Easy questions about STP.

[ciscolover]

Hi all, some easy questions about stp:

1) ¿An edge port can detect loops?

2) In each port, the bridge have a different MAC address. ¿If you connect 2 switches using 2 ports, one running STP an the other not. The loop is not detected because it not detects different macs ? 

Regards.

[InayathUlla Sharieff]

) ¿An edge port can detect loops?

Answer: YES and NO.  Bydefault no.

YES:- If you use the loop prevention mechanism on top of configuring of Edge/portfast on the edge port.

2) In each port, the bridge have a different MAC address. ¿If you connect 2 switches using 2 ports, one running STP an the other not. The loop is not detected because it not detects different macs ? 

Answer:-I dont understand what you mean to ask in this question. Anyways if one switch is running STP and other is not then there is no point of using STP in your network. How will you prevent the loops in that case?  Okay I will think that you have only one link connecting only 2 switches then you need not have to worry about anything.

But take this into consideration that in future you will add second link to the second switch what happens if you dont run STP...you know that.

HTH

Regards

Inayath

[ciscolover]

2) You can use STP to créate a tree without logical loops.... And you can use STP to loop prevention.

If you have an error and you create a loop unintencionali in your network for example a technical.

¿stp can block it? For example, you connect unintencionali a bridge using 2 ports creating a loop... A simple bridge non gestionable.

In the 2 questions I talk about loop prevention.

Regards.

[InayathUlla Sharieff]

Let me explain you how it works:-

Sw1====2 links===SW2

In this setup one of the link will be block if you are running STP.

SW1-----SW2

  \          /

    SW3

In this traingle if you dont have the STP there will be a loop. 

STP:- In this scenario the STP should be enabled and it will block one of the link.

There are lot of benifits of running STP on the network.

HTH

Regards

Inayath

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[ciscolover]

Example 1 ok...

Example 2 ok

Example 3 ok

In example 1 , if you have only STP enable in one switch the loop is not blocked... Then STP not controls loops caused by devices that not belong to the TREE...

[InayathUlla Sharieff]

By default STP is enable.if u disable the STP on one switch that means the communication of bpdu is blocking/stopped hence no more concept of blocking occurs between them.

Hth

Inayath

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[ciscolover]

I always throught STP controls loops formed in the tree by the switches with the protocol enable and also to control unwanted loops. For example, connecting an old switch without stp creating a loop connecting 2 ports to the same switch...

I throught stp could blocks when it learns the same mac by two ports...

[Madhukrishnan Gopinathan Nair]

Hello,

For the above, STP still can help you. There is a feature "bpdu gaurd" which will error-disable the port immediately if a BPDU is being received on it.

In your case, you are creating a loop by connecting a second port from the unmanaged switch (or old switch as you say)which does not have capability to run STP. So as soon as the BPDU send is received back by the switch through your second link, it will be error disabled by cisco switch if you have BPDU guard.

Hope this answers your questions

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Thanks,

Madhu

[ciscolover]

Thanks...

But, if a bridge with STP learns on 2 diferent ports the same mac, it not blocks one of them?

Regards.

[InayathUlla Sharieff]

Hi,

It depends on how the connectivity is? If the same Mac is seen on two different s ports on switch then you would see Mac flap in the logs.

Regards

Inayath

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