ACK An acknowledgment of packets sent reliably.
Active EIGRP state when a network change is seen, but on interrogation of the topology table, there is no feasible successor. The router queries its neighbors for alternative routes.
adjacency State when a router understands a partner with whom it can exchange protocol trafﬁc. In link-state routing protocols, adjacencies are formed when two neighboring routers have exchanged Hellos. Each routing protocol has different rules about adjacencies. For example, in EIGRP, an adjacency is formed on receipt of a neighbor’s Hello. In IS-IS, it is local routing information that shows the reachability of a directly connected end system (ES) or intermediate system (IS). A separate adjacency is created for each neighbor on a circuit and for each level of routing (that is, Level 1 and Level 2) on a broadcast circuit.
adjacent neighbors A neighbor is a router that is directly connected to another router. They must also have same mask and Hello parameters on the connecting link. An adjacent router is a router that has exchanged routing information with its neighbor.
administrative distance A rating of the trustworthiness of a routing information source. The higher the value, the lower the rating of trust; it is an inverse rating. This is used when there are multiple routing protocols updating the IP routing table and when a method other than metric is required to select the best route.
Administrative Domain A group of routers that share the same routing protocol within one organization. Also known as an autonomous system.
advertised distance (AD) In EIGRP, it is the cost of the path to the remote network from the neighbor (the metric from the next-hop router).
aggregated route The consolidation of advertised addresses in a routing table. Aggregating routes reduces the number of routes in the routing table, the routing update trafﬁc, and overall router overhead. This is also called “route summarization.”
area A logical set of network segments and their attached devices. Areas are usually connected to other areas by means of routers, making up a single autonomous system. Used in DECnet,
IS-IS, and OSPF, it is a subdomain within an Administrative Domain. Routers in an area maintain detailed routing information about the area’s internal composition. The routers also maintain routing information that allows them to reach other areas. In IS-IS, the area address is contained in the NET and NSAP address.
Area Border Router (ABR) In OPSF, this router is responsible for connecting two or more areas. It holds a full topological database for each area to which it is connected and sends LDA updates between the areas. These LSA updates are summary updates of the subnets within an area. A similar function of connecting areas is provided by the Level 1-2 router in IS-IS.
attribute BGP parameters used to select the best path. In essence, the attributes are the metrics used by BGP. Examples of attributes are local-preference, weight, and autonomous system-path.
autonomous switching Feature on Cisco routers that provides faster packet processing by allowing the ciscoBus to switch packets independently without interrupting the system processor.
autonomous system Deﬁnition for the organizational boundary. A collection of networks under a common administration sharing a common routing strategy. Autonomous systems can be subdivided into areas. Within the terminology of the routing protocols, it deﬁnes all the routers within an administrative domain, where each router has full knowledge of the subnets within the domain. If you are connecting directly to the Internet using BGP, the autonomous system number must be unique and obtained from the Internet addressing committees.
backup designated router (BDR) In OSPF, this is the backup to the designated router (DR), in case the DR fails. The BDR performs none of the DR functions while the DR is operating correctly.
Berkeley Software Distribution (BSD) Term used to describe any of a variety of UNIX-type operating systems based on the UC Berkeley BSD operating system.
circuit In IS-IS, this is the local routing information for a single subnet point of attachment (SNPA).
Cisco Express Forwarding (CEF) Advanced, Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic trafﬁc patterns, such as the Internet, on networks characterized by intensive Web-based applications or interactive sessions.
classful routing protocols Routing protocols that do not transmit any information about the preﬁx length. A distance vector routing protocol that will not allow VLSM or route summarization. Examples are RIP and IGRP.
classless interdomain routing (CIDR) This is the means by which the Internet assigns blocks of addresses, typically Class C addresses, and summarizes them by using the preﬁx mask.
classless routing protocols Routing protocols that include the preﬁx length with routing updates; routers running classless routing protocols do not have to determine the preﬁx themselves. Classless routing protocols support VLSM.
cluster-ID In BGP, the cluster-ID is another attribute used in conﬁguring route reﬂectors. If the cluster has more than one route reﬂector, the cluster-ID is used to recognize updates from other route reﬂectors within the cluster.
