an intermediate routing protocol to pass information about a route that is not natively defined to that intermediate routing protocol. Often used for identifying certain routes for filtering by a downstream routing process.

route map A feature used in BGP (and other IGP components) that allows for filtering or modification of routes using a variety of conditional matching.

router advertisement (RA) In IPv6, a message that an IPv6 router uses to send information about itself to nodes and other routers connected to that router.

router ID (RID) A 32-bit number that uniquely identifies the router in a routing domain.

router LSAs A Type 1 LSA that is a fundamental building block and represents all OSPF-enabled interface. Type 1 LSAs remain within the OSPF area of origination.

router solicitation (RS) An IPv6 message, part of the Neighbor Discovery Protocol (NDP), used by a host to request that the routers on the same data link announce their presence, IPv6 addresses, and all prefix/length combinations using an RA message.

Routing Information Base (RIB) The IP routing table.

routing loop A situation in which traffic is routed back in the direction that it came from or in a circular pattern through the network so that it never reaches the intended destination.

routing table The table a router uses to determine the most appropriate way to forward a packet.

S

Secure Shell (SSH) A secure protocol that can be used to remotely manage a Cisco IOS device.

security association (SA) A component of IPsec architecture that contains the security parameters agreed upon between the two endpoint devices.

seed metric A baseline value used by the destination protocol to allow for the calculation of a best path for that network prefix.

sequential protocol redistribution A method of redistribution of network prefixes between multiple routing protocols over a series of routers.

shortest path first tree (SPT) A router’s view of the topology to reach all destinations in the topology, where the local router is the top of the tree, and all the destinations are the branches of the tree.

single-point redistribution A situation in which redistribution occurs at a single point between two different routing protocols.

SNMPv2c A version of SNMP that uses community strings.

SNMPv3 A version of SNMP that can use hashing algorithms and encryption algorithms to enhance SNMP security.

SOLICIT message A message that a DHCPv6 client sends to locate DHCPv6 servers using the multicast address FF02::1:,2 which is the all DHCPv6 servers multicast address.

Source Guard A Layer 2 snooping interface feature for validating the source of IPv6 traffic.

source protocol A routing protocol and process to provide network prefixes to the routing protocol receiving the network prefixes.

split horizon A routing loop-prevention mechanism that prevents a route from being advertised out of the same interface on which it was learned.

split-horizon rule A loop-prevention mechanism that keeps iBGP routers from advertising BGP learned routes to other iBGP neighbors.

SSH See Secure Shell (SSH).

standard ACL A list of IOS global configuration commands that can match only a packet’s source IP address for the purpose of deciding which packets to discard and which ones to allow.

stateful DHCPv6 A term used in IPv6 to contrast with stateless DHCP. Stateful DHCP keeps track of which clients have been assigned which IPv6 addresses (state information).

stateless address autoconfiguration (SLAAC) A method used by an IPv6 host to determine its own IP address, without DHCPv6, by using NDP and the modified EUI-64 address format.

stateless DHCPv6 A term used in IPv6 to contrast with stateful DHCP. Stateless DHCP servers don’t lease IPv6 addresses to clients. Instead, they supply other useful information, such as DNS server IP addresses, but with no need to track information about the clients (state information).

static route A route manually configured by an administrator using the ip route or ipv6 route command.

stub See stub router.

stub area An OSPF area into which external (Type 5) LSAs are not introduced by its ABRs; instead, the ABRs originate and inject default routes into the area.

stub router A router running EIGRP that limits EIGRP DUAL algorithm computations and reduces the EIGRP query scope.

stuck in active (SIA) An event that occurs when an EIGRP query is sent to downstream neighbors and the router doesn’t receives a reply within 90 seconds.

subnets keyword A keyword used with OSPF so that classful and classless networks are redistributed.

successor The first next-hop router for the successor route.

successor route The route with the lowest-path metric to reach a destination.

summarization A method of reducing the routing table by advertising a less specific network prefix rather than multiple more specific network prefixes.

summary LSA A Type 3 LSA that contains the routes learned from another area. Type 3 LSAs are generated on ABRs.

syslog A system message log that is generated by a switch and can be collected locally or sent to and collected on a remote server.

T

TACACS+ A Cisco-proprietary protocol used to communicate with AAA servers.

Telnet An unsecure protocol that sends data in plaintext and that can be used to remotely manage a Cisco IOS device.

time-based ACL An access control list that can permit or deny defined traffic based on time of day and day of week.

time-to-live (TTL) A field in an IP header that is decremented at each pass through a Layer 3 forwarding device.

topology table A table used by EIGRP that maintains all network prefixes, advertising EIGRP neighbors for that prefix and path metrics for calculating the best path.

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

transit routing Routing in which traffic is allowed to flow from one external autonomous system through your autonomous system to reach a different external autonomous system.

time-to-live (TTL) In BGP, a value that identifies the lifetime of a BGP message in router hops. For eBGP peers, it is set to 1 by default, and for iBGP peers, it is set to 255 by default.

Type 5 LSA See external LSA.

U

Unicast Reverse Path Forwarding (uRPF) A security feature that helps limit or even eliminate spoofed IP packets on a network. This is accomplished by examining the source IP address of an ingress packet and determining whether it is valid. If it is valid, the packet is forwarded. If it is not valid, the packet is discarded.

V

variance value With EIGRP, the feasible distance (FD) for a route multiplied by the EIGRP variance multiplier. Any feasible successor’s FD with a metric below the EIGRP variance value is installed into the RIB.

virtual link A virtual tunnel that allows Area 0 to be extended further into the network and that is often used to resolve discontiguous networks.

virtual routing and forwarding (VRF) A router virtualization technology that allows you to create multiple routing tables on a single router and isolate them from each other.

VPN label A label used in an MPLS Layer 3 VPN to forward packets from one PE router to another PE router.

VPNv4 address An address made up of a route distinguisher (RD) and an IP address that is used in MPLS Layer 3 VPNs to ensure that customer information is unique within the MPLS domain.

VRF-Lite A method for creating multiple routing domains on the same routers in a network by using VRF.

W

weight A local Cisco-proprietary BGP attribute that is not advertised to any peers. A larger value is considered to be better.

well-known discretionary BGP path attributes recognized by all BGP implementations that may or may not be advertised to other peers.

well-known mandatory BGP path attributes recognized by all BGP implementations that must be advertised to other peers.

wide metrics A method of advertising and identifying interface speeds and delay that accounts for higher-bandwidth interfaces (20 Gbps and higher).