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3.Intro to the Ethernet Switching STP | RSTP Layer 2 - Coggle Diagram

- - - - Brodcast Storm
        swtching loops allow for broadcast storms to occur and duplication of frames to be recvied
        for example seding a broadcast to a SWs triangle from SWB for example will cause it to forward that broadcasr on all its interfaces to SWA and SWC which themselfs will forward thoses broadcast to themselfs and to rest of Net
        reapeating again and again >> causing Broadcast Storms that will not stop until all links bandwidth got used up
      - MAC instability
        Recving previously forwaded frames generates false MAC entries and intability within the MAC address table
        for example seding a broadcast to a SWs triangle from SWB for example which will learn the MAC address of that host through its interface > the broacast will make its way through the other SWs all the way back to SWB from another another interface >> casing the SWB to see that prev item MAC address with the 1st interface is wrong soo its deleted and new item is created to the mac address table with last interface and keeps happening on either one of the interfaces each time > keeping the SWB busy deleting and adding
  - - - Selecting the Root
        
        BPDU Bridge Protocol Data Unit
        when selecting the Root Sws need to negotiate each other > for that negotiation information sent is BPDU
        BPDU uses the IEEE 802.3 standard frame type
        
        part 3 > Timer
        BPDU are discarded when Message age exceeds Max age
        
        Max Age > represents the aging timer og BPDU
        MAX AGE is 20 (20secs) > means if the BPDU being forwarded among 20 switches > gets deleted (bigger Sw Nets are not recommanded (7 Net diamter is recommanded ))
        Eg > 2 hops(MAS Age) + 18 secs >> BPDU packet deleted > means it should be recalculated
        MAX Age rep the aging timer if a BPDU
        BPDU are discarded when Message Age Exceeds Max Afe
        
        Message Age (increase +1 each time it pass a switch) .> the aging timer of BPDU
        BPDU are discarded when message Age exceds MAX Age
        as we know the BPDU negotiate with the switches in a BPDU interval of 2 seconds which can be changed
        mwssage Age means > it takes time for each switch to recv the BPDU packet and process amd send it out >> cannot be accurately calculated soo we assign one Second >> for one Hop of the Switch (when the 1st switch send it out(root switch) > MSG AGE is 0 gets increased by 1sec for each switch we pass in the level)
        
        Hello Time
        
        Fwd Delay
        Another Timer is the Forwards delay > when switches calculate which is the best path > and some links sould be blocked > it needs time
        the BPDU sent out and in 2 seconds switches should negotiate
        For example if the root bridge sends out a message > in worst conditions itl take the longest path means the lowest child of the tree > lets say its secong hop after root which means 2sec * 2 = 4secs
        Meaning the calculations of the best path needs time >> the IEEE assigns a value of 15 secs for the best path to be calculated
        Switch should wait 15 secs to forward data >> this Timer we call it forwarding Delay
        
        part 1
        
        PID > ProtoID
        
        PVI > ProtoVersion ID
        
        BPDU Type
        
        Flags
        
        part 2 used to know which path is the best path
        
        Root ID > whhos the root
        
        Bridge ID > which switch is rsending the BPDU
        
        Priority
        
        Mac Address
        
        Port ID > which port is sending BPDU
        
        where the root path cost is equal , > a port identifier is used to determine the active alternate paths to the root
        
        RPC > root path cost > from switch to root
        
        BID Bridge ID (electing the root bridge > best path)
        
        BID are used by the STP to identify who you are (each switch)
        Bridge identifiers are used to elect the root bridge
        bridge priority can be manipulated to force root selection
        
        Mac Address
        
        Priority Value - 0>64535(64k) - 2B
        step value of priority is 4k - not all values can be used
        max used is 61440 | min is 0
        lowset 12bits of 2B cannot be used > always put to 0
        By Default each SW have the Same Pvalue > 32768 (32k)
        Smaller P wins (will be the Root Bridge)
        