Code/Length/Value (CLV) In IS-IS, these are the variable-length ﬁelds in a PDU. The Code ﬁeld speciﬁes the information in the Content ﬁeld as a number. The Length ﬁeld states the size of the Value ﬁeld. The Value ﬁeld contains the information itself.
complete sequence number packet (CSNP) In IS-IS, CSNPs describe every link in the link-state database. CSNPs are sent on point-to-point links when the link comes up to synchronize the linkstate databases. The designated router (DR), or designated intermediate system (DIS), on a multicast network sends out CSNPs every 10 seconds.
Connectionless Network Protocol (CLNP) This is the ISO protocol used to carry data and error indications at the network layer. CLNP is similar to IP and has no facilities to detect errors in data transmission. It relies on the transport layer to provide guaranteed data delivery.
Connectionless Network Service (CLNS) CLNS uses a datagram transfer service and does not require a circuit to be established before data is transmitted.
connection-oriented Software on two end nodes guarantees the transmission of network trafﬁc because a circuit setup is established before sending any data. It requires the use of sequencing, windowing, and acknowledgements.
convergence Speed of a group of internetworking devices running a speciﬁc routing protocol to agree on the topology of an internetwork after a change in that topology.
cost The metric for OSPF. It is not deﬁned in the standard with a value. Cisco uses the default of the inverse of bandwidth so that the higher the speed of the link, the lower the cost—and, therefore, the more attractive the path.
count to infinity Problem that can occur in routing algorithms that are slow to converge, in which routers continuously increment the hop count to particular networks. Typically, some arbitrary hop count limit is imposed to limit this problem.
database descriptor In OSPF, the database descriptor is referred to as DBDs or database descriptor packets (DDPs). BSCI literature uses the initialization DBD. These are packets exchanged between neighbors during the exchange state. The DBDs contain LSAs, which describe the links of every router in the neighbor’s topology table.
default routes A route that should be used if the destination network is not otherwise present in the routing table.
designated intermediate system (DIS) In IS-IS, the router (IS) on a LAN that is designated to perform additional duties. In particular, the DIS generates link-state PDUs on behalf of the LAN by treating the LAN as a pseudonode.
designated router (DR) In OSPF, this is the router responsible for making adjacencies with all neighbors on a multiaccess network, such as Ethernet or FDDI. The DR represents the multiaccess network, in that it ensures that every router on the link has the same topology database.
Diffusing Update Algorithm (DUAL) In EIGRP, this is the algorithm performed on the topology table to converge the network that provides loop-free operation at every instant throughout a route computation. This allows routers involved in a topology change to synchronize at the same time, while not involving routers that are unaffected by the change.
Dijkstra algorithm Routing algorithm that iterates on length of path to determine a set of shortest paths in a shortest-path tree. Commonly used in link-state routing algorithms. Sometimes called “shortest path ﬁrst algorithm.” This algorithm is used by OSPF.
distance-vector routing protocol Class of routing algorithms that have neighbors exchange routing tables and use the Bellman-Ford algorithm to ﬁnd shortest paths. Distance vector routing algorithms can be prone to routing loops but are computationally simpler and require fewer resources than link-state routing algorithms.
distribute list An access list that is applied to the routing protocol. It is used to control routing updates by ﬁltering out those routes that are not to be propagated. This is particularly useful in preventing routing loops in redistributed networks.
Domain Name System (DNS) System used on the Internet for translating names of network nodes into IP addresses.
dot address Refers to the common notation for IP addresses in the form n.n.n.n, where each number n represents, in decimal, 1 byte of the 4-byte IP address. This is also called “dotted notation” or “four-part dotted notation.”
dotted decimal notation Syntactic representation for a 32-bit integer that consists of four 8-bit numbers written in base 10 with periods (dots) separating them. It is used to represent IP addresses on the Internet, as in 18.104.22.168. This is also called “dotted quad notation.”
