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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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by:The_Warlock
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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
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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 167 total points
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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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by:Akinsd
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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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by:Don Johnston
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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
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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
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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
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Sorry about that. Down side of auto text on iphones
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