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Spanning Tree - Backbone Fast

Posted by markmark1983 , 16 August 2013 · 2321 views

Cuurently login on a switch with a BLK port.

Interface Role Sts Cost   Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/12    Desg FWD 19 128.14   Edge P2p
Fa0/15    Root FWD 19 128.17   P2p
Fa0/18    Altn BLK 19 128.20   P2p


Assuming the upstream switch where the BLK port is connected, has lost its connection to the root bridge.
The upstream switch will declare itself as a root to this switch, but this switch will discard it first since it is inferior.

This is the switch that has lost its connection to the root bridge, and is sending BPDU.s out port declaring it as a root:


*Mar  6 22:39:20.188: STP: VLAN0020 we are the spanning tree root
*Mar  6 22:39:22.185: %LINK-5-CHANGED: Interface FastEthernet0/15, changed state to administratively down
*Mar  6 22:39:23.192: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/15, changed state to down


the switch downstream  just listnes and do nothing until the max age timer expires

*Mar  6 22:37:31: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:33: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:35: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:37: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:39: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:41: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:43: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:45: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:47: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18

the port then now transition from blk to forwarding after the mac age timer expires, this means STP needs recalculation

*Mar  6 22:37:48: STP: VLAN0020 Fa0/18 -> listening
*Mar  6 22:37:49: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  6 22:37:49: STP: VLAN0020 Topology Change rcvd on Fa0/18
*Mar  6 22:37:49: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  6 22:38:03: STP: VLAN0020 Fa0/18 -> learning
*Mar  6 22:38:18: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  6 22:38:18: STP: VLAN0020 Fa0/18 -> forwarding

When the uplink goes back up, it just transition normally as STP, and the previous FWD port will resume BLK port after receiving the TCN from the upstream switch:


*Mar  6 22:44:10: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  6 22:44:10: STP: VLAN0020 Fa0/18 -> blocking





The backbone fast features enables the switch to switch to fwd state bypassing the LRN/LRN states once it recieves inferior BPDUs on its BLK ports

When used, backbone fast must be enabled on all switches in the network because backbone fast requires the
use of the RLQ Request and Reply mechanism in order to inform switches of root path stability. The RLQ
protocol is active only when backbone fast is enabled on a switch. In addition, the network can also run into
issues with RLQ flooding, if backbone fast is not enabled on all switches. By default, backbone fast is
disabled.


You do not need to configure backbone fast with RSTP or IEEE 802.1w because the mechanism is natively
included and automatically enabled in RSTP. For more information on RSTP or IEEE 802.1w, refer to
Spanning Tree from PVST+ to Rapidāˆ’PVST Migration Configuration Example.



tsc_lab_sw4# show spanning-tree backbonefast
BackboneFast is enabled

BackboneFast statistics
-----------------------
Number of transition via backboneFast (all VLANs)    : 0
Number of inferior BPDUs received (all VLANs)    : 0
Number of RLQ request PDUs received (all VLANs) : 0
Number of RLQ response PDUs received (all VLANs) : 0
Number of RLQ request PDUs sent (all VLANs) : 0
Number of RLQ response PDUs sent (all VLANs) : 0


EXAMPLe:( with bridges that has backbonefast disabled)

Upstream switch loses one of the root ports:


tsc_lab_sw2(config-if)#
*Mar  7 01:06:12.572: STP: VLAN0020 we are the spanning tree root
*Mar  7 01:06:14.577: %LINK-5-CHANGED: Interface FastEthernet0/15, changed state to administratively down
*Mar  7 01:06:15.583: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/15, changed state to down
*Mar  7 01:06:31.950: STP: VLAN0020 heard root 32788-0017.5af7.b880 on Fa0/21
*Mar  7 01:06:31.958: supersedes 32788-001b.8f0f.7e00
*Mar  7 01:06:31.958: STP: VLAN0020 new root is 32788, 0017.5af7.b880 on port Fa0/21, cost 38
*Mar  7 01:06:31.958: STP: VLAN0020 sent Topology Change Notice on Fa0/21


