From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Fri Oct 14 2005 - 12:05:35 GMT-3
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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