Docstoc

Ethernet

Document Sample
Ethernet Powered By Docstoc
					     Spanning Tree
protocol- Chapter 5
CCNA Exploration Semester 3
    Modified by Profs. Ward
             and Cappellino



                              1
Topics
   Redundancy in a converged network
   How Spanning Tree Protocol (STP)
    eliminates layer 2 loops
   The STP algorithm and its 3 steps
   Rapid spanning tree protocol




                                        2
Semester 3
                 LAN Design

  Basic Switch                     Wireless
  Concepts

VLANs                  STP- Ch 5


VTP                 Inter-VLAN
                    routing
                                              3
    We want:
   Redundancy accomplished at
    the ____________________
    _______________________
    _______________________
       What is redundancy?
   Multiple ________________
    _________________
   One link or device fails –
    another takes over.
   Redundancy allows flexibility
    but does have some
    challenges
                                    4
Issues with Redundancy
   Multiple ___________________
       Create problems when all the links are active:
           ____________________
           _____________________
           ____________________


        See examples on following slides…



                                                         5
 Broadcast storm
2. Flooda                    3. And so on
There’s                  B   with nothing to
broadcast loop
switching
through non-                 stop it
source ports


1. Send          A             C
  ARP
request



                     D
                                          6
Duplicate Unicast Frame Transmissions

                          B
 A is on port 3
 Don’t know B
    So flood

                  A           C
   Send
 frame to                             Frame
    B                                 arrives
            A                     B   And
                      D               again
                                          7
Loops by mistake
   Even if there are no deliberate loops for
    redundancy, there can be loops set up by
    mistake.
       For example, by improper or no labeling of
        cables in the wiring closet
   ____________
    ____________


                                                     8
Etherchannel – the exception
   EtherChannel is a grouping of Ethernet ports
    on a switch that ____________________
    ____________________________
       Multiple connections do not make a loop where
        Etherchannel is used.
       The _________________________________
        with the combined bandwidth.



                                                    9
Redundancy without loops
   There needs to be just ________________
   _________________________, but _______
    _________________ when they are needed.
   This must be done quickly and automatically.
   ______________________ does this.
   A blocked port does not include bridge
    protocol data unit (BPDU) frames that are
    used by STP to prevent loops.
                                             10
What is a spanning tree?
A   tree (extended star) topology
    A ______________________
 Spanning   all devices
    __________________________
 Spanningtree is _____________
 immediately __________________
 _______________________
                                     11
Not a spanning tree
   Not a tree - it has loops.




                                 12
Not a spanning tree
   Not spanning. Device left out.




                                     13
Spanning tree
   No loops. Includes all devices.




                                      14
Spanning tree protocol
   Used by switches to __________________
    _________________________________
   ________ unwanted links by _____________
   STP defined by ________________
   _____________ defined by _____________
   Switches __________________ –
    ________________________________


                                         15
     Spanning tree algorithm
     The switches use this algorithm to
     configure the ports _______
     ____________________
1.   Choose ___________ to be
     “____________”
2.   Choose a “_______” on ______
     _____________ closest to the root
     bridge
3.   Choose a “__________” which are
     all _______________ that are still
     ___________________________
     ___________________________
     _____________________ known
     as “non-designated” ports
                                          16
Example of port designations
                            Root bridge
      Designated port                     Designated port
       Root port                            Root port


 Root port           Designated port
                   Designated port         Non-
                                           designated
                                           port


                                                    17
1. Choose the Root Bridge
   Root bridge serves as a __________________
    ____________________________
   ______________ in the broadcast domain
    __________________________________
   Each switch has a bridge ID (___) containing priority
    value, extended system ID, followed by the MAC
    address of the switch- more to come on BID …
   A switch starts up. It sends out BPDU frames more to
    come on BPDU … containing the switch BID and the root
    ID every 2 seconds.
       At first each switch identifies itself as the root bridge.
                                                                     18
Choose the root bridge (cont…)
   ___________ Bridge Protocol Data Units (______)
    to _______________________
   The switch with the __________________________
    ________________________
       As a switch receives a BPDU, it ___________________
        ________________________________________ through
        which the BPDU was received.
         It passes on this information in its own BPDUs.

