Thursday, February 13, 2014

STP BackboneFast


Lessons Learned:
BackboneFast  - unlike UplinkFast – is used to detect and indirect failure in the network.
Where UplinkFast is used to direct a directly connected port failure.

BackboneFast:
Indirect failures should start recalculating immediately.
It will not converge as quickly as UplinkFast. It will detect our the local root port that someone else’s Root port upstream changed.

Example: 
Sw04 is root, each link between the switches is a FastEthernet link (cost of 19) except for the link Between SW03 and SW04. It’s s 2ooMB port channel, it’s cost will be 12.
So the path to the root from SW01 perspective will go through SW03 o get to SW04.

                     19 Cost
SW01  ---------------  SW02
|                                     |
| 19Cost                         | 19 Cost
|                                       |
SW03==========SW04
                     12 Cost

If we configure backboneFast on SW01 and the port-channel between SW03 ans SW04 goes down. SW03 will start to send “inferior BPDU’s towards SW03. Basically the BPDU’s will advertise a Worse cost to root through SW03.
SW01 realizes upstream that a failure has occurred. SW01 will then take it’s Max-Age Timer – by default is 20 Seconds. Sw01 will immediately expire that and start to recalculate.

The Key is that it’s used to detect indirect failures upstream but it’s only an optimization of 20 seconds.

You MUST configure BackboneFast on all switches because it’s a negotiated parameter between the switched.

From Global Config:
#Spanning-tree backbonefast

Useful commands:
#debug spaning-tree backbonefast
                                        
Note: STP will send a packet called “RLQ” (root link Query) this is specific to backboneFast to find an alternate path to the root.

Note: if the upstream sw only has one path to root and the link fails, it will advertise itself (it’s MAC) as the root.


To speed up Backbonefast – you’d have to literally change the MaxAge command on all switches. The minimum MaxAge command is 6 seconds. 

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