Ken,
Thanks for the input on this discussion. I follow and understand your
example without any problems.
Now if taking it back to the original original question - Does L3
switching require VLANs - produces this question for your example:
You state 1 fiber feed for both Science and Engineering in the Labs
building. I am then assuming that they are all connected to the same set of
switches (Layer 2) in that building.
Could you have not just simply assigned the hosts for Science to 1 IP
network and the hosts for Engineering to another IP network - then created
respective gateway interfaces for each network back on the common Layer 3
switch and accomplished the same thing??
If the answer is yes, I will followup with another question. If the
answer is no, then please explain.
Thanks!!
Stephen
----- Original Message -----
From: "Ken Diliberto"
To:
Sent: Tuesday, February 18, 2003 12:24 AM
Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
> Stephen,
>
> You're getting there. Let me give an example of how VLANs are used
> (I'd draw a picture, but it probably wouldn't look good).
>
> For this example, let's use two of the colleges on my university
> network: Science and Engineering.
>
> Each has their own block of IP addresses and want their traffic
> separate from the other. They also want flat addressing (no
> subnetting).
>
> We have three buildings: Science, Engineering and Labs. Science and
> Engineering both have computer labs in the Labs building. Each want
> their labs on their respective IP address blocks.
>
> If money were no object, this would be fairly easy with vanilla
> switches and a router with two ethernet interfaces. Multiple fiber
> feeds and two sets of switches would be everywhere.
>
> With budget limitations (for this example), we only have a single fiber
> feed to each location. That means each fiber feed needs to carry
> traffic for both networks. To keep the traffic separate, we partition
> the switch ports into two LANs: LAN 10 and LAN 20. These two LANs in
> one switch are treated as unique. To do this, the switch creates
> Virtual LANs or VLANs. The fiber feeds are now trunks because a header
> is added to each frame to identify the VLAN it belongs to.
>
> So far so good?
>
> Why would we need a router? To talk between VLANs.
>
> Do routers understand trunks? Yes.
>
> This brings up one more concept: the Router on a Stick.
>
> A router on a stick is a router with a single network connection. This
> single connection is configured as a trunk so the router can see all the
> different VLANs. If the router finds a packet on VLAN 10 with a
> destination on VLAN 20, it rewrites the headers for the destination and
> puts it back on the same trunk with VLAN 20 headers.
>
> Remember: replace "layer 3 switch" with "router" every time you see
> it. That might make more sense.
>
> Hope this helps.
>
> Ken
>
> >>> "Stephen Hoover" 02/17/03 06:55PM >>>
> I appreciate everyone's input on this subject to help me understand
> this
> concept.
>
> As far as the newbies comment goes - I most definitely am. I'm about
> as
> green as they come. I have both my CCNA and my CCDA, but my only real
> experience is installing 2 T1s (at different locations) and configuring
> NAT
> for them. I have large amount of knowledge, just no experience. It has
> been
> my goal and my dream to become a serious network engineer for the last
> 6
> years, but I just cannot seem to get a job that offers any experience.
> Everytime I get a "network" position, I just seemed to end up doing
> desktop
> support.
>
> When I first heard the term Layer 3 switching (some 4 years ago now)
> the
> first thing that popped into my mind was a switch that can route. I
> never
> even heard of a VLAN until a couple of years ago.
>
> The Cisco Study guide starts off talking about VLANs, and moves right
> into
> Inter-VLAN routing without ever really discussing Layer 3 switching as
> a
> seperate process. This is really where my confusion started. The book
> makes
> it sound like L3 switching is directly dependent on VLANs, and I just
> didn't
> see it - it wasn't something I was just willing to accept.
>
> Further more, the book states that VLANs allow for physical location
> independence, but is also says that VLANs should not cross the core -
> those
> 2 statements seem partly contradictory to me.
>
> Here is a summary of how I see VLANs now.
>
> Layer 3 switching is possible without VLANs (however the opposite is
> not
> true. Well at least not without some form of Layer 3 intervention.)
>
> VLANs simply the administration behind Layer 3 switching design.
>
> Physical location (port location) independence is ok in front of the
> layer 3 switch that is the the hosts gateway. Up to the hosts
> distribution
> switch.
>
> VLANs extending beyond the distribution layer switch across the
> core is
> generally not a good idea - possible, but not recommended. This is the
> "flat
> earth" design that Priscilla mentioned - VLANs that extend across the
> entire
> internetwork.
>
> Thanks!
> Stephen Hoover
> Dallas, Texas
>
> [snip]
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=63236&t=63147
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