The other switch with BLK ports are receiving inferior bpdus but will wait before the max age timer expires:


tsc_lab_sw4#
*Mar  7 01:04:23: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:25: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:27: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:29: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:31: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:33: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:35: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:37: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:39: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:41: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:04:41: STP: VLAN0020 Fa0/18 -> listening
*Mar  7 01:04:42: STP: VLAN0020 Topology Change rcvd on Fa0/18
*Mar  7 01:04:42: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  7 01:04:56: STP: VLAN0020 Fa0/18 -> learning
*Mar  7 01:05:11: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  7 01:05:11: STP: VLAN0020 Fa0/18 -> forwarding

The effect on traffic:


!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!.........................!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


approximately 1 minute downtime before coming backup:

When the root port on the upstream switch comes back up....the effect is teh same, as once the downstream switch receives the new TCN, immediately
puts the port to blocking , while the upstream switch is learning through the STP process.


*Mar  7 01:08:19: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  7 01:08:19: STP: VLAN0020 Fa0/18 -> blocking



!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!...............!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!



*Mar  7 01:10:04.794: %LINK-3-UPDOWN: Interface FastEthernet0/15, changed state to up
*Mar  7 01:10:07.302: set portid: VLAN0001 Fa0/15: new port id 8011
*Mar  7 01:10:07.302: STP: VLAN0001 Fa0/15 -> listening
*Mar  7 01:10:07.302: set portid: VLAN0020 Fa0/15: new port id 8011
*Mar  7 01:10:07.302: STP: VLAN0020 Fa0/15 -> listening
*Mar  7 01:10:08.309: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/15, changed state to up
*Mar  7 01:10:08.485: STP: VLAN0020 new root port Fa0/15, cost 19
*Mar  7 01:10:08.686: STP: VLAN0001 Fa0/15 -> blocking
*Mar  7 01:10:22.309: STP: VLAN0020 Fa0/15 -> learning
*Mar  7 01:10:37.316: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  7 01:10:37.316: STP: VLAN0020 Fa0/15 -> forwarding

EXAMPLE: with backbonefast enabled on all bridges:


When the uplink on the upstream bridge goes down, the BLK port on the downstream sw receives the root request:itnotice it does not need to wait for the max age timers to expire but instead proceed to STP calculation:


*Mar  7 01:16:38: STP: VLAN0020 heard root 32788-001b.8f0f.7e00 on Fa0/18
*Mar  7 01:16:38: STP: VLAN0020 Fa0/18 -> listening
*Mar  7 01:16:38: STP: VLAN0020 Topology Change rcvd on Fa0/18
*Mar  7 01:16:38: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  7 01:16:53: STP: VLAN0020 Fa0/18 -> learning
*Mar  7 01:17:08: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  7 01:17:08: STP: VLAN0020 Fa0/18 -> forwarding

Effect on traffic:


!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!...............!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!


When the uplink port comes up n the upstream bridge,


*Mar  7 01:22:11.650: %LINK-3-UPDOWN: Interface FastEthernet0/15, changed state to up
*Mar  7 01:22:14.242: set portid: VLAN0001 Fa0/15: new port id 8011
*Mar  7 01:22:14.242: STP: VLAN0001 Fa0/15 -> listening
*Mar  7 01:22:14.242: set portid: VLAN0020 Fa0/15: new port id 8011
*Mar  7 01:22:14.242: STP: VLAN0020 Fa0/15 -> listening
*Mar  7 01:22:14.250: STP: VLAN0020 new root port Fa0/15, cost 19
*Mar  7 01:22:14.460: STP: VLAN0001 Fa0/15 -> blocking
*Mar  7 01:22:15.249: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/15, changed state to up
*Mar  7 01:22:29.249: STP: VLAN0020 Fa0/15 -> learning
*Mar  7 01:22:44.256: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  7 01:22:44.256: STP: VLAN0020 Fa0/15 -> forwarding
tsc_lab_sw2#
*Mar  7 01:22:49.642: %SYS-5-CONFIG_I: Configured from console by console


!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!................!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

on the downstream switch that receive better BPDU now.


*Mar  7 01:20:25: STP: VLAN0020 sent Topology Change Notice on Fa0/15
*Mar  7 01:20:25: STP: VLAN0020 Fa0/18 -> blocking




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