>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
>
If I understand the question, the answer is correct. Where you are
getting confused is the difference in load balancing between process
switching and fast switching. Essentially, your thinking is correct
if the interface were in process switching mode, which does
per-packet load balancing. I'll make the minor nit that process
switching has no cache, but looks things up directly in the RIB
(i.e., main routing table). Formally, per-packet is deterministic
rather than statistical load balancing.
Fast switching uses per-destination load balancing. It will only have
one cached entry to any destination; the load balancing comes as a
result of having many cached entries to many destinations.
Per-destination load balancing is statistical, not deterministic, and
indeed can get unbalanced with a small number of destination. CEF
uses source-destination pair load balancing, which is still
statistical but increases the number of choices and reduces the
probability that the load will be unbalanced onto one interface in a
group.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=8143&t=8110
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