        Root Election Process
        
        Switches Powered on and they will negotiate each other with the BPDU and then compare the BID in each other to decide who is the smallest value (compare priority > then mac address)
        Afther this exchanfe switch with smallest value become the Root
        All STP SWs advertise BPDU to peers with self as root
      - calculate the best path to the root
        we know switches calculate best path to root > root selection based on BID (smallest BID wins) then the other non-root switches calclate the best path to the root using some params
        
        Spanning Tree Port Roles
        Spanning tree Supports Designated , root, and alternate(blocked link)port roles
        the Root Path cost enables port to be determined
        
        Root SW have all Designated Ports
        nonRoop Sws have one Root Port to the Root Sw and other are either Designated or Alternate (Port wiith worst Cost should be Bloacket(Alternate))
        
        Path Cost
        Root Path cost is carried in the BPDU and used to determine the shortest path to the root
        path costs depends on the link types (cost of GigabitEthernet is smaller than FastEthernet smallesr than 10Mbips Ethernet)
        
        Root Path Cost vs Path Cost
        RPC root path cost is the path cost from switch itself towards the root(sumary of all the links)
        the path cost is the ccost of one link
        
        Which value should we used > Path Cost Standards
        STP supportts various path cost standards
        the 802.1t is the default standard used ny huawei switches
        Port Speed>802.1D>802.1t>Path Cost Legacy
        10Mbps>99>1999999>1999
        100Mbps>18>199999>199
        1 Gbps>4>20000>20
        10 Gbps>2>2000>2
        
        Port ID - calculate the best path
        Where the Root path cost are equal a port identifier is used to determine the active and alternate path to the root
        Have 2 parts > Priority value | Port Number
        
        assuming we have SW triangle (SWA is root)
        between SWA and SWB we have 2 links > if the 2 links are in forwarding state > then theres a loop > one link must be bloacket
        SWA root > both its interfaces are Designated > one of the 2 should be Bloacked (link are parallel and to the same SWB > soo they have the same root path cost ) >we cannot decide
        in this case we use the Port ID (diffrent ports) > interface 1 wins on interface 2
        
        Priority value - can be changed
        
        Port Nimber
        
        After Root Bridge is selected > all Switches will calculate their own RPC root path cost
        lets say in a triangle Sw Net > we have the SWA as the Root (both ports are designated ports)
        SWB and SWC will calculate the Root path cost and find theit best path to the SWA Root (Root links) > then it should last the link between the 2 SWB and SWC where they will exchange each other lets say RPC are the same > comparing nexr the thrd field in the parameters > the Bridge ID (Priority + MAC Address) smallest wins > lets say SWB wins >> meaning SWB is the designated port > SWC should be blocked
        in a Scenario where SWC conncet 2 link with another SWD > we have BID is the same for SW on all its interfaces including the 2 link with that SWD >> SWC choose the best path using the PortID
        
        Port State Transition
        When SW is calculating which port should be forwarding data , which port should be blocked > we cannot forward data at that time > this is the state of transition in finite state machine
        For each State if somthing gone wrog or the link is pulled off > then interface is down >> getting to the Disabled State
        
        1st > Disabled > interface is down >
        
        1st State > Blocking > will be immediately changed to other state
        
        2nd State > Listening - 15 secs > Switch will send BPDU and recv BPDU
        oin this Time the SW calculate which path is the best path to the root
        it could that this time is suffcient for calculating but IEEE predict that can somthing go wring and 15 secs not enough soo in that case the IEEE add another 15 secs in the Learning State
        
        Leaning State -15secs > waiting for all SWs in the NEt to converge in that time - in this state also no data can be forwarded but if the connected host sends data to the Switch > SW will still learn the Source MAC address and puts into the source MAC address table (Accepting , Dropping but no Forwarding)
        