Dual IS-IS IS-IS that supports both OSI and IP routing information. Areas within the autonomous system can run OSI, IP, or both. However, the conﬁguration chosen must be consistent within the entire area.
Dynamic Host Configuration Protocol (DHCP) Provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them.
dynamic routes Automatic rerouting of trafﬁc based on sensing and analyzing current actual network conditions, not including cases of routing decisions taken on predeﬁned information. end node A device that is connected to the network.
end system (ES) The end node or host, which has limited routing capabilities. The ES has the OSI or IP Layer 3 protocol running and can receive and send data.
End System-to-Intermediate System (ES-IS) In IS-IS, this is the protocol by which the OSI ES and the IS communicate to dynamically learn Layer 2 adjacencies.
exchange state In OSPF, this is the method by which two neighboring routers discover the map of the network. When these routers become adjacent, they must ﬁrst exchange DBDs to ensure that they have the same topology table.
exstart In OSPF, this is the state in which the neighboring routers determine the sequence number of the DBDs and establish the master/slave relationship.
Exterior Gateway Protocol (EGP) Protocol that runs between autonomous systems. There is also a protocol with this name that was the precursor to BGP.
exterior routing A routing protocol used to exchange information between autonomous systems or organizations, used to connect organizations into the Internet. BGP and EGP are examples of exterior routing protocols.
external BGP (eBGP or eBGP-4) When BGP is used to connect different autonomous systems. fast switching A cache in the Cisco router that contains routing decisions. After the routing decision for a packet has been made, it can be cached in any one of a variety of caches. This means that the forwarding of trafﬁc through the router is greatly enhanced.
feasible condition (FC) In EIGRP, this is when a neighbor reports a path cost (AD) that is lower than the router’s FD to a network.
feasible distance (FD) In EIGRP, this describes the lowest-cost distance (metric) to a remote network. feasible successor (FS) A term used by EIGRP to describe a next-hop router with a path to the remote network that EIGRP considers a viable route, one guaranteed to be loop-free. The router that is an FS must report an AD lower than the FD of the router, thus meeting the FC.
first octet rule The mechanism by which the Layer 3 device identiﬁes the class of IP address. If the protocol is a classful protocol, the ﬁrst octet rule is the only means available to determine the network portion of an address to which it is not directly connected.
flash update A routing update sent asynchronously in response to a change in the network topology. If there is a change in the metric, the update is sent immediately without waiting for the update timer to expire. Sometimes known as triggered updates.
floating static route A ﬂoating static route is a route that, although manually conﬁgured, has been identiﬁed as a route to choose only if the dynamically learned routes fail. These routes need to have a higher administrative distance than the dynamically learned route so that they can be overridden by dynamically learned routing information. This is used to create a DDR backup to an existing link.
flooding A trafﬁc-passing technique used by switches and bridges in which trafﬁc received on an interface is sent out to all the interfaces of that device, except the interface on which the information was originally received.
Hello Hello packets are used to discover and maintain adjacencies. Messages are used to ﬁnd and maintain neighbors in the topology table. They are sent periodically and unreliably.
hierarchical design Cisco recommends that a hierarchical design be used when designing large networks. The complex problem of routing in a large network is simpliﬁed by breaking the network into a hierarchy of networks, where each level is responsible for its own routing. Cisco uses three levels, which it calls the access, distribution, and core levels.
holdtime Value set in the Hello packet. It determines how long the router waits for Hellos from a neighbor before declaring the neighbor unavailable. This information is held in the neighbor table.
host address This is a subset of the ISO NET address, which includes the domain, area, and system IDs.
IANA Address This is a classful address allocated by the IANA.
incremental update A routing update that is sent only when there is a change in the topology, not periodically when a timer expires.
Init An OSPF state in which a Hello packet has been sent from the router, which is waiting for a reply to establish two-way communication.
Integrated IS-IS Another term for Dual IS-IS. Indicates IS-IS can be used to support routing for two Layer 3 protocols (IP and CLNP) in the same network simultaneously.
