John, my understanding is that the router will continue to use the
previously cached route until such time as the cache for that route is aged
as invalid. 1/20th of the cache is aged randomly every one minute, unless
there is less than 200K of available memory, in which case 1/5th of the
cache is aged every minute. After it has aged the router will then process
switch a packet to that destination since it has no cache entry, see that
there are now 2 routes to the destination and re-build the cache with both
paths, performing per-destination balancing as that is the only available
balancing method with fast switching. I have found that in a lab
environment the aging will happen very quickly because you have very few
entries, and you will probably find that it is a minute or less before both
routes are being used. You can see this if you want by configuring 2
routers with 2 different equal cost paths to each other, but only having one
in the routing table, ie shut down one interface. Turn on ICMP debugging
and ping the other router, then bring the other interface up and do another
ping. You will probably find that the router will send all 5 pings out one
interface, and then the next 5 pings will be sent out the other interface.
You can also look at this link
http://www.cisco.com/warp/public/105/20.html#3 to find some really
interesting info on the different switching methods. Hope this helps.
-----Original Message-----
From: John Neiberger [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, June 12, 2001 1:18 AM
To: [EMAIL PROTECTED]
Subject: Route Updates and Fast Switching Cache [7:8110]
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.
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?
Thanks,
John
_______________________________________________________
Send a cool gift with your E-Card
http://www.bluemountain.com/giftcenter/
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=8133&t=8110
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
