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Distance Vector Routing Protocols
359
Foundation Topics
Distance Vector Routing Protocols
The CCNA exam requires that you be able to describe the logic behind distance vector routing
protocols, as well as describe the operation of two distance vector routing protocols, RIP and
IGRP. Configuration on a Cisco router is left until the next section.
Terminology relating to routing protocols is often misunderstood; this confusion is a direct
result of inconsistent use of terminology by authors. The terminology relating to routing
protocols and routing in this book is consistent with the courses in the Cisco CCNA training
path, as well as with most Cisco documentation. The first term that needs to be defined is
routing protocol
. This term can be contrasted with
routed protocol
. Chapter 3 provides a silly,
but I hope a memorable, story (the "Ted and Ting" story) that can help distinguish between the
first two terms. Three definitions follow:
·
A
routing protocol
fills the routing table with routing information. Examples include RIP
and IGRP.
·
A
routed protocol
is a protocol that has an OSI Layer 3 equivalent specification, which
defines logical addressing and routing. The packets defined by the network layer (Layer
3) portion of these protocols can be routed. Examples of protocols include IP and IPX.
·
The term
routing type
might appear on questions remaining from the older CCNA exam,
640-407. This term refers to the type of routing protocol--for instance, link-state.
IP routing protocols fill the IP routing table with valid, (hopefully) loop-free routes. (As you
will see later, distance vector routing protocols have many features that prevent loops, none of
which guarantees to prevent loops.) Each route includes a subnet number, the interface out
which to forward packets so that they are delivered to that subnet, and the IP address of the next
router that should receive packets destined for that subnet (if needed). An analogy to routing is
the process a stubborn man might use when taking a trip to somewhere he has never been. He
might look for a road sign referring to the destination town and pointing him to the next turn.
By repeating the process at each intersection, he should eventually make it to the correct town,
at last. Of course, if a routing loop occurs (in other words, he's lost!) and he stubbornly never
asks for directions, he could loop forever--or at least until he's out of gas!
Before discussing the underlying logic, the goals of a routing protocol should be considered.
The goals documented in the following list are common for any IP routing protocol, regardless
of its underlying logic type:
·
To dynamically learn and fill the routing table with a route to all subnets in the network.
·
If more than one route to a subnet is available, to place the best route in the routing table.
ch06.fm Page 359 Monday, March 20, 2000 5:11 PM