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When to use spanning-tree portfast trunk

Posted on 2014-12-17
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Last Modified: 2014-12-20
Usually spanning-tree portfast  is used on access port to which a computer is connected  , in order to avoid the 50 Seconds of STP. However I saw examples where they use it on a  Trunk Port. I wonder if STP runs on a Trunk and if so why would they disable it with portfast  or type edge (Nexus) ?


CISCO-IOS

spanning-tree portfast (for an access port)
spanning-tree portfast trunk (for a trunk port)


◦NX-IOS

spanning-tree port type edge (for an access port)
spanning-tree port type edge trunk (for a trunk port
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Question by:jskfan
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by:Akinsd
Akinsd earned 200 total points
ID: 40506288
You can disable spanning tree (enable portfast) on a trunk port if there is only one way in and out of the switch, or you are very certain no loops can occur.

 STP
Notice how spanning tree disable the left port in Figure A. With portfast enabled (spanning tree disabled), a loop will definitely occur.

However, there is no consequence enabling STP in figure B as there is no alternate path.

See short simple articles I wrote about STP below
http://www.experts-exchange.com/Networking/Misc/A_11209-Root-Bridge-Election.html

http://www.experts-exchange.com/Networking/Protocols/A_11315-Root-Port-Election-STP-or-RSTP.html


I hope this helps
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by:jskfan
ID: 40506306
I believe I have seen example where the SWITCH port that connects to ESX server is configured as Portfast Trunk.

Usually ports connected to computers are configured as Access ports with portfast, but ESX uses the concept of Vlans too, but I wonder if it can cause loop to Cisco switch that it is connecting to ?
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Akinsd earned 200 total points
ID: 40506319
There won't be a loop if there's only one link or a set of bundled links (Etherchannel) to the Cisco switch. Technically, there's only one connection to the switch and no alternate path.

Just keep the concept simple

If there are 2 or more paths to the switch, you want to use spanning tree. If there's only one, spanning tree is useless at that point and it's only wasting processor resource.

Spanning tree was beneficial in the past but people are moving away from it. Same way car manufactures don't put real tires as spares in cars anymore and use donuts instead.

Why build a bridge that will only be used if the major bridge fails. It is simply a waste of resource. All spanning tree does is disable a link if another link is active. The disabled link will only become active if the main link fails. What if the main link never fails? That means the backup link will never be used.

Etherchannel is a better redundant technology where multiple links are bundled to function as one link. All links are used and if 1 link fails, a switch over is not necessary as the load will be shared by the remaining links.

I hope you get the picture.

STP2
See how redundant ports are blocked in figure A but all ports are active in figure B. I cannot afford to enable portfast in figure A but I can confidently enable it in figure B. All 4 links in B are bundled and function as one link. I still have redundancy and higher bandwidth in B.
The ports could be trunk or access ports and it won't matter.
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by:Predrag Jovic
Predrag Jovic earned 100 total points
ID: 40506378
Trunk don't have to be used to connect switch to another switch. You can use portfast trunk on connection to some end devices like servers or ip phones.
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by:Don Johnston
Don Johnston earned 200 total points
ID: 40508173
If you have a router on a stick or a trunk going to a server, you could use it there.

But... since the whole purpose of portfast is to transition a newly initialized port to forwarding as quickly as possible, using it there doesn't seem very useful since routers and servers a) rarely get power cycled and b) they typically take longer than 30 seconds to boot up.
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Author Comment

by:jskfan
ID: 40508197
I was not sure about the Network Adapter on ESX server that it understands Trunk. So the ESX Network Adapter is not like regular computer 's Network Adapter ?

if a Switch is connected to a computer, and configure the Link as Trunk , would the computer still communicate with switch as if it the link was configured as Access port ?
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by:Don Johnston
Don Johnston earned 200 total points
ID: 40508206
You can trunk to a VMWare server.

If a switchport is configured as a trunk but the connected computer is not, then the computer will communicate over the native VLAN.
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Author Comment

by:jskfan
ID: 40508215
If a switchport is configured as a trunk but the connected computer is not, then the computer will communicate over the native VLAN.

OK
It will be just like an access port with no specific Vlan
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by:Don Johnston
Don Johnston earned 200 total points
ID: 40508236
That's one way of putting it... but I wouldn't (put it that way).

Because there is a specific VLAN.  There's always a VLAN.  :-D

In this case, the computer is communicating over the native VLAN.
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Author Closing Comment

by:jskfan
ID: 40508243
Thank you Guys!
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by:Don Johnston
ID: 40508252
You can disable spanning tree (enable portfast)

BTW, portfast does not disable spanning-tree.  It simply bypasses the initial listening/learning states.  Spanning-tree is still sending and processing BPDU's on a port with portfast enabled on it.
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by:Akinsd
ID: 40510478
@ Don Johnson
It is easier to just say disable. From years of experience, it has proven to help people understand the process faster.
There's about 300page documentation on spanning tree. Most engineers don't care about the details.  The most common concept for spanning tree is whether redundant port is blocked or not. To disable ports being blocked, you enable portfast.
Just "keep it simple"
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Expert Comment

by:Don Johnston
ID: 40510880
It is easier to just say disable.
It's also easier to say that airplanes fly because of anti-gravity engines.  While that may make it easier explaining an airplane to children, it's not very useful when dealing with student pilots.  

From years of experience, it has proven to help people understand the process faster.
And not a month goes by that I don't have someone in class that has a misunderstanding on a fundamental concept in networking that I have to correct.

Your statement was an absolute. Meaning there was no preface of "the behavior is similar to..." or "it works like..." Leading to the assumption that portfast disables spanning tree... which it does not.

There's about 300page documentation on spanning tree.
373 pages to be exact (1998 revision).  :-)

Most engineers don't care about the details.
I think that you'll find a lot of people here (if they're not engineers) do care about the details.

To disable ports being blocked, you enable portfast.
Not correct.  A port with portfast enabled will block if the spanning-tree algorithm determines it should block.

Just "keep it simple"
I prefer to "keep it correct".
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