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Loop Avoidance

Loop Avoidance. Redundant topology eliminates single points of failure. Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems. Broadcast Storms. Host X sends a broadcast. Switches continue to propagate broadcast traffic over and over.

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Loop Avoidance

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  1. Loop Avoidance • Redundant topology eliminates single points of failure. • Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems.

  2. Broadcast Storms • Host X sends a broadcast. • Switches continue to propagate broadcast traffic over and over.

  3. Multiple Frame Copies • Host X sends a unicast frame to router Y. • MAC address of router Y has not been learned by either switch yet. • Router Y will receive two copies of the same frame.

  4. MAC Database Instability • Host X sends a unicast frame to router Y. • MAC address of router Y has not been learned by either switch. • Switches A and B learn the MAC address of host X on port 0. • The frame to router Y is flooded. • Switches A and B incorrectly learn the MAC address of host X on port 1.

  5. Spanning-Tree Protocol • Provides a loop-free redundant network topology by placing certain ports in the blocking state.

  6. Spanning-Tree Operation • One root bridge per network • One root port per nonroot bridge • One designated port per segment • Nondesignated ports are unused

  7. Spanning-Tree Protocol Root Bridge Selection • Bpdu = Bridge Protocol Data Unit (default = sent every two seconds) • Root bridge = Bridge with the lowest bridge ID • Bridge ID = • In the example, which switch has the lowest bridge ID?

  8. Spanning-Tree Port States • Spanning-tree transits each port through several different states:

  9. Spanning-Tree Port States (Cont.)

  10. Spanning-Tree Path Cost

  11. Spanning-Tree Example

  12. Spanning-Tree Switch#show spanning-tree vlan 1 VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address 0001.96DC.1A62 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32770 sys-id-ext 1) Address 0010.1116.A3A4 Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/1 Desg FWD 19128.3 Shr Fa0/2 Root FWD 19128.3 Shr Switch(config)#spanning-tree vlan 1 priority 4096

  13. VTP Modes • Creates VLANs • Modifies VLANs • Deletes VLANs • Sends/forwards advertisements • Synchronizes • Saved in NVRAM • Creates VLANs • Modifies VLANs • Deletes VLANs • Forwards advertisements • Does not synchronize • Saved in NVRAM • Forwards advertisements • Synchronizes • Not saved in NVRAM

  14. VTP Operation • VTP advertisements are sent as multicast frames. • VTP servers and clients are synchronized to the latest revision number. • VTP advertisements are sent every 5 minutes or when there is a change.

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