Term
STP intelligently chooses which ports
block, with two goals in mind: |
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Definition
■ All devices in a VLAN can send frames to all other devices. In other words, STP does not
block too many ports, cutting off some parts of the LAN from other parts.
■ Frames have a short life and do not loop around the network indefinitely. |
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Term
| Three Classes of Problems Caused by Not Using STP in Redundant LANs |
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Definition
Broadcast storms
MAC table instability
Multiple frame
transmission |
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Term
| STP uses three criteria to choose whether to put an interface in forwarding state: |
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Definition
■ STP elects a root switch. STP puts all working interfaces on the root switch in forwarding
state.
■ Each nonroot switch considers one of its ports to have the least administrative cost
between itself and the root switch. The cost is called that switch’s root cost. STP places
its port that is part of the least root cost path, called that switch’s root port (RP), in forwarding
state.
■ Many switches can attach to the same Ethernet segment, but in modern networks, normally
two switches connect to each link. The switch with the lowest root cost, as compared
with the other switches attached to the same link, is placed in forwarding state. |
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Term
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Definition
The STP bridge ID (BID) is an 8-byte value unique to each switch. The bridge ID consists
of a 2-byte priority field and a 6-byte system ID, with the system ID being based on a universal
(burned-in) MAC address in each switch. Using a burned-in MAC address ensures that
each switch’s bridge ID will be unique |
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Term
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Definition
STP defines messages called bridge protocol data units (BPDU), which switches use to
exchange information with each other. The most common BPDU, called a Hello BPDU, lists
many details, including the sending switch’s BID. By listing its own unique BID, switches can
tell which switch sent which Hello BPDU. |
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Term
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Definition
■ The lowest priority
■ If that ties, the lowest switch MAC address |
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Term
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Definition
| is based on a lower root cost to get to the root switch |
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Term
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Definition
The designated port (DP) on each LAN segment is the switch port that advertises
the lowest-cost Hello onto a LAN segment. When a nonroot switch forwards a Hello, the
nonroot switch sets the root cost field in the Hello to that switch’s cost to reach the root. In
effect, the switch with the lower cost to reach the root, among all switches connected to a
segment, becomes the DP on that segment. |
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Term
| The root switch Hello BPDU |
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Definition
| Sends every 2 seconds by defaults |
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Term
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Definition
| Default value 2 seconds.The time period between Hellos created by the root. |
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Term
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Definition
Default value 1o times Hello.How long any switch should wait, after ceasing to hear Hellos,
before trying to change the STP topology. |
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Term
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Definition
Default value 15 seconds.Delay that affects the process that occurs when an interface
changes from blocking state to forwarding state. A port stays in an
interim listening state, and then an interim learning state, for the
number of seconds defined by the forward delay timer. |
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Term
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Definition
■ Listening: Like the blocking state, the interface does not forward frames. The switch
removes old stale (unused) MAC table entries for which no frames are received from
each MAC address during this period. These stale MAC table entries could be the cause
of the temporary loops.
■ Learning: Interfaces in this state still do not forward frames, but the switch begins to
learn the MAC addresses of frames received on the interface. |
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Term
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Definition
■ It elects the root switch using the same parameters and tiebreakers.
■ It elects the root port on nonroot switches with the same rules.
■ It elects designated ports on each LAN segment with the same rules.
■ It places each port in either forwarding or blocking state, although RSTP calls the blocking
state the discarding state. |
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Term
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Definition
■ Adds a new mechanism to replace the root port, without any waiting to reach a forwarding
state (in some conditions)
■ Adds a new mechanism to replace a designated port, without any waiting to reach a forwarding
state (in some conditions)
■ Lowers waiting times for cases in which RSTP must wait. |
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Term
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Definition
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Term
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Definition
PortFast allows a switch to immediately transition from blocking to forwarding, bypassing
listening and learning states. However, the only ports on which you can safely enable
PortFast are ports on which you know that no bridges, switches, or other STP-speaking
devices are connected. Otherwise, using PortFast risks creating loops, the very thing that the
listening and learning states are intended to avoid.PortFast is most appropriate for connections to end-user devices. |
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Term
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Definition
The Cisco BPDU Guard feature helps defeat these kinds of problems by disabling a port
if any BPDUs are received on the port. So, this feature is particularly useful on ports that
should be used only as an access port and never connected to another switch. |
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