Network Glossary S

SAFE Blueprint An architecture and set of documents that defines Cisco’s best recommendations for how to secure a network.

same-layer interaction The two computers use a protocol with which to communicate with the same layer on another computer. The protocol defined by each layer uses a header that is transmitted between the computers to communicate what each computer wants to do.

scheduler A queuing tool’s logic by which it selects the next packet to dequeue from its many queues.

sequence number (OSPF) In OSPF, a number assigned to each LSA, ranging from 0x80000001 and wrapping back around to 0x7FFFFFFF, which is used to determine which LSA is most recent.

sequence number A term used with WFQ for the number assigned to a packet as it is enqueued into a WFQ. WFQ schedules the currently lowest SN packet next.

Service Interworking The process, defined by FRF.5 and FRF.8, for combining ATM and FR technologies for an individual VC.

service policy A term referring to the MQC service-policy command, which is used to enable a policy map on an interface.

service set identifier Defines a particular wireless LAN. The SSID configured in the radio card must match the SSID in the access point before the station can connect with the access point.

Set In the context of SNMP, the Set command is sent by an SNMP manager, to an agent, requesting that the agent set a single identified variable to the stated value. The main purpose is to allow remote configuration and remote operation, such as shutting down an interface by using an SNMP Set of an interface state MIB variable.

SF See Superframe.

shaped mode The operating mode of shaped round-robin that provides a low-latency queue with policing.

shaped round-robin A packet-scheduling algorithm used in Cisco switches that provides similar behavior to CBWFQ in shared mode and polices in shaped mode.

shaping rate The rate at which a shaper limits the bits exiting the shaper.

shared distribution tree In PIM-SM, the path of the group traffic that flows from the RP to the routers that need the traffic. It is also called the root-path tree (RPT), because it is rooted at the RP.

shared mode The operating mode of shaped round-robin that provides behavior like CBWFQ with bandwidth allocated between different traffic classes by a relative amount rather than absolute percentage of the available bandwidth.

shortest-path tree switchover In the PIM-SM design, the process by which a PIM-SM router can build the SPT between itself and the source of a multicast group and take advantage of the most efficient path available from the source to the router as long as it has one directly connected group member. Once it builds an SPT, it sends a PIM-SM (S, G) RP-bit Prune toward the upstream router on the shared tree.

single-rate, three-color policer Policing in which a single rate is metered, and packets are placed into one of three categories (conform, exceed, or violate).

single-rate, two-color policer Policing in which a single rate is metered, and packets are placed into one of two categories (conform or exceed).

signal-to-noise ratio The difference between the measured signal power and the noise power that a particular receiver sees at a given time. Higher SNRs generally indicate better performance.

Slow Start A method for how a TCP sender grows its calculated CWND variable, thereby growing the allowed window for the connection. Slow Start grows CWND at an exponential rate.

Slow Start Threshold A calculated TCP variable, used along with the TCP CWND variable, to dictate a TCP sender’s behavior when it recognizes packet loss. As CWND grows after packet loss, the TCP sender increases CWND based on Slow Start rules, until CWND grows to be as high as the SSThresh setting, at which point TCP Congestion Avoidance logic is used. Essentially,

SSThresh is the threshold at which Slow Start logic ends.

SLSM See static length subnet masking.

SMI See Structure of Management Information.

Smoothed Round-Trip Time With EIGRP, a purposefully slowly changing measurement of round-trip time between neighbors, from which the EIGRP RTO is calculated.

smurf attack A style of attack in which an ICMP Echo is sent with a directed broadcast (subnet broadcast) destination IP address, and a source address of the host that is being attacked. The attack can result in the Echo reaching a large number of hosts, all of which reply by sending an Echo Reply to the host being attacked.

SN See sequence number.

SNMP agent A process on a computing device that accepts SNMP requests, responds with SNMP-structured MIB data, and initiates unsolicited Trap messages back to an SNMP management station.

SNMP manager A process on a computing device that issues requests for SNMP MIB variables from SNMP agents, receives and processes the MIB data, and accepts unsolicited Trap messages from SNMP agents.

SNR See signal-to-noise ratio.

socket A 3-tuple consisting of an IP address, port number, and transport layer protocol. TCP connections exist between a pair of sockets.

soft reconfiguration A BGP process by which a router reapplies routing policy configuration (route maps, filters, and the like) based on stored copies of sent and received BGP Updates.

software queue A queue created by Cisco IOS as a result of the configuration of a queuing tool.

solicited node multicast In IPv6, an address used in the Neighbor Discovery (ND) process. The format for these addresses is FF02::1:FF00:0000/104, and each IPv6 host must join the corresponding group for each of its unicast and anycast addresses. source DR A designated router that is directly connected with a source of the multicast group.

source registration In the PIM-SM design, the process by which a source DR, after it starts to receive the group traffic, encapsulates the multicast packets in the unicast packets and sends them to the RP.

source-based distribution tree Method by which a dense-mode routing protocol distributes multicast traffic from a source to all the segments of a network. Also called shortest-path tree (SPT), because it uses the shortest routing path from the source to the segments of the network.

source-specific addresses The range 232.0.0.0 through 232.255.255.255 that is allocated by IANA for SSM destination addresses and is reserved for use by source-specific applications and protocols.

source-specific multicast IGMPv3 is designed to support source filtering. IGMPv3 allows a host to indicate interest in receiving multicast packets only from specific source addresses, or from all but specific source addresses, sent to a particular multicast destination address.

sparse-mode protocol A multicast routing protocol that forwards the multicast traffic only when requested by a downstream router.

