Spanning Tree Notes

This note below is from creating (200) Vlans and using the default PVSTP+

%SPANTREE_VLAN_SW-2-MAX_INSTANCE: Platform limit of 128                                 instances exceeded. No instance created for VLAN128


MST essentially takes the best features of IEEE 802.1D Spanning-Tree, AKA Common Spanning-Tree, and the Cisco extensions to STP, PVST, PVST+, Rapid PVST+, and combines them.

##### MST0    vlans mapped:   201-4094
Bridge        address 000c.ceab.bc80  priority      32768 (32768 sysid 0)
Root          this switch for the CIST
Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured    hello time 2 , forward delay 15, max age 20, max hops    20

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/9            Desg FWD 200000    128.9    P2p
Fa0/10           Desg FWD 200000    128.10   P2p
Fa0/13           Desg FWD 200000    128.13   P2p
Fa0/14           Desg FWD 200000    128.14   P2p

##### MST1    vlans mapped:   1-100
Bridge        address 000c.ceab.bc80  priority      32769 (32768 sysid 1)
Root          this switch for MST1

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/9            Desg FWD 200000    128.9    P2p
Fa0/10           Desg FWD 200000    128.10   P2p
Fa0/13           Desg FWD 200000    128.13   P2p
Fa0/14           Desg FWD 200000    128.14   P2p

##### MST2    vlans mapped:   101-200
Bridge        address 000c.ceab.bc80  priority      24578 (24576 sysid 2)
Root          this switch for MST2

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/9            Desg FWD 200000    128.9    P2p
Fa0/10           Desg FWD 200000    128.10   P2p
Fa0/13           Desg FWD 200000    128.13   P2p
Fa0/14           Desg FWD 200000    128.14   P2p

Similar to CST and PVST, MST uses a cost value derived from the inverse bandwidth of the interface (higher bandwidth means lower cost). The root port is chosen based on the lowest end-to-end cost to the root bridge. The show spanning-tree mst command shows the local cost values of the outgoing ports on the local switch.

sw2#sh spanning-tree mst 0 detail

##### MST0    vlans mapped:   1-4094
Bridge        address 0017.5900.fe80  priority      32768 (32768 sysid 0)
Root          address 000c.ceab.bc80  priority      32768 (32768 sysid 0)
              port    Fa0/13          path cost     200000  
Regional Root address 0013.1a06.6580  priority      32768 (32768 sysid 0)
                                      internal cost 400000    rem hops 18
Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured    hello time 2 , forward delay 15, max age 20, max hops    20

FastEthernet0/9 of MST0 is alternate blocking
Port info             port id         128.11  priority    128  cost      200000
Designated root       address 000c.ceab.bc80  priority  32768  cost           0
Design. regional root address 000c.ceab.bc80  priority  32768  cost           0
Designated bridge     address 000c.ceab.bc80  priority  32768  port id    128.9
Timers: message expires in 4 sec, forward delay 0, forward transitions 2
Bpdus sent 16, received 444
When MST is enabled, Rapid Spanning-Tree Protocol (RSTP) is automatically enabled. RSTP is an IEEE standard defined in 802.1w that speeds up convergence through a reliable handshaking process. RSTP defines new port “roles” to automatically allow for the functionality built into Cisco proprietary features such as PortFast and UplinkFast.
RSTP “edge” ports behave the same as PVST PortFast enabled ports. However, in order to maintain backwards compatible configurations Cisco’s implementation of RSTP does not automatically elect edge ports as the standard suggests. Instead a port must be configured as an edge port with the spanning-tree portfast command.

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Protected ports - are used to prevent traffic from being exchanged at layer 2 between two or more ports that are in the same VLAN. Traffic received in a protected port cannot be sent out another protected port, however traffic received in a protected port can be sent out a non-protected port. This feature is a much smaller subset of the Private VLAN feature, and cannot span between multiple physical switches.
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Storm control is used to limit the amount of unicast, multicast, or broadcast traffic received in a port. The most common application of this feature is to prevent broadcast storms, but it can also be used to police individual ports not to exceed a desired rate.
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Another static feature of the CAM table is the ability to Null route MAC addresses. Since static entries always override dynamically learned entries, if the drop keyword or an unused interface is used in the mac-address-table static command traffic destined to that MAC address will be dropped.