Routing Table : closer look
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Sample Routing Table
Output Interface or
next hop IP
Static Route
Dynamic Routing
Protocol (RIP)
Directly Connected
Network
Administrative Distance/Metric
Routing Table structure
Insert Routing Table
Level 1 Routes (1)
• A level 1 route is a route with a subnet mask
equal to or less than the classful mask of the
network address.
– 192.168.1.0/24 is a level 1 network route, because
the subnet mask is equal to the network's classful
mask.
– /24 is the classful mask for class C networks, such
as the 192.168.1.0 network.
Level 1 Routes (2)
• Default route - A default route is a static route
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with the address 0.0.0.0/0.
Supernet route - A supernet route is a network
address with a mask less than the classful mask.
Network route - A network route is a route that
has a subnet mask equal to that of the classful
mask.
The source of the level 1 route can be a directly
connected network, static route, or a dynamic
routing protocol.
Level 1 Routes (3)
• The level 1 route 192.168.1.0/24 can be further
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defined as an ultimate route.
An ultimate route is a route that includes:
– a next-hop IP address (another path)
– an exit interface
• The directly connected network 192.168.1.0/24 is a
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level 1 network route because it has a subnet mask
that is the same as its classful mask.
This same route is also an ultimate route because it
contains the exit interface Serial 0/0/1.
Parent and child Routes :Classful
Network (1)
Parent Routes
Child Routes
Parent and child Routes :Classful
Network (2)
• A level 1 parent route is a network route that does
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not contain a next-hop IP address or exit interface
for any network.
A parent route is actually a heading that indicates
the presence of level 2 routes, also known as child
routes.
A parent route is created whenever a route with a
mask greater than the classful mask is entered into
the routing table (subnetting).
The subnet is the level 2 child route of the parent
route.
Parent and child Routes :Classful
Network (3)
• A level 2 route is a route that is a subnet of a
classful network address.
• Like a level 1 route, the source of a level 2
route can be a directly connected network, a
static route, or a dynamic routing protocol.
Parent and child Routes :Classful
Network (4)
• Level 1 Parent Route contains the following
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information :
172.16.0.0 - The classful network address for our
subnet.
/24 - The subnet mask for all of the child routes.
If the child routes have variable length subnet
masks (VLSM), the subnet mask will be excluded
from the parent route and included with the
individual child routes.
is subnetted, 1 subnet - This part of the route
specifies that this is a parent route and in this
case has one child route, that is, 1 subnet.
Parent and child Routes :Classful
Network (5)
• Level 2 Child Route contains the following
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information:
C - The route code for directly connected network.
172.16.3.0 - The specific route entry.
is directly connected - Along with the route code of
C, this specifies that this is a directly connected
network with an administrative distance of 0.
FastEthernet0/0 - The exit interface for forwarding
packets that match this specific route entry.
Parent and child Routes :Classless
Network
Routing Table Look up Process (1)
The router examines level 1 routes, including network routes and supernet routes,
for the best match with the destination address of the IP packet.
Routing Table Look up Process (2)
If the best match is a level 1 ultimate route - a classful network,
supernet, or default route - this route is used to forward the packet.
Routing Table Look up Process (3)
If the best match is a level 1 parent route, proceed to Step 2.
Routing Table Look up Process (4)
The router examines child routes (the subnet routes) of the parent route
for a best match
Routing Table Look up Process (5)
If there is a match with a level 2 child route, that subnet will be
used to forward the packet.
Routing Table Look up Process (6)
If there is not a match with any of the level 2 child routes, proceed to Step 3.
Routing Table Look up Process (7)
Is the router implementing classful or classless routing behavior?
Routing Table Look up Process (8)
Classful routing behavior: If classful routing behavior is in effect,
terminate the lookup process and drop the packet.
Routing Table Look up Process (9)
Classless routing behavior: If classless routing behavior is in effect, continue
searching level 1 supernet routes in the routing table for a match,
including the default route, if there is one.
Routing Table Look up Process (10)
If there is now a lesser match with a level 1 supernet or default routes,
the router uses that route to forward the packet.
Routing Table Look up Process (11)
If there is not a match with any route in the routing table, the router drops the packet.
Longest Match: Level 1 Network
Routes
• The best match or longest match is the route in the routing
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table that has the most number of left-most matching bits with
the destination IP address of the packet.
The route with the most number of equivalent left-most bits, or
the longest match, is always the preferred route.
The router first examines level 1 routes for the best match.
