STP

Unanswered Question
Aug 17th, 2010

Its a small doubt, please clarify- When STP Max age timer expires, we can conclude that Root Bridge is down . what conclusion we can made when STP Hello timer expires on a non root bridge? If a non root bridge doesnt get a Configuraton BPDU with in a Hello interval , i.e for 10 seconds, then will the non-root bridge will wait till max age timer expires or will non-root bridge take action before max age timer expires?

I have this problem too.
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Peter Paluch Tue, 08/17/2010 - 08:19

Hello,

The Hello timer dictates the frequency of sending Configuration BPDUs and is set to 2 sec by default. If the Hello timer expires, a new BPDU is originated and sent out. Note that if a switch is not authorized to originate a BPDU then it does not use the Hello timer at all. In Legacy 802.1D STP, only root bridge originated BPDUs and they were only relayed by other switches. However, other switches did not originate their own Configuration BPDUs. If the STP driver failed on the root bridge, there would be 20 seconds (max_age) of no BPDUs in the network at all.

The RSTP changes this a bit: each switch in RSTP originates its own BPDUs on Designated ports. So when the Hello timer expires on any switch, it sends out a Configuration BPDU.

Best regards,

Peter

tecknology Tue, 08/17/2010 - 08:41

Thanks you very much for the response.

So in 802.1D if a non root bridge doesnt receive configuration BPDUs in Hello time interval, will the non-root bridge take any actions or will it wait untill max age timer expires?

Peter Paluch Tue, 08/17/2010 - 10:19

Hello,

So in 802.1D if a non root bridge doesnt receive configuration BPDUs in
Hello time interval, will the non-root bridge take any actions or will
it wait untill max age timer expires?

It will wait until max_age timer expires. Remember, because a non-root bridge does not generate its own BPDUs, the Hello timer is of no use for such a switch. The Hello timer is used exclusively to control the sending of BPDUs.

Best regards,

Peter

krahmani323 Tue, 08/17/2010 - 08:43

Hello,

Its a small doubt, please clarify- When STP Max age timer expires, we  can conclude that Root Bridge is down . what conclusion we can made when  STP Max Age timer expires on a non root bridge?

=> Check how STP recovers from an Indirect Link failure

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


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"If a non root bridge doesnt get a Configuraton BPDU within 10 Hello  intervals , i.e for  10 x 2seconds = 20 seconds,

then will the non-root bridge will wait  till max age timer expires Yes, IF in PVST(+) mode WITHOUT Backbone Fast (Cisco proprietary) feature enabled.

or will non-root bridge take action before  max age timer expires?" Yes IF in PVST(+) mode WITH Backbone Fast feature enabled (if enabled pay attention to enable it on all switch participating in stp).

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Backbone fast is a Cisco proprietary feature that, once enabled on all       switches of a bridge network, can save a switch up to 20 seconds (max_age) when       it recovers from an indirect link failure.

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

****************

In RSTP the feature is standardized : natively included and automatically enabled in RSTP.

Regards.

Karim

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