Spanning Tree Protocol Defined in IEEE 802.1d, a protocol used on LAN bridges and switches to dynamically define a logical network topology that allows all devices to be reached, but prevents the formation of loops.

SPF algorithm The algorithm used by OSPF and IS-IS to compute routes based on the LSDB.

SPF calculation The process of running the SPF algorithm against the LSDB, with the result being the determination of the current best route(s) to each subnet.

split horizon Instead of advertising all routes out a particular interface, the routing protocol omits the routes whose outgoing interface field matches the interface out which the update would be sent.

spread spectrum A technology that enables frequency reuse. Two variants exist: frequency hopping (FHSS) and direct sequence (DSSS). Both techniques spread the signal power over a relatively wide portion of the frequency spectrum over time, to reduce interference between systems.

SRR See shaped round-robin.

SRTT See Smoothed Round-Trip Time.

SSID See service set identifier.

SSM See source-specific multicast.

SSThresh See Slow Start Threshold.

stateful autoconfiguration A method of obtaining an IPv6 address that uses DHCPv6. See also stateless autoconfiguration.

stateless autoconfiguration 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. See also stateful autoconfiguration.

static length subnet masking A strategy for subnetting a classful network for which all masks/prefixes are the same value for all subnets of that one classful network.

sticky learning In switch port security, the process whereby the switch dynamically learns the MAC address(es) of the device(s) connected to a switch port, and then adds those addresses to the running configuration as allowed MAC addresses for port security.

storm control A Cisco switch feature that permits limiting traffic arriving at switch ports by percentage or absolute bandwidth. Separate thresholds are available per port for unicast, multicast, and broadcast traffic.

STP See Spanning Tree Protocol.

straight-through cable Copper cable with RJ-45 connectors in which the wire at pin 1 on one end is connected to pin 1 on the other end; the wire at pin 2 is connected to pin 2 on the other end; and so on.

strict priority A queuing scheduler’s logic by which, if a particular queue has packets in it, those packets always get serviced next.

Structure of Management Information The SNMP specifications, standardized in RFCs, defining the rules by which SNMP MIB variables should be defined.

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 network (OSPF) A network/subnet to which only one OSPF router is connected.

stub router (EIGRP) A router that should not be used to forward packets between other routers. Other routers will not send Query messages to a stub router.

stub router (OSPF) A router that should either permanently or temporarily not be used as a transit router. Can wait a certain time after OSPF process start, or after BGP notifies OSPF that BGP has converged, before ceasing to be a stub router.

stuck-in-active The condition in which a route has been in an EIGRP active state for longer than the router’s Active timer.

sub-AS The term referring to a group of iBGP routers in a confederation, with the group members being assigned a hidden ASN for the purposes of loop avoidance.

subnet A subset of a classful IP network, as defined by a subnet mask, which used to address IP hosts on the same Layer 2 network in much the same way as a classful network is used.

subnet broadcast address A single address in each subnet for which packets sent to this address will be broadcast to all hosts in the subnet. It is the highest numeric value in the range of IP addresses implied by a subnet number and prefix/mask.

subnet ID 16 bits between the interface ID and global routing prefix in an IPv6 global address, used for subnet assignment inside an enterprise.

subnet mask A dotted-decimal number used to help define the structure of an IP address. The binary 0s in the mask identify the host portion of an address, and the binary 1s identify either the combined network and subnet part (when thinking classfully) or the network prefix (when thinking classlessly).

subnet number A dotted-decimal number that represents a subnet. It is the lowest numeric value in the range of IP addresses implied by a subnet number and prefix/mask.

subnet zero When subnetting a class A, B, or C address, the subnet for which all subnet bits are binary 0.

successor route With EIGRP, the route to each destination for which the metric is the lowest of all known routes to that network.

summary route A route that is created to represent one or more smaller component routes, typically in an effort to reduce the size of routing and topology tables.

Superframe An early T1 framing standard.

superior BPDU Jargon used by STP mostly when discussing the root election process; refers to a Hello with a lower bridge ID. Sometimes refers to a Hello with the same bridge ID as another, but with better values for the tiebreakers in the election process.

supplicant The 802.1X driver that supplies a username/password prompt to the user and sends/receives the EAPoL messages.

SVC See switched virtual circuit.

switched interface An interface on a Cisco IOS–based switch that is treated as if it were an interface on a switch.

switched virtual circuit A VC that is set up dynamically when needed. An SVC can be equated to a dial-on-demand connection in concept.

synchronization In BGP, a feature in which BGP routes cannot be considered to be a best route to reach an NLRI unless that same prefix exists in the router’s IP routing table as learned via some IGP.