beef410's version from 2018-05-21 01:57


Question Answer
STP IEEE802.1d
STP StatesBlocking Listening Learning Forwarding Disabled
RSTP StatesDiscarding Learning Forwarding - Backup / Alternate - Port will show STP states
Root PortThe port that receives the best BPDU on a bridge is the root port. This is the port that is the closest to the root bridge in terms of path cost
Loop PreventionSTP prevents L2 switching(bridging) loops
Root BridgeOriginates Hello BPDU's, decides timers,
BPDUOnly switches send BPDU. Config(Hello) BPDU / TCN top change notification
BIDBridge ID 2byte priority and 6byte MAC IE 32768:FF:FF:FF:FF:FF:FF. Default 32768 + vlan id
Root Bridge SelectionPrimary and secondary should be most powerful switches
Root Bridge ElectionLower BID wins, superior vs inferior BPDU. Until lower BID is received switch presumes itself Root. Continual process, lower BID new device will become Root
show spanning-tree vlanInt Role Sts Cost Priority Type, Root and Bridge info
LISPariticpates in root bridge election, cannot send or receive frames / learn MACs
LRNCan learn MACs cannot forward
FWDDoes everything
Hello Timedefault 2 sec, how often Root Bridge originates BPDU's
Forward Delaydefault 15 sec, length of LIS and LRN states. Aids in loop prevention / flapping ports
Max Agedefault 20 sec, how long switch keeps superior BPDU before discarding it. (how long it waits till it looks for another root)
Root Path CostIncrements at arrival FastE 19, Gig 4, 10gig 2, 10mbps 100
TiebreakerLowest Root BID -> Lowest Root pathcost to root bridge -> Lowest sender BID -> Lowest sender port ID
Port RoleDesg (usable) Altn (potential loop, used as alternate in blocking state)
PVSTAllows for per vlan load balancing <(config-if)# spanning vlan cost x>. Cisco proprietary, must use ISL trunking
PVST+Allows 802.1q trunking instead of only ISL.
PrimarySet switch as root. If previous root was >24576 new priority will be 24576 + vlan ID. If previous root was <24576 subtract 4096 from previous root's priority
PortFastFor access layer switches, STP lrn/lis can timeout DHCP. Skips lis/lrn straight to blk/fwd. Can apply to trunk ports, won't activate till port is made access.
(config)# spanning portfast defaultEnable PortFast on all access ports.
BPDUguard<(config-if)# spanning bpduguard enable>If a BPDU arrives the port will shutdown err-disable
RootGuard<(config-if)# spanning guard root>, if superior-than-root BPDU comes through port will enter Root Inconsistent state. Port stays up and operate normally if inferior BPDU comes through
(config)# spanning portfast bpduguard defaultEnable BPDU guard on all PortFast enabled interfaces
Edge PortRSTP considers a change topology relevant when a non-edge port enters forwarding
RSTP timersNo max age, port ages out after 3 missed hello BPDU's/6s
switch#sh spanning summVlan port totals w/ state and mode PVST/RPVST
P2PPoint to point - full duplex
Sharedpoint to point - half duplex
Loop guardIf a port stops seeing BPDU's its max age will expire and it will begin to transition to fwding. In absence of BPDU's STP will consider link loop-free. loopguard will put the port into inconsistent if it stops seeing hellos to prevent this. Port inconsistent is effectively equivalent to blocking.
(config-if)#spanning-tree guard loopenables loop guard on a per interface basis.
RSTP v STPWhen a top change occurs in STP it is reported to the root bridge then the root bridge updates its BPDU's. In RSTP the port floods top change BPDU's to all RSTP ports. This is why RSTP will resume in 6s while STP will resume in 30-60.
system ID extensionUsed in PVST to identify vlan

Path Costs

Question Answer
100Mbs Fast Ethernet19
10Mbs Ethernet100

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