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Advanced Routing
Lesson 3 – Understanding Routing Tables

Lesson Content
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The Routing Table
Routing Table Sources
Interpreting the entries in the routing table
Directly Connected Routes
Directly connected IPv6 Routes
Static Routes
Default Static Routes
Default Static IPv6 Routes
Routing Table Terms
Ultimate Route
Level 1 Route
Level 1 Parent Route
Level 2 Child Route
Route Lookup Process
IPv6 Routing Table Entries

The Routing Table
• A routing table is a file stored in RAM that contains information
about:
Directly connected routes
Remote routes
Network or next hop associations

Routing Table Sources
The show ip route command is used to display the contents of the
routing table:
• Local route interfaces - Added to the routing table when an
interface is configured
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• Static routes - Added when a route is manually configured and the
exit interface is active
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Routing Table Sources
(Cont…)

Interpreting the entries in
the routing table

Directly Connected Routes
A newly deployed router, without any configured interfaces, has an
empty routing table
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Directly connected IPv6 Routes
• The show ipv6 route command shows the ipv6 networks and
routes installed in the routing table
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• Static routes must be manually updated if the topology changes
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• Configure a static route to a specific network using the ip route network

mask {next-hop-ip | exit-intf} command
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• Configure a default static route using the ip route 0
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0
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0
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0 {exitintf | next-hop-ip} command
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Directly
connected, dynamically
learned, and link local
routes are ultimate
routes
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Level 1 Route (Cont…)

Level 1 Parent Route

A level 1 parent route is a
level 1 network route
that is subnetted
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Level 1 Parent Route (Cont…)

Level 2 Child Route
A level 2 child route is a
route that is a subnet of
a classful network
address
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Level 1 parent routes
contain level 2 child
routes
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2
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4
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If the best match is a level 1 ultimate route, then this route is used to
forward the packet
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The router examines child routes (the subnet routes) of the parent route
for a best match
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If there is not a match with any of the level 2 child routes, proceed to the
next step
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7
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The router continues searching level 1 supernet routes in the routing
table for a match, including the default route, if there is one
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If there is not a match with any route in the routing table, the router
drops the packet
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The route with the
greatest number of
equivalent far left bits, or
the longest match, is
always the preferred
route
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• IPv6 is classless by design, all routes are effectively level 1 ultimate
routes
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End of Lesson 3



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