        1 more item...
      - Root Election Process
        all STP switches advertise BPDU to peers with self as root
        Port Role Establishment Process
        
        the Bridge ID and root Path Cost are used to elect port roles
        
        Port State Transition
        
        Disabled / Blocking / Listening / Learning / Forwarding
        
        Topology change Process (method to solve MAC address table when topology is changing )
        
        topology chanfe notification informs root of the topology change
        Root Fllushes enteries using BPDU with TC bit set
        
        Topology Change MAC Refresh
      - Root Faiiure
        Non Root bridges wait for MAX age before assuming loss of root
        Re-Convergene is then initiated, beginning with root election
        swtches A B C , A is root switch
        Indirect Link Failiure
        SW B begins root election but BPDU is ignored by switch C
        Root BPDU os propagated to SW B after MAX agre Expires
        Direct link Failiure
        
        Switch B detects failiure and switches alternate port to root port
        STP converges after 2x forward delay (30 secs by default)
        
        Topologuy change MAC Instability
        change in the STP topology may invalidate MAC table enteries
        MAC table enteries expire only after 300 secs by default
      - how STP decisions are made for example blocking a link
        idea of STP is to find a tree > spanning tree in the topology (Sws topology)
        most important thing in the tree is the root >
        
        choose a root (switch as a root of the tree)
        
        all the others calculate the best path to the root >> if this path is not the really the best from switch to the root and there is a path better then the path will be blocked
      - After the STP Calculations > Network is Stable and some links are in forwarding state and Some Links are Bloacked > if Some error happens then how will STP react to these conditions
        
        Root Failiure
        if the Root Bridge is down
        Eg>SW triangle
        as we knwo after the STP calculation > only the Root bridge SWA will send out BPDU
        when other Non-Root Bridges SWB/SWC recv the BPDU from the root port > it will forward it for the next switch
        NonRoot Bridger wait for max age before assuming loss of root
        if the root bridge is down > SWB and SWC will wait for the BPDU (it will not be sent to other switches) but just wiat for the MAx Age Byy default > 20 secs
        After 20 secs BPDU timed out then the BPDU is deleted > then SWB and SWC know that the Bridge root is down >> begin to Calculate new STP
        ReConvergence is then initiated, beginning with Root Election
        in the finite state machine the new STP calculation needs 2 ForwardingDelay 2*15secs > 30 secs [Learning State(15) + ListeningState(15)]
        the Total covergence MaxAge(20) + 2*15 >> 50secs
        
        Indirect Link Failiure
        SW Triangle SWA is Root STP calculations has been done SWB has one root link and one Designated link and SWC has one Root link and one Blocked link (not forwarding data) after SWB and SWC have already exchanged each other..
        the link which is down is on SWB but SWC dosent know if somthing wrong happened bicuz the link connceting to SWB is alternate(Bloacked)
        Afther the link on SWC is down > SWB can detect that but not in same way back
        if the root link on SWB is down then SWB will loses the root port > SWB will imediately recalculate the STP / SWB does not know there is another path to the Root Bridge cuz SWC link is bloacked >> SWB will elect itself as the RootBridge > and it sends BPDU packets to the SWC which accept cuz even in bloacked link BPDU packet can be recved but still SWC drop it cuz its not better than the SWA BPDU (lower priority) > SWC wil not respond , but at that time SWB is still waiting for the respond (MAXAGE>20secs) then the SWB will elect the port btw SWC and SWB as the root port (This Convergence need 50 secs)
        
        Direct Link Failiure
        SW Triangle (SWA is the Root (and there is 2 links between SWA and SWB))
        means the Failiure Link is on the Switch that has alternate port
        assuming alternate port is on SWB , in this scenarion we have 2 links between SWA and SWB (root and alternate) (equal links)
        Aftter the calculation the second Second link is Blocked on SWB, then somthing happend and 1st link the forwarding link iis down
        After forwarding link (root) is down > SWB knows that the only root port is lost > So SWB recalculate the STP IMMEDiaTelY(no maxAge waiting) then we find that the 2nd link which blocked is the best path to the root and this calculation also need to pass the finite machine (2 forward delay) 30 seconds
        (SWB detectes failiure and switches alternate port to root port + STP Converges failiure after 2x forward delay (30 secs by default))
        
