Fundimental of spanning tree

Basic concepts

Basic concepts of spanning tree:
1, BPDU is the packet that used to communicated between switches
2, Root bridge is the switch which has lowest number of priority, mac address
3, Switch port can be root port, designated port, when a port is not a designated or root port it will be in blocking mode
4, ports statues: blocking, listening, learning, forwarding
5, hello interval is by default 2 secs, max age is 10 * hello interval, thus 20 sec by default,listening period: 15 sec; learning period: 15 sec
6, when tcn (topology change notification) happened,a blocking port will take 30 secs to 50 secs to turn to forwarding state depending on the topology change scenario

It is not always that a topology change will cause stp recalculation(root bridge re-selections), but all bridge who received tcn packet will age-out its CAM(mac address table) in 15 secs, in the meanwhile, blocked ports on the bridge will take 30 sec to 50 secs to go to forwarding state ( but not all blocked ports can necessarily go to forwarding state, it is possible that some blocked port will stay blocked even after topology change).

Spanning tree convergence

1, each switch declare self as root bridge by sending its own hello BPDU, BPDU will include bridge ip, priorty
2, Once switch received superior BPDU from peer, it will stop sending its own BPDU, instead it will relay this superior BPDU (with lower valude of priority.mac) by adding cost of interface.
interface cost 100M:19 10M:100
3, After root bridge is selected, root bridge will generate BPDU packet every 2 sec by default, other switches will relay this packet by adding cost.
4, The port from which BPDU is received will be selected as root port. If there are more than one ports receiving BPDU packets, the port that has the lowest cost (shortest path) will be selected as root port, the the other port will be blocked (alternative port)

Topology change in spanning tree

Topology change will in most cases not cause stp algorithm recalculation, only when root bridge is lost stp recalculation is triggered.

Scenario 1:Topology change on port-fast port (up or down)
Switch will not send out TCN (topology change notification)

Scenario 2: Topology chanage on NONROOT switch, linkdown on designate port in switch A
1, switch A will generate TCN bpdu packet, the send TSN through its root port
2, NONROOT switches who received TCN will send TCN up via its root port, and send TCA(acknowledge) back to the orignal port; at the same time, these switches will set cam timeout to 15sec (learning period)
3, Finally Root bridge will received TCN packet, it will generate topology change BPDU, and flood to the rest of the switches who has not got TCN packets yet.
4, All switches who received topology change packet will reset its CAM timeout to 15 sec (remove mac address from the table after 15 secs)
5, MAC address will be relearned immediately in most cases.

Scenario 3: Topology chanage on NONROOT switch,linkdown on root port in switch B
1, Switch B will delare it is root bridge by sending hello packets out to the rest of the ports.
2, The rest of the switches that is connecting to B but no other link towards root bridge will received BPDU from switch B, but no more BPDU from root bridge will be relayed to them. After MAX age timeout (10 * hello packet interval) 20 secs by default, these switches will acknowledge that root bridge is losted, they will restart spanning tree convergence. It will take max age (secs) + listening (15 secs) + learning (15 secs) for new convergence is in place.

spanning tree features

BPDU guard: Switch will set interface to err state when switch received BPDU from that interface

BPDU filter: Switch will drop the BPDU from the interface where BPDU filter in enabled, but will not put interface into err state

Root guard: Switch will put interface to err state when switch received BPDU from that interface, which is superior than the current root bridge.

portfast: Switch will not send TCS message when the interface with port-fast enabled has change from up to down or from down to up.

UPlinkfast & Backbone fast, will be described in separated page

loop guard will be described in separated page

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