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Bronze

portfast

Hi everybody

How is everybody doing?

I just want to confirm few things with regard to portfast.

1)My book says if portfast enabled port receives a bpdu, it will shutdown .

When I performed the lab  I noticed that portfast enabled port does not shutdown, stp just blocks that port if the received bpdu is superior.

Cisco says portfast enabled port should have only one host .  The question is what if we have two hosts connected via hub to portfast enabled port?

When i performed this lab , I observed portfast enabled does not react and portfast feature remained enabled

2)  Enabling portfast feature does not disable transmission of bpdu.   Even if switch port is in access mode, switch continues to transmit bpdu.

Is there any way we can stop such bpdu to conserve bandwidth?

3 )Unfortunately I don't have  resources to see how a port configured with " switchport host" command reacts to following situations:

If a port is configured with switchport host command, what will happen if:

1)  more than one hosts are connected to such port? Will it shutdown?

2) if bpdu is received on such port?  will it shutdown?

Thanks and have a great weekend.

2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: portfast

1)My book says if portfast enabled port receives a bpdu, it will shutdown .

When  I performed the lab  I noticed that portfast enabled port does not  shutdown, stp just blocks that port if the received bpdu is superior.

Cisco  says portfast enabled port should have only one host .  The question is  what if we have two hosts connected via hub to portfast enabled port?

When i performed this lab , I observed portfast enabled does not react and portfast feature remained enabled

--> PORTFAST only transitions quickly by skipping listening and learning states. It doesnt do anything with regards to reception of bpdu. It wont block anything by itself. It you connect a switch to the portfast enabled ports there may be a spanning tree loop. If you connect 2 hosts using HUB with the portfast on one port that would not affect anything . If you use two ports to connect then it will create a loop.

2)  Enabling portfast feature does not disable transmission of  bpdu.   Even if switch port is in access mode, switch continues to  transmit bpdu. Is there any way we can stop such bpdu to conserve bandwidth?

--> Yes you can stop sending BPDUs using bpdufilter feature only when portfast is on. It stops sending and receiving on BPDUs.

Interface Command:

spanning-tree bpdufilter enable


3 )Unfortunately I don't have  resources to see how a port configured  with " switchport host" command reacts to following situations:  If a port is configured with switchport host command, what will happen if:

a)  more than one hosts are connected to such port? Will it shutdown?

b) if bpdu is received on such port?  will it shutdown?

--> switchport host enables portfast and port to be access port.  You can connect multiple hosts to such port but if you use a switch then spanning tree loops will occur Hub will create broadcast issues (ccna stuff). If BPDU is received on such port it wont shut it down as its just a normal portfastport.

To shut it down when bpdu is received you can use "bpduguard" feature of portfast so that port gets errdisabled and doesnt let anyone in.

Interface Command:

spanning-tree bpduguard enable

Happy weekend to you too.

Let me know if this helped. Thanks.

Nandan Mathure

Super Bronze

Re: portfast

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

The other posts have well covered the details, but I just want to emphasis the key to understanding Portfast is it's a STP port that skips initial listening and learning while blocking the port (as described by Nandan).

Documentation about using this setting for hosts is because this is what Portfast was provided for, to allows hosts to join the network without waiting for STP to unblock the port.  Just a single edge host isn't going to create a loop and earlier hosts used to have difficulity with the delay between getting link-up and being able to actual "talk" to anything (as original Etherent was shared.  I.e. there was no port delay with 10Base2 or 10Base5 or 10BaseT shared hubs.  (I.e. Link-up, host is hot on the network.)

So, Portfast allows a host to come on the network a bit quicker because you're telling the port it shouldn't need to worry about loops.

Interestingly, though, Portfast doesn't disable STP, again it just skips initial link-up listening and learning while blocking.  As noted in the other posts, if Portfast detects a STP loop it will still block the port.  The problem, though, is your infrastructure get take quite a hit until (and if) a looped network is blocked by STP.  Without Portfast, a loop isn't allowed (all this assumes we have BPDUs, etc.) because we wait to unblock a port when we know it is safe to do so.  With Portfast the port is unblocked and we might have to deal with a loop flood and then try to block the port.

With rapid-STP, Portfast also doesn't send TCNs with link up/down status change while port is still in Portfast (edge) mode.

If this makes sense, then you can see why you could connect multiple downstream hosts via a hub or "dumb" switch (or switch with STP disenabled or filtered) and a Portfast doesn't care.  I.e. it really doesn't care about hosts.

