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pvst spanning tree

Posted on 2013-10-26
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Last Modified: 2013-10-28
Experts,

Suppose I have (3) cisco 3560 switches brand new taken out of the box.  I power them on and connect them to each other via trunk links. I dont do anything to the Priority so they will use their MAC's to calculate the bridge IDs. they will use PVST by default.  Let's say switch 2 has the lowest MAC address.  If I have 10 VLANS,  can I  assume bridge 2 will be the root bridge for all the VLANS?

Question #2: Does a switch need to have the "int vlan" to qualify to be a candidate for root bridge?

Please, not links explaining STP. I know what it's about and just want to clear up some dumb questions.
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Question by:trojan81
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8 Comments
 
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Assisted Solution

by:Robert Sutton Jr
Robert Sutton Jr earned 668 total points
ID: 39603782
Question #1: If you allow the network to determine this by itself, then yes that is the way the selection should take place via lowest MAC address and same priority on all switches.


#2:  Not necessarily, but typically yes.
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by:Akinsd
Akinsd earned 664 total points
ID: 39604234
In addition to the answer above, all switches have interface vlans out of the box. They all belong to VLAN (Interface Vlan 1) unless you create other interface vlans and re-assign switch ports
VLAN 1 (default VLAN) also cannot be renamed or deleted.
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Don Johnston earned 668 total points
ID: 39604310
Let's say switch 2 has the lowest MAC address.   If I have 10 VLANS,  can I  assume bridge 2 will be the root bridge for all the VLANS?
No need to assume. It's an absolute.  :-)  In other words, yes.
Question #2: Does a switch need to have the "int vlan" to qualify to be a candidate for root bridge?
Nope. SVI's (or VLAN interfaces) are only necessary for processing Layer-3 traffic.
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Expert Comment

by:Akinsd
ID: 39604815
Just to clarify somethings here

SVI (Switched Virtual Interface) as the name implies is a virtual interface and can be layer 2 or layer 3. This is on every switch by default.
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Expert Comment

by:Don Johnston
ID: 39605548
SVI (Switched Virtual Interface) as the name implies is a virtual interface and can be layer 2 or layer 3. This is on every switch by default.
I guess that would depend on one's definition of "SVI".  I have never heard of anyone using that term to describe anything other than a VLAN interface.
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by:Akinsd
ID: 39605604
VLAN = Virtual Local Area Network

SVI = Switched Virtual Interface

The interface VLANs use is Virtual because they themselves are virtual, but are not by themselves SVI

By Layer 2 or 3, I meant "can be on layer 2 or layer 3 switch".

It allows the switch to function in OSI layer 3, (tcp/ip)
When you assign a management ip address to a switch for telnetting or SSHing for example, (whether on a L2 or L3 switch), you are using the SVI

Please keep it simple.

SVI = a Virtual Interface on a Switch (VIS if you will).
Interface is a platform for exchange

http://www.cisco.com/en/US/prod/collateral/routers/ps5853/prod_white_paper0900aecd8064c9f4.html
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by:Don Johnston
ID: 39605612
By Layer 2 or 3, I meant "can be on layer 2 or layer 3 switch".
The omission of the word "on" is rather significant.
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by:Akinsd
ID: 39605620
Sorry about that. Down side of auto text on iphones
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