Saturday, January 31, 2009

VLANs and spanning tree

VLANs are a feature of ethernet switches which makes them act like multiple "virtual switches". Each VLAN is a separate broadcast domain and could be configured with a separate subnet. That way could could have separate subnets for separate purposes (IT, accounting, network management) on one physical switch. This saves money and cabling while decreasing complexity.

Spanning tree is a protocol which allows you to build redundant loops out of ethernet switches without suffering a bandwidth outage due to looping ethernet frames. Spanning tree blocks ports in your switch mesh to change a topology of loops into a non-looping tree. Then if you suffer a link outage, spanning tree will reconverge in a new fully operational tree. This reconvergence make take significant time (30-50 seconds) with the old spanning tree protocol.

More modern improvements to the spanning tree protocol, including RSTP (rapid spanning tree) and MST (multiple spanning tree) will be covered in a later episode.

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Blogger Darrell Root said...

Diagram is at

January 31, 2009 at 10:03 AM  
Blogger Jeff said...

Hello Darrell,
I am planning on buying a couple of routers to pratice for the new CCENT certification. Is the Cisco 2501 ok to use for this. If not what model do you recommend?

February 3, 2009 at 11:58 AM  
Blogger Darrell Root said...

You definitely want 1RU routers for home so you don't use up too much power or space. They should each have 2 interfaces that can interconnect so you can build a triangle.

25xx routers are incredibly old. But cheap. You can learn a ton of IOS on them. If they have 16 megs of flash and 16 megs of DRAM you can even load early IPv6 code on them.

26xx routers are better (especially if you get one with a fast ethernet interface, where you can configure 802.1q subinterfaces). But even they don't support OSPFv3 for IPv6.

Of course, a modern 28xx router would be best. But the newer the router, the higher the price.
You'd have to pay real money for a 28xx.

The most important thing is to get routers with multiple ethernet interfaces so you can put them in a triangle.

My first episode from 2006 talks about what type of gear to get (although it's a bit dated).

February 6, 2009 at 8:20 PM  
Blogger Brandon said...

Awesome videos! I find myself spending HUGE amounts of time trying to find previous vodcasts or a topic that was previously done. Can you include the search bar gadget or Labels gadget??!!!

Let me know, thanks! Again awesome instruction

January 27, 2010 at 6:10 PM  
Blogger Darrell Root said...

The easiest way to list all the videos is to subscribe in itunes. I'm taking a break from the videos but next time I do a burst I'll see if I can find a better blogger template with a better overall index.

Thanks for the compliment on the videos.

January 27, 2010 at 8:50 PM  

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