In our example, there is a match between the destination IP address
192.168.1.2 and the level 1 ultimate route of 192.168.1.0/24.
R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0
R1 uses this route and forwards the packet out interface Serial 0/0/0.
Longest Match: Level 1 Parent and
Level 2 child routes (1)
Longest Match: Level 1 Parent and
Level 2 child routes (2)
• The first match that occurs is with the level 1
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parent route, 172.16.0.0.
Remember, with non-VLSM subnets the classful
mask of the parent is now displayed.
Before any child routes (subnets) are examined for
a match, there must first be a match with the
classful address of the parent route.
• Because the first route entry is a level 1 parent
route that matches the destination address (Step
1b of the route lookup process), the route lookup
process moves to Step 2.
Longest Match: Level 1 Parent and
Level 2 child routes (3)
• Because there is a match with the parent
route, the level 2 child routes will be
examined for a match.
• However, this time the actual subnet mask
of /24 is used for the minimum number of
left-most bits that must match.
Route look up with VLSM (1)
Route look up with VLSM (2)
• As with non-VLSM networks, if there is a match between
the packet's destination IP address and the classful mask
of the level 1 parent route, the level 2 child routes will be
searched.
• The only difference with VLSM is that child routes display
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their own specific subnet masks.
These subnet masks are used to determine the number of
left-most bits that must match the packet's destination IP
address.
For example, for there to be a match with the 172.16.1.4
child route, a minimum of 30 left-most bits must match
because the subnet mask is /30.
Routing behavior (1)
• Routing behavior influences the process of
searching for the preferred route using the
no ip classless or ip classless commands.
• Classless and classful routing behaviors are
not the same as classless and classful
routing protocols.
Routing behavior (2)
• Classful and classless routing protocols
affect how the routing table is populated.
• Classful and classless routing behaviors
determine how the routing table is
searched after it is populated.
• In the figure, the routing sources (including
classful and classless routing protocols) are
the inputs used to populate the routing
table.
Classful Routing Behavior (1)
Classful Routing Behavior (2)
Classful Routing Behavior (3)
Classful Routing Behavior (4)
Classful Routing Behavior (5)
• Because router R2 is using classful routing behavior,
no ip classless, the router will not search beyond
the child routes for a lesser match.
Classful Routing Behavior (6)
• The routing table process will not use the default
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route, 0.0.0.0/0, or any other route.
A common error is to assume that a default route
will always be used if the router does not have a
better route.
In the example, R2's default route is not
examined nor used, although it is a match.
This is often a very surprising result when a
network administrator does not understand the
difference between classful and classless routing
behavior.
Classless Routing Behavior (1)
Classless Routing Behavior (2)
Is the router implementing classful or classless routing behavior?
Classless Routing Behavior (3)
Classful routing behavior: If classful routing behavior is in effect,
terminate the lookup process and drop the packet.
Classless Routing Behavior (4)
Classless routing behavior: If classless routing behavior is in effect, continue
searching level 1 supernet routes in the routing table for a match,
including the default route, if there is one.
Classless Routing Behavior (5)
If there is now a lesser match with a level 1 supernet or default routes,
the router uses that route to forward the packet.
Classless Routing Behavior (6)
If there is not a match with any route in the routing table, the router drops the packet.
Classless Routing Behavior (7)
Classless Routing Behavior (8)
Classless Routing Behavior (9)
Classless Routing Behavior (10)
Classless Routing Behavior (11)
Classless Routing Behavior (12)
• Because we are using classless routing behavior
(ip classless), the router continues searching the
routing table, beyond this parent route and its
child routes.
Classless Routing Behavior (13)
• S* 0.0.0.0/0 is directly connected, Serial0/0/1
• The mask is /0, which means that zero or no bits need to match. A
default route will be the lowest-bit match. In classless routing
behavior, if no other route matches, the default route will match.
Classless Routing Behavior (14)
• In this case the router will use the default route,
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because it is the best match.
The packet will be forwarded out the Serial 0/0/1
interface.
Classful VS Classless routing
behavior in the real world
• Remember that classful and classless routing
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behaviors are independent from classful and
classless routing protocols.
A router could be configured with classful routing
behavior (no ip classless) and a classless routing
protocol, such as RIPv2.
A router could also be configured with classless
routing behavior (ip classless) and a classful
routing protocol, such as RIPv1.
In today's networks, it is recommended to use
classless routing behavior so that supernet and
default routes can be used whenever needed.