On your question about conserving bandwidth by filtering out BPDUs, on typical LAN circuits, and switches, the "insurance" of having STP active is well worth the slight performance hit to block an inadvertent loop.  In theory if your topology is loop free (i.e. no redundancy) you don't need STP, but all it takes is one "oops" and your whole network can crash and burn.  Features like STP, even in non-redundant L2 topologies, is an important safeguard and one reason to use "smart" or manageable switches that support it rather than $99 48 port non-managed switches.

5 REPLIES

Re: portfast

1)My book says if portfast enabled port receives a bpdu, it will shutdown .

When  I performed the lab  I noticed that portfast enabled port does not  shutdown, stp just blocks that port if the received bpdu is superior.

Cisco  says portfast enabled port should have only one host .  The question is  what if we have two hosts connected via hub to portfast enabled port?

When i performed this lab , I observed portfast enabled does not react and portfast feature remained enabled

--> PORTFAST only transitions quickly by skipping listening and learning states. It doesnt do anything with regards to reception of bpdu. It wont block anything by itself. It you connect a switch to the portfast enabled ports there may be a spanning tree loop. If you connect 2 hosts using HUB with the portfast on one port that would not affect anything . If you use two ports to connect then it will create a loop.

2)  Enabling portfast feature does not disable transmission of  bpdu.   Even if switch port is in access mode, switch continues to  transmit bpdu. Is there any way we can stop such bpdu to conserve bandwidth?

--> Yes you can stop sending BPDUs using bpdufilter feature only when portfast is on. It stops sending and receiving on BPDUs.

Interface Command:

spanning-tree bpdufilter enable


3 )Unfortunately I don't have  resources to see how a port configured  with " switchport host" command reacts to following situations:  If a port is configured with switchport host command, what will happen if:

a)  more than one hosts are connected to such port? Will it shutdown?

b) if bpdu is received on such port?  will it shutdown?

--> switchport host enables portfast and port to be access port.  You can connect multiple hosts to such port but if you use a switch then spanning tree loops will occur Hub will create broadcast issues (ccna stuff). If BPDU is received on such port it wont shut it down as its just a normal portfastport.

To shut it down when bpdu is received you can use "bpduguard" feature of portfast so that port gets errdisabled and doesnt let anyone in.

Interface Command:

spanning-tree bpduguard enable

Happy weekend to you too.

Let me know if this helped. Thanks.

Nandan Mathure

Bronze

portfast

Thanks Nandan for your response.

portfast

>> It you connect a switch to the portfast enabled ports there will be a spanning tree loop.

Not necessarily, a loop will exist only if a cable is physically looped.

Re: portfast

@jimmysands73

Yea thats right. corrected the wording. Thanks

Super Bronze

Re: portfast

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

The other posts have well covered the details, but I just want to emphasis the key to understanding Portfast is it's a STP port that skips initial listening and learning while blocking the port (as described by Nandan).

Documentation about using this setting for hosts is because this is what Portfast was provided for, to allows hosts to join the network without waiting for STP to unblock the port.  Just a single edge host isn't going to create a loop and earlier hosts used to have difficulity with the delay between getting link-up and being able to actual "talk" to anything (as original Etherent was shared.  I.e. there was no port delay with 10Base2 or 10Base5 or 10BaseT shared hubs.  (I.e. Link-up, host is hot on the network.)

So, Portfast allows a host to come on the network a bit quicker because you're telling the port it shouldn't need to worry about loops.

Interestingly, though, Portfast doesn't disable STP, again it just skips initial link-up listening and learning while blocking.  As noted in the other posts, if Portfast detects a STP loop it will still block the port.  The problem, though, is your infrastructure get take quite a hit until (and if) a looped network is blocked by STP.  Without Portfast, a loop isn't allowed (all this assumes we have BPDUs, etc.) because we wait to unblock a port when we know it is safe to do so.  With Portfast the port is unblocked and we might have to deal with a loop flood and then try to block the port.

With rapid-STP, Portfast also doesn't send TCNs with link up/down status change while port is still in Portfast (edge) mode.

If this makes sense, then you can see why you could connect multiple downstream hosts via a hub or "dumb" switch (or switch with STP disenabled or filtered) and a Portfast doesn't care.  I.e. it really doesn't care about hosts.

On your question about conserving bandwidth by filtering out BPDUs, on typical LAN circuits, and switches, the "insurance" of having STP active is well worth the slight performance hit to block an inadvertent loop.  In theory if your topology is loop free (i.e. no redundancy) you don't need STP, but all it takes is one "oops" and your whole network can crash and burn.  Features like STP, even in non-redundant L2 topologies, is an important safeguard and one reason to use "smart" or manageable switches that support it rather than $99 48 port non-managed switches.

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