   Eventually all switches agree that the switch with the
    lowest BID is the root bridge.
   Administrator can set the priority to fix the selection
                                                       19
BPDU
   The _______________________________
    ________________________________
   The BPDU message is encapsulated in an
    Ethernet frame.
   The ______________ MAC address in the
    BPDU frame is a _________________ for
    the ___________________________


                                         20
BPDU Frame contains 12 fields:
 2 bytes   Protocol ID     Administrative
 1 byte    Version         uses
 1 byte    Message type
 1 byte    Flags
 8 bytes   Root ID         BID and path
 4 bytes   Cost of path    information- used
                           to identify the root
 8 bytes   Bridge ID       bridge and the
 2 bytes   Port ID         cost
 2 bytes   Message age     Timer fields used
 2 bytes   Max age         to determine how
                           frequently BPDU
 2 bytes   Hello time      messages are sent
 2 bytes   Forward delay                          21
Bridge ID

   Used to ______________________ on a network
   The bridge ID consists of:
       ____________________________
         Lowest priority- ie. Lowest BID- becomes root bridge
         By default the priority is 32768
             Value range is 1 - 65536
       ___________________________________
         Identifies the VLAN with which the BPDU is associated
       ________________________
         MAC address used as ___________________________
         MAC address with the lowest hexadecimal value has the
          lower BID
             Best to configure the desired root bridge and not count on MAC
                                                                           22
              address
Configure priority
   Set a _____________________ if a specific switch
    is to become the root bridge
   Method 1:
       SW1#spanning-tree vlan 1 root primary
         Sets value to 24576 or 4096 less than lowest priority
          detected.
       SW1#spanning-tree vlan 1 root secondary
         Sets value to 28672. This switch should become the root
          bridge if the primary root bridge fails.
   Method 2:
       SW1#spanning-tree vlan 1 priority 24576
         command gives more granular control over the bridge
          priority value by allowing the NA to configure the priority
   How would we verify bridge priority?
                                                                    23
2. Select root ports
   After root bridge designation _________________________
    _________________________________________________
    __________________________ in the broadcast domain.
   Every non-root bridge (Switch) selects a root port
       Only _________________________________
       This is the _____________________________ to the root bridge
           Path information is determined by _______________________
            _________ along the path from the destination to the root bridge.
   What is cost?...




                                                                         24
Finding the cost of a link
   Default port costs are defined by the speed at which
    the port operates
   Set by IEEE.
       Costs may change as newer, faster Ethernet is developed.
   Port cost is configurable by the NA

Link speed              Revised cost         Previous cost
10 Gbps                 2                    1
1 Gbps                  4                    1
100 Mbps                19                   10
10 Mbps                 100                  100
                                                             25
Changing the cost of a link
   SW1(config)#int fa0/1
   SW1(config-if)#spanning-tree cost 25
   SW1(config-if)#end

   To revert back to default value use “no”…
       SW1(config)#int fa0/1
       SW1(config-if)#no spanning-tree cost
       SW1(config-if)#end
                                                26
    What is the best path?
   In the example, the path
    cost from switch S2 to
    the root bridge switch S1,
    over path 1 is 19, while
    the path cost over path 2
    is 38.
       Because path 1 has a
        lower overall path cost to
        the root bridge, it is the
        preferred path.
       STP then configures the
        redundant path to be
        blocked, preventing a loop
        from occurring.
                                     27
What if root ports have the same cost?
   Switch uses the customizable port priority value. If
    those are the ____________?
   _____________________________
        The port ID is ________________________________
         port. This gives each port a unique number
   Example: by default
    F0/1 has port priority
    value of 128 and .1 as
    port ID giving: 128.1

    F0/2 has 128.2
                                                           28
                                         X
                                        F0/2        F0/1
        F0/1 would be the root port…
Configure port priority
   SW2(config-if)#spanning-tree port-priority 112
   Priority values range from 0 - 240, in
    increments of 16.
   The ____________________________
   ________________________
       Becomes ________________________
   ____________________________________
    _____________________________ by STA to
    prevent looping
                                              29
3. Select designated/non-
designated ports
   ____________ ports - All _______________
    __________________________________
       For _____ bridges, ______________________
        ports.
       For ___________bridges, a designated port is the
        _______________________________________
        ________________________ as needed.
       Only ___________________________________
   ______________ ports - All ports configured
    to be in a _______________ to prevent loops
                                                    30
Select designated/non-
designated ports cont…
   If two switches are connected to the same LAN
    segment, which switch port will be the designated
    and which the non-designated port?
       Switches on the LAN segment in question exchange BPDU
        frames, which contain the switch BID.
       Lowest path cost to the root bridge will be the designated
        port. If _____________________?
         Switch with the ____________ has its port configured as a
           _______________________
         Switch with the higher BID- non-designated port

                 See example on next slide…
                                                              31
Example…




           32
Summary of Port Roles
   STP makes:
       Root ports- forwarding
       Designated ports- forwarding
       Non-designated ports- shut down or blocked




                                                     33
Five Port states in traditional STP
   Blocking – _______________ BPDU frames. Non-
    designated port which _________________
    ___________________________________
   Listening - ____________________ BPDU frames.
   Learning - _____________________ BPDU frames.
    _____________________ in preparation of
    participating in frame forwarding
   Forwarding – Fully active, _________________
   Disabled – ___________________________

