From: istong@stong.org
Date: Fri Oct 14 2005 - 21:13:37 GMT-3
Howard,
Great points. Just the other day a coworker asked how a
router can "route" packets without a routing protocol or
without specifying ip routing in the config. The answer was
as you mentioned - the router can forward packets based on
statics (and I added also via connected interfaces)
Ian
www.ccie4u.com
> May I pause/digress slightly to review some terminology
> that may be confusing things? Cisco terminology for
> things that routers do sometimes are obsolescent by
> current IETF definitions, but have come to be accepted
> within the Cisco world, especially when they have been in
> older training or products.
>
> The core functionality of devices called "routers" (e.g.,
> see RFC 1812), is forwarding packets based on IP header
> information. The first routers from the early vendors did
> all related processing in a single CPU. This processor
> both "drew the map" (i.e., built the routing table from
> hardware, static and dynamic information) and then
> "directed traffic", forwarding packets based on the
> routing table.
>
> In current IETF terminology, "routing" always involves a
> "control" plane that builds the routing table (i.e.,
> routing information base) and possibly separate
> forwarding information base(s), and a "forwarding"
> (sometimes "data") plane that actually moves packets from
> ingress to egress. In the never-ending search for higher
> performance, Cisco first split the control and forwarding
> planes into different processors of the AGS+.
>
> This first dual-path implementation was called autonomous
> switching, and continued into the 7000, where hardware
> upgrades resulted in a functionally equivalent mode
> called silicon switching. On the 7500, optimum switching
> still used two processors, one for control and one for
> forwarding, but the introduction of VIPs allowed one (plus
> hot standby) control processor and multiple forwarding
> processors.
>
> Where am I going with this? I am going to the observation
> that the command "no ip routing" really should be "no ip
> forwarding". The presence or absence of a routing
> protocol is independent of whether or not the router will
> forward based on IP header information. An entire routing
> table could be built from static routes, and the router
> would happily forward.
>
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Ian Stong
http://www.ccie4u.com
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