Spanning Tree Protocol

Spanning Tree Protocol

1. Issues without STP

   • Time-to-live field – A value in an IP packet’s header that is decremented by one, each time the packet enters a router interface
   • Switches have no TTL field
   • Broadcast Storm – A broadcast frame (i.e a frame destined for FFFF.FFFF.FFFF.FFFF) circulates endlessly around a Layer 2 topology, because the frame has no TTL field. PC’s look at this traffic and don’t do anything else
     • Switches flood broadcasts out every port except the port it originated from. They just keep flooding
     • Switch processing will be high
   • MAC Address Table Corruption
     • Gets rid of correct entries in the table for incorrect entries because the frame comes back into a different interface
     • Duplicate frames are received

2. Identifying STP Port States

   • Root Bridge – An STP topology has a single root bridge. The bridge (or switch) with the lowest bridge ID (BID) is elected as the root bridge
   • Default Priority is 32768, if tie, then the lowest switch MAC becomes root
   • Port States
     • Root Port – The port on a non-root bridge that is closest to the root bridge, in terms of cost. Only one per switch. If costs tie, go with port with the lowest port ID
     • Designated Port – The port on a network segment that is closest to the root bridge, in terms of cost. Can be multiple
     • Non-Designated Port – Ports that block traffic, in order to preserve a loop-free Layer 2 topology. Any left-over port
     • Disabled Port – A port that is administratively shut down

3. Port Speed and STP Port Cost

   • 10 Mbps – 100
   • 100 Mbps – 19
   • 1 Gbps – 4
   • 10 Gbps – 2

4. STP Convergence Times

   • Bridge Protocol Data Unit (BPDU) – A type of packet exchanged in an STP topology that is used to determine which switch is the root bridge
   • Blocking State – 20 seconds waiting for BPDU
   • Listening State – 15 seconds (listening to BPDUs, trying to learn its place in the network)
   • Learning State – 15 seconds (starts populating its MAC address table)
   • Forwarding State – Starts forwarding traffic

5. STP variants

   • IEEE 802.1D
     • All VLANs use a common STP topology
     • Common Spanning Tree (CST)
   • Per VLAN Spanning Tree (PVST)
     • Used over ISL Trunks
   • Per VLAN Spanning Tree Plus (PVST+)
     • Used over IEEE 802.1Q trunks
   • Multiple Instances Spanning Tree Protocol (MISTP)
   • Multiple Spanning Trees Protocol (MSTP)
     • Used over IEEE 802.1S
   • Rapid Spanning Tree Protocol (RSTP)
     • Used over IEEE 802.1q
     • Faster convergence
   • Rapid PVST+
     • Faster convergence

6. Verification

   • Show spanning-tree
   • Show spanning-tree vlan 100
   • Extended sys-id is the VLAN
   • Priority plus VLAN gives bridge ID priority