John Neiberger wrote:
>
> After reading a practice test question and answer I'm confused about the
> operation of fast switching, specifically when a route has already been
> cached when a new equal-cost route is learned via a different interface.
> For example:
>
> Route A learns of 192.168.1.0/24 via e0 with a metric of 1000. Fast
> switching is enabled so this route is cached. Then the router learns of
> 192.168.1.0/24 with a metric of 1000 via e1. My thinking is that the cache
> would be invalidated and recreated with two entries but the test engine
> answer stated that routing would not change because the route was cached
and
> the cache would not be invalidated.
>
With the exception of CEF, all Cisco switching modes are demand-based.
Only after a packet is successfully forwarded to a destination (via
process-switching), will IOS add a route cache entry for _that destination_.
So it's not as if the existing of a route/prefix causes a cache entry to
be created; rather it's traffic generated.
In your example above, all traffic to 192.168.1.22 would go via e0, as
long as that remains valid. Traffic for a second host in 192.168.1.0/24
will go via e1. And so on.
CEF does add entries to the FIB and AdjTable that directly correspond to
routing table and ARP cache entries. It's not demand-based.
> Any thoughts? I'd test this myself but at the moment I only have two
> routers at home. Do any of you have any experience with this?
Actually you can test this with just two routers if you can create
two equal-cost paths with them. But you must create the traffic with
a third device -- a host connected to a LAN interface. Switching modes
only apply to packets that are in fact switched; i.e., forwarded.
Traffic that originates within the router or terminates in the router
is not fast-switched, nor process-switched -- it's not switched at all!
If a ping is sourced from the router with two equal paths, the pings
will alternate paths on a per-packet basis.
Marty Adkins Email: [EMAIL PROTECTED]
Mentor Technologies Phone: 240-568-6526
133 National Business Pkwy WWW: http://www.mentortech.com
Annapolis Junction, MD 20701 Cisco CCIE #1289
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=8153&t=8110
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