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Alternate/Backup Port

Posted on 2016-08-20
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Last Modified: 2016-08-21
I am trying to understand  in which scenario Alternate/Backup Ports can be used.
http://www.9tut.com/rapid-spanning-tree-protocol-rstp-tutorial
on the above Link they give example of a HUB , but I do not think anyone uses a Hub nowadays. However what if instead of a HUB as shown on the Link, we put  SwitchX with 2 uplinks connected to SW2
SwitchX is connect to other Downstream LAN

sw
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Question by:jskfan
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14 Comments
 
LVL 83

Assisted Solution

by:David Johnson, CD, MVP
David Johnson, CD, MVP earned 200 total points
ID: 41764027
with a switch wouldn't you want 1 port to sw2 and one to sw3.. This will give you redundancy if sw2 goes offline.
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LVL 18

Assisted Solution

by:Garry Glendown
Garry Glendown earned 200 total points
ID: 41764064
Using multiple links between two switches makes sense (mainly) to either increase capacity (ensure you are using some sort of aggregation protocol like LACP instead of spanning tree), or if you physically (or due to commercial restraints) can run a ring link to another switch, but have different cable routings between the two devices. Otherwise, as David stated, you will split the network if the whole switch in between goes down.
Apart from that, when running a whole ring do make sure you have your loop prevention protocol (ST, RST, PVST, ...) set up uniformly between all devices, otherwise you will quickly notice ;)
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LVL 31

Expert Comment

by:Predrag
ID: 41764067
Backup and alternative ports are port roles in RSTP.
Alternative port is port in discarding state and it is just another potential path to the root bridge. If some of the currently active paths to the root bridge becomes unavailable port will change state to forwarding and become new designated port or root port depending on what link on your topology drawing gets broken.
Backup port is port in discarding state that is connected to the shared network segment (like hub - so you will rarely see these today).

You can read Cisco's explanation - Understanding Rapid Spanning Tree Protocol (802.1w)

It is more interesting with this topology:
RST port roles
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LVL 47

Assisted Solution

by:Craig Beck
Craig Beck earned 200 total points
ID: 41764068
This is theoretical guys, just for learning and understanding how STP works.  :-)

Sometimes you cant connect to two different switches.

Using a switch would only really offer performance enhancements in this scenario as you can largely control STP functionality at SW2, as would be the case if the switch was unmanaged,for example.

In your scenario you'd have a DP at SW2 which would be a RP at the new switch (assuming it's managed) and the second port would be a DP at SW2 and a AP at the new switch.

A backup port would be where a redundant link is connected to the same hub and would replace the RP if it failed, after following the transition states, while an AP is a link that can immediately take over from the RP if it fails. A BP doesn't immediately transition into the RP as N AP does.
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LVL 31

Expert Comment

by:Predrag
ID: 41764200
I forgot to mention above - backup port is always located on switch with designated port (as it can be seen on above picture (taken from Juniper site)).
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Author Comment

by:jskfan
ID: 41764231
Craig Beck
This is theoretical guys, just for learning and understanding how STP works.  :-)

Correct..

It is hard to Master one thing while you are doing Other things
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Author Comment

by:jskfan
ID: 41764233
Well, Alternate Port is just Blocking Port in STP
The gray area is the Backup Port, since HUBs are no longer used nowadays... Just thought to throw this question to see if Backup Ports are used in any other scenario than HUBs
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Author Comment

by:jskfan
ID: 41764276
If you turn off STP on one Switch that is connected to another switch with 2 links. the other Switch will have one Forwarding Port and one Backup Port. Because when you turn off STP on a Switch , it will operate as a HUB
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LVL 31

Accepted Solution

by:
Predrag earned 1400 total points
ID: 41764283
Well, Alternate Port is just Blocking Port in STP
Basically, yes, that would be blocked port in STP.
The gray area is the Backup Port, since HUBs are no longer used nowadays...
Blue circle around B, but can look gray on some displays.
Because when you turn off STP on a Switch , it will operate as a HUB
If STP is turned off on the switch - it will still operate as switch (each port is separate collision domains). To turn switch to hub you need to turn off MAC learning on ports (ports in the same VLAN with MAC learning turned off are one collision domain).
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LVL 57

Expert Comment

by:giltjr
ID: 41764284
Although HUB are not used today, some inexpensive switches don't support STP so the will produce loops within a network just like a HUB.
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LVL 31

Expert Comment

by:Predrag
ID: 41764287
Additionally, to really function as HUB also ports on the switch should be configured in half duplex mode (forgot it above and comment is not editable anymore).
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Author Comment

by:jskfan
ID: 41764349
So to Make a Switch works as a HUB.
-Turn off STP
- execute this command : no mac address-table learning vlan #
-configure each connected port to be Half Duplex
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LVL 31

Expert Comment

by:Predrag
ID: 41764360
Yes.
MAC address learning can be also disabled per port:
no mac-address-table learning interface <type> <port>
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Author Closing Comment

by:jskfan
ID: 41764366
Thank you Guys !
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