Solved: What is the use of BPDU?

veerendrav2 · ‎06-04-2015

Hi all,

Imagine there is a switch having 3 port(Fa0/1, Fa0/2, Fa0/3). Fa0/1 is connected to a PC, Fa0/2 is connected to another switch and in the same way Fa0/3 is connected to another switch.

Then I configured Fa0/1 as FastPort.

1) Now what is use of sending BPDUs to Fa0/1, even though if switch sent, it won't come back.So, on the Fa0/1 there is not possibility of loops.

2) Ok, Fa0/1 is FastPort and there is a BPDU gaurd i.e. Fa0/1 won't participate in STP protocol(Disabling the STP on that port). The PC will get IP address from DHCP server immediately.So, the PC sends broadcast message to DCHP server via Fa0/1. what about Fa0/2 and Fa0/3, they won't forward right!, because the there in STP's Initial states(assume those ports are not in forwarding state). So, what is use of Fastport and BPDU Gaurd.

Maybe I misunderstood the FastPort concept, if yes, please correct me. :-)

Thanks in advance ;-)

Mark Malone · ‎06-04-2015

Hi look at it from a bigger concept, imagine there were 100 layer 2 switches in your network with hundreds of pc,printers etc , everyone of these ports will communicate back to a central root switch which processes the bpdus and takes part in the overall calculation to prevent loops in your network at layer 2 , that's a lot of traffic back and forth using resources on the switches and links involved

If you use portfast and bpduguard it takes these ports out of the STP calculation when something goes wrong and devices go down reducing the amount of resources used and putting these devices straight into forwarding mode as they should be as there edge devices

Only switch ports that connect to other switches should be sending bpdus regarding each other so STP can prevent loops , there's no need for hundreds of other devices to be passing this traffic also if we can prevent it

I understand what your saying regarding the edge port coming up quicker than the designated port so that it may not be responsive until its STP calculation is complete thats correct but you could be running rstp as well which could speed up the process but its more regarding the overall layer 2 setup

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Mark Malone · ‎06-04-2015

Hi look at it from a bigger concept, imagine there were 100 layer 2 switches in your network with hundreds of pc,printers etc , everyone of these ports will communicate back to a central root switch which processes the bpdus and takes part in the overall calculation to prevent loops in your network at layer 2 , that's a lot of traffic back and forth using resources on the switches and links involved

If you use portfast and bpduguard it takes these ports out of the STP calculation when something goes wrong and devices go down reducing the amount of resources used and putting these devices straight into forwarding mode as they should be as there edge devices

Only switch ports that connect to other switches should be sending bpdus regarding each other so STP can prevent loops , there's no need for hundreds of other devices to be passing this traffic also if we can prevent it

I understand what your saying regarding the edge port coming up quicker than the designated port so that it may not be responsive until its STP calculation is complete thats correct but you could be running rstp as well which could speed up the process but its more regarding the overall layer 2 setup

veerendrav2 · ‎06-04-2015

Even thought I'm not convinced with your answer, that makes sense to me. Thanks Mark! ;-)