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Chapter 6: Routing
·
To notice when routes in the table are no longer valid, and to remove those routes from the
routing table.
·
If a route is removed from the routing table and another route through another neighboring
router is available, to add the route to the routing table. (Many people view this goal and
the previous one as a single goal.)
·
To add new routes, or to replace lost routes with the best currently available route, as
quickly as possible. The time between losing the route and finding a working replacement
route is called
convergence
time.
·
To prevent routing loops.
Comparing Routing Protocols
Several routing protocols for TCP/IP exist. IP's long history and continued popularity have
called for the specification and creation of several different competing options. So, classifying
IP routing protocols based on their differences is useful--and also is a fair topic for exam
questions.
One major classification of IP routing protocols is whether they are optimized for creating
routes inside one organization or routes between two or more interconnected organizations.
Exterior routing protocols are optimized for use between routers from different organizations.
Border Gateway Protocol (BGP) and Exterior Gateway Protocol (EGP) are the two options for
exterior routing protocols; BGP is the most popular and the more recently developed of the two.
(EGP is not technically a routing protocol but is a "reachability" protocol; it is obsolete.)
The CCNA exam focuses on interior routing protocols. If you are interested in pursuing CCIE-
ISP, CCIE-R/S, or CCNP certification, understanding exterior routing protocols is very
important. An excellent learning tool and reference to IP routing and routing protocols is the
Cisco Press book
Routing TCP/IP, Volume I
, by Jeff Doyle.
Routing protocol
is the term used to describe the programs and processes used to exchange and
learn routing information. Other documents call these same programs and processes
routing
algorithms
. Personally, I prefer the term
type of routing protocol
, yet a third term for the same
concept. Terminology counts; for the CCNA exam, remember all three terms.
One type of routing protocol is the
link-state protocol
. Link-state protocols use a topological
database that is created on each router; entries describing each router, each router's attached
links, and each router's neighboring routers are included in the database. Each router builds a
complete map of the network. The topology database is processed by an algorithm called the
Dijkstra shortest path first
(SPF)
algorithm for choosing the best routes to add to the routing
table. This detailed topology information along with the Dijkstra algorithm helps link-state
protocols avoid loops and converge quickly.
ch06.fm Page 360 Monday, March 20, 2000 5:11 PM