Interior Gateway Protocol (IGP) In the past, the term “gateway” was used to deﬁne a router. This is a routing protocol that runs within an autonomous system.
interior routing protocol A protocol used to route information between routers within an autonomous system or organization.
intermediate system (IS) A router. The IS is a device capable of directing trafﬁc to remote destinations.
Intermediate System-to-Intermediate System (IS-IS) The OSI routing protocol that learns the location of the networks within the autonomous system so that data can be forwarded to the remote hosts.
internal BGP (iBGP or iBGP-4) When BGP is used to connect routers resident in the same autonomous system.
internal router An OSPF router that has all its interfaces in the same area.
internet Short for internetwork. Not to be confused with the Internet. See also internetwork.
Internet The Internet is commonly deﬁned as a network of networks. It is a worldwide mesh of hundreds of thousands of networks, owned and operated by millions of people all over the world, all connected to thousands of ISPs. It provides the World Wide Web (WWW), a series of Web pages offering a range of services and information.
Internet Assigned Numbers Authority (IANA) Responsible for address allocation in the Internet.
internetwork A collection of networks interconnected by routers and other devices that functions (generally) as a single network. It is sometimes called an internet, which is not to be confused with the Internet.
IPv4 IP version 4 is the current version of IP that is used extensively throughout the world. It is deﬁned in RFC 791.
IPv6 IP version 6 is a replacement for the current version of IP (version 4). IPv6 includes support for ﬂow ID in the packet header, which can be used to identify ﬂows. IPv6 used to be called “IPng (next generation).”
IS-IS domain A group of routers running the IS-IS protocols for exchanging routing information.
Layer 3 switching Used in the context of VLANs, the mechanism by which a switch will route between VLANs. It also refers to routers, when the routing decision has been made and the result has been cached. The subsequent lookup involves switching (for example, fast switching), but on a Layer 3 decision.
Level 1 (L1) Used in IS-IS to reference a router or a link. These links or routers are internal to the area. The routers receive routing information for their area only and have no knowledge of the other areas’ networks. To reach other areas, Level 1 routers maintain a default route to the nearest Level 2 router.
Level 1-2 (L1-2) Used in IS-IS to reference a router or a link that connects areas. This router connects a Level 1 area to the Level 2 backbone. It has a Level 1 routing table to route to ES and IS in its own area by system ID. It maintains a Level 2 preﬁx table to route to other areas.
Level 2 (L2) Used in IS-IS to reference a link or a router. These routers are connected only to the backbone and provide transit trafﬁc between areas.
link A physical connection to a neighbor. This link is then transmitted to all the other routers in the area via the LSP.
link-state advertisement (LSA) In OSPF, this is a packet describing a router’s links and the state of those links. There are different types of LSAs to describe the different types of links. It is a broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables.
link-state database In OSPF, this is the raw material for constructing a topology map. It is a copy of every LSA in the area, giving a list of every router, its links, and the state of the links. From this information, it can create a SPF tree of every network and every path to each network.
link-state packet (LSP) In IS-IS, this is a packet that describes a router’s links. There are separate LSPs for Level 1 and Level 2 updates.
link-state request (LSR) In IS-IS, this is when the router receives a CSNP complete with LSP, and it compares the LSP against the topological database. If either the LSP entry is not present or the entry is older than the DBD, the router will request further information via an LSR.
link-state routing algorithm A routing algorithm in which each router broadcasts or multicasts information regarding the cost of reaching each of its neighbors to all nodes in the internetwork. Compare with distance vector routing protocol.
link-state update (LSU) Update sent in response to the LSR. It is the LSP that was requested.
logical AND The operation where two bits are compared and the result is TRUE only if both output bits are TRUE. The mechanism by which a subnet is derived from an IP host address. The router ANDs the subnet mask, in binary, onto the host address, in binary. The result of the logical AND is the subnetaddress.
neighbor A router on the same link with which an adjacency is formed and routing information is then exchanged.
neighbor table An EIGRP list of every neighbor, including the IP address, the outgoing interface, the holdtime, smooth round-trip time (SRTT), and uptime, or how long since the neighbor was added to the table. This table is built from information on Hellos received from adjacent routers (neighbors).
