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Bronze

alternate and blocked port in rstp

hi every body!

I have some confusions about the alternate port and back up port.

Cisco documents define " a backup port receives more useful bpdu from the same bridge it is on and is a port blocked"

That means backup port can only exists on the switch which has designated port,connected to shared medium.

But does it not mean that back port can not be on the same switch which has alternate port connected to shared medium?

So we have a switch which has two ports are connected to shared medium, one of the port is alternate port , what would be the other port?

Thanks a lot!

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: alternate and blocked port in rstp

Hello Sarah,

nice point this

yes the second port is put in blocking state.

the backup exists for the designated port on the shared medium so in your scenario the second port on the switch that loses the designated port election is simply put in blocked state.

It makes sense.

Hope to help

Giuseppe

5 REPLIES
Hall of Fame Super Silver

Re: alternate and blocked port in rstp

Hello Sarah,

the alternate port is the port that will be promoted to root if the current root port fails.

the backup port is a port that is connected to a LAN segment where the same switch has already a better port : example

if I connect fas2/10 and fas2/11 to a hub they share the same segment.

the first port due to its lower index will be the designated port for the LAN segment the second one is classified as backup port and is placed in blocking state if now the first port fails RSTP knows that a backup port for the same segment exists and from the STP point of view no change is necessary just to promote the backup port to designated port.

Actually I've never seen a backup port because in modern networks we have two types of links: point-to-point switch links or user access ports (including router interfaces or server interfaces that for STP are treated like access ports).

At the beginning I was also confused then in a document I found network diagrams showing what a backup port is and I understood what they are.

Hope to help

Giuseppe

Bronze

Re: alternate and blocked port in rstp

Thanks Giuseppe! Could you please help me understand the following:

say a switch has alternate port f0/1 on the shared medium.if we connect another port say f0/2 to the shared medium, what would that port be? it cant be backup port as this switch is not designated bridge on the medium.

thanks a lot!

Hall of Fame Super Silver

Re: alternate and blocked port in rstp

Hello Sarah,

nice point this

yes the second port is put in blocking state.

the backup exists for the designated port on the shared medium so in your scenario the second port on the switch that loses the designated port election is simply put in blocked state.

It makes sense.

Hope to help

Giuseppe

Re: alternate and blocked port in rstp

If the second port is blocked, it's going to be an alternate port, not backup for sure. It could be root port too.

In STP, there were only "blocked" ports. A port can only block when it periodically receives BPDUs. BPDUs are originated from the root bridge, so you can say that a blocked port keeps hearing from the root bridge. If you ever lose your root bridge (the port connecting you to the root), you are going to elect a new one, and a port receiving BPDUs is going to be selected as a new root port. So basically, one of the blocked ports will become the new root port in that case.

RSTP introduced the distinction between two kinds of blocked ports:alternate and backup. An alternate port receives better BPDUs from another bridge, while a backup port receives better information from the bridge itself.

I hope this long introduction will make easy the explanation of the difference between the two now;-)

Suppose that bridge X has a root port, an alternate port and a blocked port. The root port goes down. What port will it select as a root port? X has lost connectivity to the root. It is looking for a path to the root. This path is advertised by BPDUs. And two ports are receiving BPDUs.

- the alternate port receives some information from bridge Y that advertises a path to the root

- the backup port receives some information from bridge X that advertises a path to the root.

So now, is it clear which port X should select as a root port? Certainly not the backup port, because we know that X has lost the path to the root. So the information received on this backup port is based on the root port that has just gone away. The new root port can only be selected out of the group of alternate ports. That's the reason to be for alternate ports vs backup ports.

The group of alternate ports is what was called the "uplink group" when cisco introduced uplinkfast. That's a group of ports that can be immediately selected as root port in case of failure of the root port.

Regards,

Francois

Hall of Fame Super Silver

Re: alternate and blocked port in rstp

Hello Francois,

your explanation is a 5++

Best Regards

Giuseppe

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