S

shortest path first.

See [SPF. shortest path first. An algorithm used by IS-IS and OSPF to calculate the shortest path tree to each reachable destination in the network.]

SIA. stuck-in-active.

A route in EIGRP that has been active for 3 minutes.

single point of failure.

Any point in a network where losing a single link or device can make some destinations (servers or end devices) unreachable.

Smooth Round Trip Time.

See [SRTT. Smooth Round Trip Time. A weighted average of the amount of time it takes for a packet to be acknowledged; used by EIGRP in determining how long to wait for an acknowledgement before taking further action.]

SONET. Synchronous Optical Network.

A redundant ring network media standardized by the CCITT.

source routing.

When the ingress device in a network (possibly a router, LSR, or the originating host) determines the best path through the network and uses labels or other fields to direct the packet along that path.

SPF. shortest path first.

An algorithm used by IS-IS and OSPF to calculate the shortest path tree to each reachable destination in the network.

spoofing.

Changing the source address of a packet so that it appears to be originating from a trusted host or so that the source of an attack cannot be traced.

SRTT. Smooth Round Trip Time.

A weighted average of the amount of time it takes for a packet to be acknowledged; used by EIGRP in determining how long to wait for an acknowledgement before taking further action.

stream.

A flow of packets between two devices.

stream merge

Combining two or more streams into one FEC.

stub site.

A site through which no traffic should flow; only traffic to and from the stub site should flow along links to and from the site.

stubby area.

An OSPF area into which no external routes (type 5 LSAs) are advertised.

stuck-in-active.

See [SIA. stuck-in-active. A route in EIGRP that has been active for 3 minutes.]

subnet.

In the original meaning, a part of a major network; currently, this term is used interchangeably with network.

subnet mask

See [mask. A set of four octets that separates the network portion of the IP address from the host portion of the IP address.]

suboptimal routing

Occurs when a router chooses a path through the network, which incurs extra hops or slower links than the best path.

successor.

The EIGRP neighbor this router is using to forward packets to a given destination.

summarize.

To combine multiple destinations, advertisements, or prefixes into one destination by shortening the subnet mask.

summary-address.

A command used to configure address summaries on interfaces in IOS.

SVC. switched virtual circuit.

A switched point-to-point link, common on ATM networks but also supported on other media, such as Frame Relay and X.25.

switched virtual circuit.

See [SVC. switched virtual circuit. A switched point-to-point link, common on ATM networks but also supported on other media, such as Frame Relay and X.25.]

Synchronous Optical Network.

See [SONET. Synchronous Optical Network. A redundant ring network media standardized by the CCITT.]