NetFlow A Cisco solution that enhances the speed of transmission by caching routing decisions.
Network Address Translation (NAT) Mechanism for reducing the need for globally unique IP addresses. NAT allows an organization with addresses that are not globally unique to connect to the Internet by translating those addresses into globally routable address space.
network congestion A condition in which excessive trafﬁc on the network is the cause of delays and packet loss.
network entity title (NET) In IS-IS, this is the ISO address of the system, but not to the process destination within the system. The NET describes both the area and system ID of a system in the IS-IS network but excludes the NSEL, which is set to 0×00. If the NSEL identiﬁes the process within the system, the ISO address is called the “NSAP address.”
network protocol data unit (NPDU) See protocol data unit (PDU).
network selector (NSEL) In IS-IS, this is sometimes referred to as the SEL ﬁeld. This ﬁeld describes the service at the network layer to which the packet is to be sent. NSEL is similar to the Protocol ﬁeld in IP.
network service access point (NSAP) In IS-IS, this describes a service at the network layer to which the packet is to be directed. The NSAP is the NET address with the NSEL ﬁeld set to a positive value, a value other than 0×00.
originator ID This is a BGP attribute. It is an optional nontransitive attribute that is created by the route reﬂector. The attribute contains the router ID of the router that originated the route in the update. The purpose of this attribute is to prevent a routing loop. If the originating router receives its own update, it ignores the update.
Overload (OL) bit In IS-IS, the OL is set on an LSP if the router cannot store the entire link-state database. When other routers receive LSPs with this bit set, they will not send the router any transit trafﬁc for fear that its routing table is incomplete. If the router is making decisions using incomplete data, its decisions might result in suboptimal paths or even routing loops. Trafﬁc destined for the router can still be sent to the directly connected interfaces of a router transmitting the OL bit in its LSPs.
partial sequence number packet (PSNP) In IS-IS, PSNPs are sent on point-to-point links to explicitly acknowledge each LSP the router receives. A router on a broadcast subnetwork sends a PSNP requesting the LSPs it needs to synchronize its link-state database.
passive In EIGRP, an operational route is passive. If the path is lost, the router examines the topology table to ﬁnd an FS. If there is an FS, it is placed in the routing table; otherwise, the router queries its neighbors, sending the route into active mode.
poison reverse Routing updates that speciﬁcally indicate that a network or subnet is unreachable, rather than implying that a network is unreachable by not including it in updates. It is used to avoid routing loops.
prefix list The preﬁx list is used to control how BGP learns or advertises updates. These replace distribute lists for BGP.
prefix mask The preﬁx mask identiﬁes the number of bits in the subnet mask. It is written in the /xx format after the address. It is used in supernetting and route aggregation.
Priority A Cisco tool for OSPF allowing the designated router to be manually elected or, conversely, prevented from taking part in the DR/BDR election.
private addressing Private addressing is the means by which an organization can address its network without using a registered address from the Internet. This saves considerable address space on the Internet and eases restrictions within the organization. These addresses are deﬁned in RFC 1918. They include networks 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
protocol data unit (PDU) A unit of data passed from one layer of the OSI model to the same level of the OSI model on another node. Each layer preﬁxes the PDU to indicate the sending OSI layer, so the network layer sends NPDUs and the data-link layer sends DLPDUs.
pseudonode In IS-IS, this is the LAN identiﬁer for a broadcast subnetwork. The pseudonode makes the broadcast medium appear as a virtual router and the routers appear as connected interfaces. The adjacency to a pseudonode is represented by the system ID of the DIS plus the circuit ID of the link. The pseudonode reduces the amount of resources needed to manage the adjacencies, because the DIS generates link-state PDUs on behalf of the pseudonode. These LSPs are sent to all the connected ISs. The routers connected to the broadcast medium no longer have to maintain the adjacencies to all other routers on the medium (thus reducing memory, CPU, and bandwidth
query Sent from the EIGRP router when it loses a path to a network. If there is no alternate route (feasible successor), it will send out queries to neighbors inquiring whether they have a feasible successor. This makes the route state change to active. The queries are sent reliably.
