Re: question on port fast and bpdu guard

To put it simply, because they would have an interface that after STP
convergence was not either a root port (RP) or designated port (DP)
and therefore, the STP algorithm places those interfaces in a blocked
state. If it was my interview I would have asked who the root bridge
in the topology was so that I could draw out exactly the state of each
port.

Let's assume SWA is the root bridge. Links would have these states:

SWA/SWB Link: DP
SWA/SWC Link: DP

SWB/SWA Link: RP
SWC/SWA Link: RP

Now what happens next would depend on some things....for SWD to choose
a root port it will pick the interface with the lowest cost path to
the root bridge. In our topology we will say all links are equal.
Therefore, SWD has two equal cost patch to the root, so it will
evaluate lowest sending BID. Let's say SWB has the lower BID here. So
then...

SWB/SWD Link: DP
SWD/SWB Link: RP

SWC/SWD Link: DP
SWD/SWC Link: BLOCKING <--- not root or designated.

P.S. the blocking link KNOWS it is not the DP due to the information
sent in the BPDU's on the segment in this case from SWC. SWC has a
superior BPDU and wins the day for the right to be DP on that segment

HTH!

On Thu, Apr 15, 2010 at 12:18 PM, imran ali <immrccie_at_gmail.com> wrote:
> Thanks you Anthony & Cristian,
>
> i have the following topology
>
> SWA--------------------------------------------SWB
> | |
> | |
> | |
> | |
> | |
> SWC--------------------------------------------SWD
>
>
> IS there a loop in the network.?? yes as each switch has tow different paths
> to reach the other switch ..
>
> *how does SWC or SWD know about the loop in the topology ..*
>
> This was my interview question...i simply said " by examining BPDU'S.." but
> he needed more detailed answers..
>
> i am looking for more technical answer..
>
> Thanks
>
> On Thu, Apr 15, 2010 at 8:56 PM, Cristian Matei
> <cristian.matei_at_datanets.ro>wrote:
>
>> Hi Ali,
>>
>> BPDU Guard and BPDU Filter can be applied in the following 2 ways:
>> -at the global level; once enabled here it will get applied
>> only to PORTFAST enabled ports (I say enabled and not configured as a
>> configured portfast port may lose its portfast status in certain
>> conditions)
>> -at the interface level; once enabled here it's just applied
>> regardless of the portfast status of the port (so portfast is not required
>> in this case)
>>
>> Now PORTFast itself is not "needed" for STP or RSTP; it just improves
>> end-to-end STP/RSTP convergence time; for best end-to-end convergence time,
>> PORTFAST is required to be explicitly configured in both STP and RSTP.
>>
>> Regards,
>> Cristian.
>>
>> -----Original Message-----
>> From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
>> imran ali
>> Sent: Thursday, April 15, 2010 5:49 PM
>> To: Cisco certification
>> Subject: question on port fast and bpdu guard
>>
>> HI gurus,
>>
>> does enabling bpdu guard requires a port fast to be enabled.. i mean bpdu
>> guard can be enabled independent of port fast status ?.
>>
>> further why portfast is required in traditional STP topology and not in
>> RSTP
>> networks..??
>>
>> can i enable portfast feature in RSTP netorks
>>
>> Thanks,
>>
>>
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>>
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>
>
>

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