Page 634

Glossary

595

Spanning-tree frames called bridge protocol data units (BPDUs) are sent and
received by all switches in the network at regular intervals. The switches par-
ticipating in the spanning tree don't forward the frames; instead, they're pro-
cessed to determine the spanning-tree topology itself. Cisco Catalyst series
switches use STP 802.1d to perform this function. See also: BPDU, learning
bridge, MAC address, spanning tree, and spanning-tree algorithm.

SPF

Shortest Path First algorithm: A routing algorithm used to decide on

the shortest-path spanning tree. Sometimes called Dijkstra's algorithm and
frequently used in link-state routing algorithms. See also: link-state routing
algorithm.

SPID

Service Profile Identifier: A number assigned by service providers or

local telephone companies and assigned by administrators to a BRI port.
SPIDs are used to determine subscription services of a device connected via
ISDN. ISDN devices use SPID when accessing the telephone company switch
that initializes the link to a service provider.

split horizon

Useful for preventing routing loops, a type of distance-

vector routing rule where information about routes is prevented from
leaving the router interface through which that information was received.

spoofing

1) In dial-on-demand routing (DDR), where a circuit-switched

link is taken down to save toll charges when there is no traffic to be sent,
spoofing is a scheme used by routers that causes a host to treat an interface
as if it were functioning and supporting a session. The router pretends to
send "spoof" replies to keepalive messages from the host in an effort to con-
vince the host that the session is up and running. See also: DDR. 2) The
illegal act of sending a packet labeled with a false address, in order to deceive
network security mechanisms such as filters and access lists.

spooler

A management application that processes requests submitted to it

for execution in a sequential fashion from a queue. A good example is a print
spooler.

SPX

Sequenced Packet Exchange: A Novell NetWare transport protocol

that augments the datagram service provided by network layer (Layer 3)
protocols, it was derived from the Switch-to-Switch Protocol of the XNS
protocol suite.

www.sybex.com