Stp Configurations - IBM 8271 Nways F12 User Manual

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HAPTER PANNING

STP Configurations

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Bridge A is selected by STP as the Root Bridge, because it has the
lowest Bridge Identifier. The Designated Bridge Port for LAN A is port 1
on Bridge A. Each of the other four bridges have a Root Port (the port
closest to the Root Bridge). Bridge X and Bridge B can offer the same
path cost to LAN B.
In this case Bridge B's port is chosen as the Designated Bridge Port,
because it has the lowest Bridge Identifier. Bridge C's port is chosen as
the Designated Bridge Port for LAN C because it offers the lowest Root
Path Cost (the route through Bridge C and B has a cost of 200, the
route through Bridge Y and B has a cost of 300). You can set the path
cost of a bridge port to influence the configuration of a network with a
duplicate path.
Once the network topology is stable, all the bridges listen for special
Hello BPDUs transmitted from the Root Bridge at regular intervals. If the
bridge does not receive a Hello BPDU after a certain interval (the Max
Age time), the bridge assumes that the Root Bridge, or a link between
itself and the Root Bridge, has gone down. The bridge then initiates a
reconfiguration of the network topology.
You can adjust timers to determine how quickly a network reconfigures
and therefore how rapidly the network recovers from a path failure
using an SNMP Network Manager.
Figure 6-5 shows two possible STP configurations using IBM 8271
Nways units:
Configuration 1 — Redundancy for Backbone Link
In this configuration, a Model E24 and a Model F24 both have STP
enabled and are connected by two links. STP discovers a duplicate
path and disables one of the links. If the enabled link breaks, the
disabled link becomes re-enabled, therefore maintaining connectivity.
Configuration 2 — Redundancy through Meshed Backbone
In this configuration, four Model F24 units are connected such that
there are multiple paths between each one. STP discovers the
duplicate paths and disables two of the links. If an enabled link
breaks, one of the disabled links becomes re-enabled, therefore
maintaining connectivity.