Cabletron Systems 2.2 Manual De Usuario
Bridging Basics
2-3
Bridging
possible bridge path to the end station. The original APE frame contains no
routing information (e.g., bridge numbers and ring numbers). As the frame is
propagated along all available paths to the destination station, each bridge along
the way adds its own bridge and ring numbers to the packetÕs RIF before
forwarding it, thereby providing route information.
routing information (e.g., bridge numbers and ring numbers). As the frame is
propagated along all available paths to the destination station, each bridge along
the way adds its own bridge and ring numbers to the packetÕs RIF before
forwarding it, thereby providing route information.
In response to each received APE packet, the destination station directs a reply to
the sending station. On receiving the replies, the sending station ideally assumes
that the Þrst returned reply contains the most efÞcient route. The sending station
then stores the route information and uses it to send subsequent transmissions to
the same station.
the sending station. On receiving the replies, the sending station ideally assumes
that the Þrst returned reply contains the most efÞcient route. The sending station
then stores the route information and uses it to send subsequent transmissions to
the same station.
Because APE frames do increase network trafÞc, some sites may use STE explorer
frames as an alternate method of route discovery. With STE exploration, a
Spanning Tree Algorithm (either conÞgured automatically via BPDUs or
manually via management) is maintained for the sole purpose of determining
how to direct an explorer frame during route discovery.
frames as an alternate method of route discovery. With STE exploration, a
Spanning Tree Algorithm (either conÞgured automatically via BPDUs or
manually via management) is maintained for the sole purpose of determining
how to direct an explorer frame during route discovery.
During the discovery process, a source station will send out STE explorer frames
into a bridged topology. If a bridge is in a forwarding state according to Spanning
Tree, it will forward an explorer frame onto its attached LAN segment (appending
the Bridge and LAN Segment IdentiÞers in the appropriate area of the RIF); if the
bridge is Þltering, it will discard the explorer frames. In this fashion, only a single
explorer frame will reach each individual LAN segment.
into a bridged topology. If a bridge is in a forwarding state according to Spanning
Tree, it will forward an explorer frame onto its attached LAN segment (appending
the Bridge and LAN Segment IdentiÞers in the appropriate area of the RIF); if the
bridge is Þltering, it will discard the explorer frames. In this fashion, only a single
explorer frame will reach each individual LAN segment.
Ultimately, the destination station will receive only a single STE packet, and will
respond with APE packets (that return to the sending station on all possible
bridge paths) or an STE packet (that returns to the sending station via in the
reverse route of the STE explorer packet).
respond with APE packets (that return to the sending station on all possible
bridge paths) or an STE packet (that returns to the sending station via in the
reverse route of the STE explorer packet).
Although the Spanning Tree Algorithm determines the bridge path an STE takes
to the destination station, during future communication between the stations,
bridges along the route will use Source Routing to forward the packet (i.e., the
bridges will read the Routing Information Field in the header of speciÞcally
routed frames to decide whether to forward them).
to the destination station, during future communication between the stations,
bridges along the route will use Source Routing to forward the packet (i.e., the
bridges will read the Routing Information Field in the header of speciÞcally
routed frames to decide whether to forward them).
About Source Route-Transparent Bridges
Because network topologies have developed in which bridges must be able to
handle network trafÞc from end stations which support source routing and others
which do not, a hybrid type of bridgeÑSource Route-Transparent (SRT) Ñ
combines elements of both bridging methods.
handle network trafÞc from end stations which support source routing and others
which do not, a hybrid type of bridgeÑSource Route-Transparent (SRT) Ñ
combines elements of both bridging methods.
An end stationÕs network drivers can be conÞgured in software to use a bit setting
in the source address portion of a data frame to indicate whether the station is to
operate in a Source Route or Transparently bridged network environment. The
Routing Information Indicator (RII) bit of the source address is set to 1 if the
station is to use Source Routing; if the station is to operate in a Transparently
bridged environment, the RII bit is left unchanged (i.e., at 0).
in the source address portion of a data frame to indicate whether the station is to
operate in a Source Route or Transparently bridged network environment. The
Routing Information Indicator (RII) bit of the source address is set to 1 if the
station is to use Source Routing; if the station is to operate in a Transparently
bridged environment, the RII bit is left unchanged (i.e., at 0).