        Topology Change MAC Instability
        SW Triangle hostB connected to SWB anf hostC connected to SWC
        After the calculations The SWC link btw SWB and SWC is blocked
        the communication between hostA and hostB is made throughout the SWBAC path > SWC MAC Adddress Table >> 2 interfaces with MAC Address towards (>SWA,>SWB)
        the output interface(>towards SWB)> when this link is down < then after the recalculating > the SWC link is up as weve seen in the InDirectLink Failiure Soluution
        the data btw hostA and hostB should now go the way SWBC link but what it seeems that MAC table still point to SWA soo the frame still goes passed SWA soo SWA cannot forward (SWAC link down ) >> frame dropped >> Changes in the STP topology may invalidate MAC table entries
        MAC address table Expiraiton time is 300secs > long time for communication between hostA and hostB to be disruped
        Solution > the idea is to delete the MAC address items immediately after the topology is changed
        
        Topology Change Process
        mechanism > when the topology is changed at a certain SW of the Tree > another Type of BPDU > TCN BPDU > TopologyChangeNotification BPDU
        after ToppChange > SW detects it and sends a TCN BPDU towards the root and when the upper layer SW (in the Tree) gets this information >> it sends a configuration BPDU(with the flag bit that the topology change Aknowledgement >> meaning that this is a notification that tells SW > Topology Changed ) > then SW aknowledge it back and forward that to upper layer SW same thing step by step until we get to Root >> after the root know > in the next long period >> it will send out the Conf BPDU with the (TC TopologyChange bit = 1 ) informing all the SWs back that toplogy changed > MAC address table on SW then cannot be used > MAC Table items must be deleted > new MAC address table should bed learned
        Root Flushes MAC entries using BPDU with TC bit set (inform all SWs MAC address table should be timed out) > items wil be deleted immediately
- - - - RSTP Port Roles
        other than the main 3 ports that were in STP
        
        Roles > Description
        Backup > a backup path to doownstream nodes, where links exist on the same LAN segment as the designated port
        Alternate > an Alternate path to the root bridge that differs from the path provided by the root port of the switch
      - RSTP Edge Ports
        Systems that do not participate in RSTP connect to edge ports
        Edge ports do not recv BPDU and can instantly forward data(dosent take part in STP calculations >> no need to wait 15secs)
      - Port States of RSTP
        State > port Role
        
        Discarding
        Disabled / Alternate / Backup / Root / Designated
        
        Learning
        Root / Designated
        
        Forwarding
        Root / Designated
    - - Unused fields of the STP BPDU are active within RSTP
        New capabilities are introduced as part of RSTP
        f;ags are changed / ...
      - Designated switches generate their own BPDU at Hello Time , regardless of whether an RST BPDU has been recved which diffrent from the STP process
      - RSTP Convergence
        All RSTP enabled switches begin as root and send RST BPDU
        Ports are set to a designated role and a discarding state
      - RST BPDU Proposal
        proposals are sent in RST BPDU during root election
        a switch will ignore a proposal if it has a better bridge ID
      - RSTP sychrronization process
        Upon recving a superior BPDU > Swtch the Current Switch will cease to send RST BPDU cotaining proposals and begin to sychronize
      - RST BPDU Agreement
        once all downStreams non-edge designated ports have been blocked > current switch will send agreement with the RST BPDU
      - RSTP Converged Link
        The downStream port is again unblocked and a new round of sychronization occurs between non-root Switchs
      - Link/Root Failiure
        Loss of upstream RST BPDU signals link/device failiure
        Proposal and agreement based convergence will ensue
      - Topology Change Process
        During the sending of an agreement , addresses are flushed for all ports except the port on which the RST BPDU was received >> for this we neet the TCN Topology change notification informs Nets on topoChanges immediately without having to inform the root