query scoping A technique where query propagation is limited to avoid Stuck in Active (SIA).
redistribution Allowing routing information discovered through one routing protocol to be distributed in the update messages of another routing protocol. It is the process of exchanging routing updates between different routing protocols. This can be done only between protocols that support the same protocol suite at Layer 3, for example, EIGRP and OSPF for TCP/IP.
Reliable Transport Protocol (RTP) In EIGRP, this is a mechanism used to determine requirements that the packets be delivered in sequence and are guaranteed.
reply A response to the query. If a router has no information to send in a reply, it will send queries to all its neighbors.
Retransmission Timeout (RTO) In EIGRP, timer calculated in reference to the SRTT. RTO determines how long the router waits for an ACK before retransmitting the packet.
route reflector This is the BGP router that ignores the BGP spilt-horizon rule and is conﬁgured to forward routes from other identiﬁed iBGP clients. This removes the necessity for a fully meshed iBGP network, which preserves network resources.
route reflector client A client for which a route reﬂector ignores the BGP split-horizon rule. It is a router that has a TCP session with its iBGP peer. It forwards routes to the route reﬂector, which propagates them on to other routers. The client does not have peer connections with other clients.
route reflector cluster A cluster is the group of a route reﬂector and clients. There can be more than one route reﬂector in a cluster.
route table The routing table for EIGRP. A path is moved from the topology table to the routing table when a feasible successor is identiﬁed. See also routing table.
routed protocol Protocol that carries data and can be routed by a router. A router must be capable of interpreting the logical internetwork as speciﬁed by that routed protocol. Examples of routed protocols include AppleTalk, DECnet, IPX, and IP.
Routeing Domain Routeing Domain is the same as the Administrative Domain. It deﬁnes the boundaries of a network of interconnected routers operated and managed by the same administrative group.
routing function Process of ﬁnding a path to a destination host. Routing is very complex in large networks because of the many potential intermediate destinations that a packet might traverse before reaching its destination host.
routing loop A loop in which the routing information is fed back to the originating router as if from another router. This often happens when redistribution is conﬁgured. It can lead to confusion in the network because when the originating router loses the route, it might believe that there is an alternative path. Occurs when routers have misinformation about the network and, instead of sending trafﬁc to the destination, pass the packets between themselves in the belief that the other router knows the path.
routing protocol Protocol that accomplishes routing through the implementation of a speciﬁc routing algorithm. Examples of routing protocols include IGRP, OSPF, and RIP.
routing table Table stored in a router or some other internetworking device that keeps track of the available networks, the best paths to those destinations, and in some cases, metrics associated with those routes.
runaway congestion A condition in which the results of network congestion cause the network to generate more trafﬁc and compound the problem.
seed metric The metric that is given to a route when it enters the routing protocol. Most routes start with a metric of 0 because they ﬁrst become known to the routing protocol to which they are directly connected. However, if they are redistributed into the routing protocol, there is no starting point from which to increment the route metric. Therefore, the default metric is conﬁgured to provide a seed metric for the redistributed routes.
SEL See network selector (NSEL)
sequence number PDU (SNP) In IS-IS, SNPs are used to acknowledge the receipt of LSPs and to synchronize link-state databases.
server A node or software program that provides services to clients.
ships in the night (SIN) Routing protocols that do not interact.
silicon switching Switching based on the Silicon Switch Engine (SSE), which allows the processing of packets independent of the Silicon Switch Processor (SSP) system processor. Silicon switching provides high-speed, dedicated packet switching.
smooth round-trip time (SRTT) In EIGRP, this is the time that the router waits after sending a packet reliably to hear an acknowledgment. This is held in the neighbor table and is used to calculate the RTO.
