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OECG – Chapter 3 Definitions


CST – common spanning tree. A single STP instance applied to multiple VLANs

STP – spanning tree protocol, IEEE 802.1D. A protocol that runs on a layer 2 network with the purpose of maintaining a loop-free topology. Because loops are not our friend.

MST – multiple spanning tree, IEEE 802.1S. Uses 802.1W RSTP and allows for running spanning-trees in a trunked 802.1Q environment. Maps to regions. Within regions, STP instances are created in which you map VLAN’s to instances. An MST region appears as a single switch to switches outside the region. MST will run an IST (internal spanning-tree) to non-MST region members.

RSTP – rapid spanning tree, IEEE 802.1W. Uses BPDU’s as with 802.1D, but leverages some available bits to send messages allowing the network to converge faster. Does away with the listening state. Has a number of new “port state” concepts.

Hello timer – the interval between Hellos issued from the root bridge, 2 seconds by default.

Maxage timer – the interval a switch waits while not hearing Hellos before converging, 20 seconds by default.

Forward Delay timer – the interval between states, 15 seconds by default. Also the time used to age out CAM entries during a topology change.

blocking state – a port receiving Hellos, but not the least-cost path. This port does not forward.

forwarding state – a port that is transmitting and receiving traffic.

listening state – a port state unique to 802.1D where the port doesn’t send traffic and doesn’t learn MAC addresses, but does wait for STP convergence and CAM flushing
learning state – a port that’s learning MAC’s, but not sending/receiving traffic
disabled state – a port that one of us admin types has shut down
alternate state – an RSTP state, meaning a port that is eligible to be a root port. Not to be confused with your state of mind after the ingestion of banned narcotics.
discarding state – an 802.1W state that covers 802.1D’s disabled, blocking and listening.
backup state – an RSTP state, meaning a port that is eligible to be the designated port.

Root Port – the port on the switch with the lowest computed cost back to the root bridge.

Designated Port – the only port on a segment that forwards the best hellos onto that segment.

superior BPDU – a new Hello that would make the spanning-tree believe the originator should be the new root bridge.

PVST+ – per VLAN spanning-tree plus, a Cisco-proprietary method of running one spanning-tree instance per-VLAN through 802.1Q trunks.

UplinkFast – a Cisco-proprietary enhancement to STP that determines an alternate root port, and begins forwarding traffic through that root port immediately in the event the root fails. Also make the switch unlikely to become a transit switch by jacking up port costs and bridge priority.
BackboneFast – a Cisco-proprietary enhancement to STP that issues a root link query (RLQ) to determine by asking a neighbor the state of the root bridge immediately upon missing an expected Hello. This allows for faster convergence, because the switch proactively determines the state of root, rather than waiting for maxage to expire.

PortFast – a Cisco proprietary enhancement to STP that allows a port to transition to forwarding immediately upon linking up.

RootGuard – puts a port that receives a superior BPDU into a “loop inconsistent” state.

BPDUGuard – puts a port into err-disable state if receiving an unexpected BPDU.

UDLD – unidirectional link detection. Saves the planet by disabling a link if one half of the link is working and other is not. UDLD is a layer 2 messaging protocol. To be effective, both sides of the link should speak UDLD. Features normal and aggressive modes.

LoopGuard – puts a port that isn’t receiving the expected BPDU’s into a “loop-inconsistent” state.

LACP – IEEE 802.1AD. Link aggregation control protocol. A port-channeling standard.

PAgP – Cisco proprietary. Port Aggregation Protocol. A port-channeling method.

By Ethan Banks
Ethan Banks On productivity.

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