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BackboneFast

Chris Zhang
Level 1
Level 1

Hi,

Is it true that, when BackboneFast is enabled, the switch that lost connection to its root bridge will do two things.

      1. It will send an RLQ reply out all other ports connected (does it also send it to the root port? it will have no effect anyway due to loss of connection?)
      2. It will send inferior BPDU claiming it is the new root bridge to all other ports connected (except the root port?)

Without BackboneFast, it will only do point two above?

Suppose what I said above is not entirely wrong - but please correct me on spot. The other benefit, brought by BackboneFast, in addition to point one from above, is to get rid of the max-age timing constraints concerning both points one and two? e.g. The switch that lost connection to root bridge will send an RLQ reply and an interior BPDU to claim itself as the new root bridge, without having to wait for the max-age to expire - waiting for the BPDU from its current root bridge to lose effect.

AND in addition, for the downstream switch, it will also act immediately, when receiving either an RLQ reply or an inferior BPDU on its blocked port, such that it will relay the superior BPDU(from the current root bridge) to the switch that lost connection to the current root bridge in order to find a new root port for that switch?

I know the above may not sound like a straight forward question to answer, but could anyone please pick the false statement from above?

Thanks.

Chris

2 Replies 2

Ganesh Hariharan
VIP Alumni
VIP Alumni

Hi,

Is it true that, when BackboneFast is enabled, the switch that lost connection to its root bridge will do two things.

      1. It will send an RLQ reply out all other ports connected (does it also send it to the root port? it will have no effect anyway due to loss of connection?)
      2. It will send inferior BPDU claiming it is the new root bridge to all other ports connected (except the root port?)

Without BackboneFast, it will only do point two above?

Suppose what I said above is not entirely wrong - but please correct me on spot. The other benefit, brought by BackboneFast, in addition to point one from above, is to get rid of the max-age timing constraints concerning both points one and two? e.g. The switch that lost connection to root bridge will send an RLQ reply and an interior BPDU to claim itself as the new root bridge, without having to wait for the max-age to expire - waiting for the BPDU from its current root bridge to lose effect.

AND in addition, for the downstream switch, it will also act immediately, when receiving either an RLQ reply or an inferior BPDU on its blocked port, such that it will relay the superior BPDU(from the current root bridge) to the switch that lost connection to the current root bridge in order to find a new root port for that switch?

I know the above may not sound like a straight forward question to answer, but could anyone please pick the false statement from above?

Thanks.

Chris

Hi Chris,

Just go through this link it has both the secnarios what you have asked hope that helps !!

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800c2548.shtml

Ganesh.H

lamav
Level 8
Level 8

"1. It will send an RLQ reply out all other ports connected (does it also send it to the root port? it will have no effect anyway due to loss of connection?)"

Chris, it is the root bridge that sends RLQ responses. Or it can be a switch that receives the RLQ request and has lost its connection to the root. So, when a switch receives an RLQ, it will respond if:

1.) It is the root.

2.) It has lost its connection to the root.

Otherwise, it will not respond but simply forward the RLQ reques towrad the root.

Remember that the whole point of the RLQ request is to ensure that the root is still reachable.

"2. It will send inferior BPDU claiming it is the new root bridge to all other ports connected (except the root port?)"

Correct. The switch that has experienced the direct link failure to the root will lose its path to the root and claim itself as the new root.

That link you were given is excellent, by the way.

HTH

Victor

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