SPF tree A tree of the topological network. It can be drawn after the SPF algorithm has been run. The algorithm prunes the database of alternative paths and creates a loop-free shortest path to all networks. The router is at the root of the network, which is perceived from its perspective.
split horizon Routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops.
static route A route that is explicitly conﬁgured and entered into the routing table.
stub area This is an OSPF area that does not accept external summary routes. A default route is used to direct trafﬁc to networks unknown in the stub area. This conﬁguration allows for small routing tables and limited SPF calculation and thus a limitation on the amount of resources required.
Stuck in Active (SIA) In EIGRP, state reached when a router has sent out queries and is waiting for ACKs from all its neighbors. The route is active until all the ACKs have been received; if they do not appear after a certain time, the router is SIA for the route.
subnet mask A 32-bit number that is associated with an IP address; each bit in the subnet mask indicates how to interpret the corresponding bit in the IP address. In binary, a subnet mask bit of 1 indicates that the corresponding bit in the IP address is a network or subnet bit; a subnet mask bit of 0 indicates that the corresponding bit in the IP address is a host bit. The subnet mask then indicates how many bits have been borrowed from the host ﬁeld for the subnet ﬁeld. It sometimes is referred to simply as “mask.”
subnetwork The data-link layer.
subnetwork dependent layer Interfaces with the data-link layer and hides the different kinds of data-link layers from the network layer. This sublayer transmits and receives PDUs from the subnetwork, translates DLPDUs into NPDUs, and hands them to the appropriate OSI process. The subnetwork dependent layer is also responsible for creating and maintaining adjacencies through the exchange of IS-IS Hello PDUs.
subnetwork independent layer Interfaces with the transport layer and provides it with network services. It describes how CLNS creates and maintains knowledge of the network by exchanging and processing routing information so that data can be transmitted efﬁciently to remote destination hosts and handed to the transport layer.
subnetwork point of attachment (SNPA) The data-link layer offers two services: the physical connection to the medium and the services offered to the physical layer and network layer. The SNPA refers to these services. The SNPA address is the physical address (for example, the MAC address on a LAN).
suboptimal path A path that is not the best path. Sometimes a less desirable path is chosen.
successor The next-hop router that passes the FC. It is chosen from the FSs as having the lowest metric to the remote network.
summarization When subnets or IANA addresses are included in a larger address, where the address is written with a preﬁx or mask that includes the subnets. For example, 172.16.128.0/24– 172.16.240.0/24 is represented by the subnet 172.16.128.0/17.
supernet A summarization of classful networks. For example, a group of Class C addresses 22.214.171.124 through 126.96.36.199 could be summarized into the address 188.8.131.52 with a mask of 255.255.240.0 (/20).
switching function Forwarding packets from an inbound interface to an outbound interface.
synchronization rule This BGP rule states that a router cannot forward a route to an eBGP peer unless the route is in its local IP routing table. This requires that the IGP and BGP routing tables are synchronized. This is to prevent BGP from advertising routes that the autonomous system cannot direct to the destination. BGP synchronization is on by default but is commonly turned off.
topology table In EIGRP, a table that contains all the paths advertised by neighbors to all the known networks. This is a list of all the successors, feasible successors, the feasible distance, the advertised distance, and the outgoing interface. DUAL acts on the topology table to determine successors and feasible successors by which to build a routing table.
triggered update See flash update.
Type/Length/Value (TLV) TLV is the same as a CLV, but some literature refers to the variablelength ﬁelds as Type/Length/Value in accordance with the IP terminology.
update An EIGRP packet containing change information about the network. It is sent reliably. It is sent only when there is a change in the network to affected routers.
variable-length subnet mask (VLSM) The capability to specify a different subnet mask for the same network number on different subnets. VLSM can help optimize available address space. Some protocols do not allow the use of VLSM. See also classless routing protocols.
virtual LAN (VLAN) A logical (rather than physical) grouping of devices into a common broadcast domain. A VLAN is deﬁned on a LAN switch and is applied to the appropriate switch ports. This means that the devices associated with the logical network do not have to be geographically local to one another.