Port roles RSTP Rapid Spanning Tree Protocol:Alternate ports and backup ports

inRSTP (Rapid Spanning Tree Protocol)Learning which,We will know RSTP different fromPVST (Per Vlan Spanning Tree)One of the places,It isPort Roles difference。As different port states believe everyone is familiar with,But the problem of port role,Many books and tutorial included in the classroom are a passing,In retrospect,Students can not help knowing a lot of questions,Let's take a look at this,Among the Rapid Spanning Tree Protocol,Exactly what role the port and the two often talked aboutAlternate ports and backup portsWhat is emerging。

Rapid Spanning Tree Protocol RSTP port role of

  • Root Port
  • Designated Port
  • Alternate and Backup Port

According to the official White Paper Cisco《Understanding Rapid Spanning Tree Protocol (802.1in)》View,RSTP port role has above four。

With port and designated port will not say,This and PVST same,Nor is it the place we are interested in。Next we sayAlternate and Backup Port,Alternate ports and backup ports。


In PVST each Vlan Spanning Tree Protocol is in 802.1D,There is a blocking state,In fact, the alternate port and backup port is corresponding to the state,802.1w introduces these two roles。


When a port receives fromNon-selfThe BPDU,And this BPDU priority than the specified port to receive only the difference,Then the port will be transformed into alternate ports,As an alternative route to the bridge with,Once the route designated port failure,Then this will immediately enable alternative path。

When a port receives fromitselfThe BPDU,And this carries a higher priority BPDU,The port will be transformed into a backup port,As a gateway to a network backup path,This is not the path to a backup path to the root oh。

So this time it was said,Two switches using two wires inserted together how it is to replace the port,In order to see how the backup port it? Actually,You just need to switch any two cable shorted interfaces,Then modify the duplex to half, you can see the ~

Keke,In fact, in a shared network,Backup ports occur only possible,Because it may appear that the switch network to a network with redundant line,of course,Duplex mode switch is judged,Full duplex - point;Half-duplex - sharing network。


it is good,Then we do a few small experiments to verify the backup port role。

I use a switch a hub,I used them between two straight-through cable,And the spanning-tree mode switch to Rapid Spanning Tree Protocol,Wait convergence。

f0 switch / 6 connectors received f0 / 5 port BPDU,Because of its higher priority,So f0 / 6 entered a backup port role。This time we try to change the priority f0 / 5 port:

This is the time to see f0 / 5 has become a backup port。

Because I was lazy ah ...... ...... so do not replace port verified ............ ah。

《Understanding Rapid Spanning Tree Protocol (802.1in)"allowableHereDownload inside a detailed graphic description of it,Baidu LibraryYou can find the official Chinese version。

[warning]Another:Why, in the ieee will have to replace the role of the port of existence? Because now all Cisco devices running 802.1D802.1w The 802.1D compatibility mode [/warning]

Original article written by Gerber drop-off:R0uter's Blog » Port roles RSTP Rapid Spanning Tree Protocol:Alternate ports and backup ports

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