[rescue] routers on a stick

[James Fogg]

When you run vlan's you need to route between the vlans in order for anything
to see into the other vlans. One way to do this is to run a trunking protocol
(say ISL or 802.1q) over a single ethernet (or fddi/giga-ethernet/TR/etc) link.
This one link has all of the vlans (and therefore all the ip networks) on it.
It becomes a monster of a bottleneck. Another issue is that even the best
routers are really, really slow compared to switching. This approach is called
"router on a stick" (the single link is the stick).

Another way to do it is to use a router with lots-o-interfaces. Place one
interface in each vlan. This is expensive, and still a bottleneck (because
routers are slow compared to switches).

The real solution is to use layer 3 switching. This is real coolness. Normally,
a switch makes a list (table) of MAC addresses available on each port and pushes
packets toward the port that has the destination MAC address on its table. Some
genious figured out that if you made the switch IP aware you could use the
layer 3 address the same way you use MAC addresses. This allows you to use
switching hardware (very, very fast) for routing.

btw.. why vlans? think "broadcast domain" (same idea as collision domain,
different layer).

On Tue, 03 Jul 2001, THOU SPAKE:
> What exactly does the phrase router on a stick mean?  Obviously it is a
> derogotory term for a type of router, but beyond that...
> 
> --
> Joshua Boyd

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	 James D. Fogg, Network Engineer
	Vicinity Corporation - Lebanon, NH

     DESK (603) 442-1751 - CELL (603) 252-1864
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            EMAIL jfogg at vicinity.com

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