                                             34
States and timers
   Amount of time
    that a port stays in
    the various port
    states depends on
    the ___________
   During a topology
    change
       A port temporarily              Possibly forward or remain in
        _______________                 blocked state
        _______________
        ____________ for
        a specified period
                              Up to ____________________
        called the "forward
        delay interval.“      __________ again- full
                              convergence             35
BPDU timers
   Timers are optimised for a 7-switch diameter
    network.
       _______________________________________
        _______________________ to travel from the
        ________________ on the broadcast domain
   The network has __________________
    before switches forward user data.
   Typically timers and the diameter should not
    be adjusted though they are configurable
                                               36
Cisco PortFast
   An access port leading to a workstation or server
    does not need to go through the STP modes
    because it will not be closed down.
       The switch port can be configured with _____________
        ____________________________________________
   If a switch is connected later and the port receives a
    BPDU, STP can put the port into a blocking state
    and then through the modes.
       Feature called ______________________
   To configure portfast, enter interface config mode
    for the port where portfast is going to be used and
    then the command:
         spanning-tree portfast
                                                           37
Verify spanning tree


                       Root bridge



                       This switch




                              38
Topology change notification (TCN)
   After the network converges, the root bridge sends
    out BPDUs, but the other switches do not normally
    send BPDUs back.
   If there is a ______________________________
    ______________ called the topology change
    notification (________) ______________________
   Each switch that receives the TCN sends an
    ____________________ and sends a TCN towards
    the root bridge until the root bridge receives it.
   The ____________________________ with the
    topology change (TC) bit set into the whole network
                                                   39
STP developments




                   40
    PVST+
   ________________ can be implemented.
   Separate instance of spanning tree runs for
    each VLAN




                                                  41
PVST+ cont…
   The VLAN needs to be identified, so each
    BID has 3 fields:
       Bridge Priority
       _____________________________________
       MAC address.
   Original BID just had bridge priority and MAC
    address
   PVST+ is the default spanning-tree
    configuration for a Catalyst 2960 switch.

                                               42
PVST+ cont…
   When the priority and extended system ID
    are prepended to the switch MAC address,
    each VLAN on the switch can be represented
    by a _________




                                           43
Rapid Spanning Tree Protocol
   __________________________________ with it.
       RSTP (IEEE 802.1w) is an evolution of STP (IEEE 802.1D)
   Much _____________________
   ____________________________
       2 in version field indicates it is RSTP
   Sends BPDUs with its current information every 2
    seconds.
       Does not use timers in the same way as STP
   3 missed BPDUs taken to mean loss of the link. (6
    seconds)
   _____________________________
       RSTP does not have a __________________________
                                                           44
_______________ in RSTP
   A port that will ______________________
    __________________________
       Immediately goes to forwarding state.
   Same idea as Cisco’s PortFast.
   An edge port becomes a normal spanning-
    tree port if it receives a BPDU
   Configuring an edge port uses the PortFast
    keyword as before.
       spanning-tree portfast

                                                45
Link types
   ___________ are categorized into 2 link
    types, ____________________________
       A _________________ between two switches is
        regarded as a _________________
       A link ____________________ is regarded as a
        ____________________
   Ports on a ________________ are able to
    _________________________
   Designated ports
       Make the most use of the link type parameter.
                                                        46
Port states
• There are only _____________ in RSTP that
correspond to the three possible operational states.
  • The 802.1D ________________ states are ________
  into a unique 802.1w ________________________

Operational       STP              RSTP
Enabled           Blocking         Discarding
Enabled           Listening        Discarding
Enabled           Learning         Learning
Enabled           Forwarding       Forwarding
Disabled          Disabled         Discarding
                                                47
RSTP Port Roles
   ____ - A ___________
    that has been elected
    for the spanning-tree
    topology
   _______________ - A
    ___________________
    _________________
   Alternate - An _______
    ___________________
    This path is different
    than using the root port.
   Backup - A
    ___________________
    to a segment where
    ___________________
    __________________
   _________ - Not strictly
    part of STP, a network
    administrator can
    ___________________
                                48
Design considerations
   Root bridge should be a ______________ in
    the ______________ of the network.
   ________ the number of _______________
   Use VTP ____________.
   Use ________________________
   _________________ even if no blocked
    ports or physical redundancy exist


                                          49
Troubleshooting Suggestions
   Before you troubleshoot a bridging loop, you
    need to know at least these items:
       Topology of the bridge network
       Location of the root bridge
       Location of the blocked ports and the redundant
        links
       How the network looks when it works correctly
   Use the “_________” command as needed

                                                     50

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:4
posted:1/18/2012
language:English
pages:50