routing table : closer look w.lilakiatsakun. sample routing table static route dynamic routing...

Routing Table : closer lookRouting Table : closer look

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Sample Routing TableSample Routing Table

Static Route

Dynamic Routing Protocol (RIP)

Directly Connected Network

Administrative Distance/Metric

Output Interface or next hop IP

Routing Table structureRouting Table structure

Insert Routing TableInsert Routing Table

Level 1 Routes (1)Level 1 Routes (1)• A level 1 route is a route with a subnet mask equal to or less than A level 1 route is a route with a subnet mask equal to or less than

the classful mask of the network address. the classful mask of the network address. – 192.168.1.0/24 is a level 1 network route, because the subnet mask 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 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.class C networks, such as the 192.168.1.0 network.

• A level 1 route can function as a:A level 1 route can function as a:• Default route - A default route is a static route with the address Default route - A default route is a static route with the address

0.0.0.0/0.0.0.0.0/0.• Supernet route - A supernet route is a network address with a Supernet route - A supernet route is a network address with a

mask less than the classful mask. mask less than the classful mask. • Network route - A network route is a route that has a subnet Network route - A network route is a route that has a subnet

mask equal to that of the classful mask. A network route can also mask equal to that of the classful mask. A network route can also be a parent route. Parent routes will be discussed in the next be a parent route. Parent routes will be discussed in the next section.section.

• The source of the level 1 route can be a directly connected The source of the level 1 route can be a directly connected network, static route, or a dynamic routing protocol.network, static route, or a dynamic routing protocol.

Level 1 Routes (2)Level 1 Routes (2)

• The level 1 route 192.168.1.0/24 can be further The level 1 route 192.168.1.0/24 can be further defined as an defined as an ultimate routeultimate route. .

• An ultimate route is a route that includes:An ultimate route is a route that includes:– either a next-hop IP address (another path)either a next-hop IP address (another path)– and/or an exit interface and/or an exit interface

• The directly connected network 192.168.1.0/24 is The directly connected network 192.168.1.0/24 is a level 1 network route because it has a subnet a level 1 network route because it has a subnet mask that is the same as its classful mask. mask that is the same as its classful mask.

• This same route is also an ultimate route because This same route is also an ultimate route because it contains the exit interface Serial 0/0/1. it contains the exit interface Serial 0/0/1.

• C 192.168.1.0/24 is directly connected, C 192.168.1.0/24 is directly connected, Serial0/0/1Serial0/0/1

Parent and child Parent and child Routes :Classful Network (1)Routes :Classful Network (1)Parent Routes

Child Routes

Parent and child Parent and child Routes :Classful Network (2)Routes :Classful Network (2)

• A level 1 parent route is a network route that A level 1 parent route is a network route that does not does not contain a next-hop IP address or exit interfacecontain a next-hop IP address or exit interface for any for any network. network.

• A parent route is actually a heading that indicates the A parent route is actually a heading that indicates the presence of level 2 routes, also known as child routes. presence of level 2 routes, also known as child routes.

• A parent route is created whenever a route with a A parent route is created whenever a route with a mask greater than the classful mask is entered into mask greater than the classful mask is entered into the routing table (subnetting). the routing table (subnetting).

• The subnet is the level 2 child routeThe subnet is the level 2 child route of the parent of the parent route. route.

• In this case, the level 1 parent route that was In this case, the level 1 parent route that was automatically created is: 172.16.0.0/24 is subnetted, automatically created is: 172.16.0.0/24 is subnetted, 1 subnets1 subnets


• A level 2 route is a route that is a subnet of a A level 2 route is a route that is a subnet of a classful network address. classful network address.

• Like a level 1 route, the source of a level 2 Like a level 1 route, the source of a level 2 route can be a directly connected network, a route can be a directly connected network, a static route, or a dynamic routing protocol. static route, or a dynamic routing protocol.

• In this case, the level 2 route is the actual In this case, the level 2 route is the actual subnet route that was added to the network subnet route that was added to the network when we configured the FastEthernet 0/0 when we configured the FastEthernet 0/0 interface:interface:

• C 172.16.3.0 is directly connected, C 172.16.3.0 is directly connected, FastEthernet0/0FastEthernet0/0


• Level 1 Parent RouteLevel 1 Parent Route contains the following contains the following information :information :

• 172.16.0.0172.16.0.0 - The classful network address for - The classful network address for our subnet.our subnet.

• /24/24 - The subnet mask for all of the child - The subnet mask for all of the child routes. routes.

• If the child routes have variable length subnet If the child routes have variable length subnet masks (VLSM), the subnet mask will be masks (VLSM), the subnet mask will be excluded from the parent route and included excluded from the parent route and included with the individual child routes. with the individual child routes.

• is subnettedis subnetted, 1 subnet - This part of the route , 1 subnet - This part of the route specifies that this is a parent route and in this specifies that this is a parent route and in this case has one child route, that is, 1 subnet.case has one child route, that is, 1 subnet.


• Level 2 Child RouteLevel 2 Child Route contains the following contains the following information:information:

• CC - The route code for directly connected network. - The route code for directly connected network.

• 172.16.3.0 172.16.3.0 - The specific route entry.- The specific route entry.

• is directly connectedis directly connected - Along with the route code of - Along with the route code of C, this specifies that this is a directly connected C, this specifies that this is a directly connected network with an administrative distance of 0.network with an administrative distance of 0.

• FastEthernet0/0FastEthernet0/0 - The exit interface for forwarding - The exit interface for forwarding packets that match this specific route entry. packets that match this specific route entry.


• The level 2 child route is the specific route entry The level 2 child route is the specific route entry for the 172,16.3.0/24 subnet. for the 172,16.3.0/24 subnet.

• Notice that the subnet mask is not included with Notice that the subnet mask is not included with the subnet, the level 2 child route. the subnet, the level 2 child route.

• The subnet mask for this child route (subnet) is the The subnet mask for this child route (subnet) is the /24 mask included in its parent route, 172.16.0.0. /24 mask included in its parent route, 172.16.0.0.

• Level 2 child routes contain the route source and Level 2 child routes contain the route source and the network address of the route. the network address of the route.

• Level 2 child routes are also considered ultimate Level 2 child routes are also considered ultimate routes because they will contain the next-hop IP routes because they will contain the next-hop IP address and/or exit interfaceaddress and/or exit interface

Parent and child Parent and child Routes :Classless NetworkRoutes :Classless Network

Routing Table Look up Process Routing Table Look up Process (1)(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.


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.


If the best match is a level 1 parent route, proceed to Step 2.


The router examines child routes (the subnet routes) of the parent route for a best match


If there is a match with a level 2 child route, that subnet will be used to forward the packet.


If there is not a match with any of the level 2 child routes, proceed to Step 3.


Is the router implementing classful or classless routing behavior?


Classful routing behavior: If classful routing behavior is in effect, terminate the lookup process and drop the packet.


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.


If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.


If there is not a match with any route in the routing table, the router drops the packet.

Longest Match: Level 1Longest Match: Level 1 Network RoutesNetwork Routes

• The best match or longest match is the route in the routing The best match or longest match is the route in the routing table that has the most number of left-most matching bits table that has the most number of left-most matching bits with the destination IP address of the packet. with the destination IP address of the packet.

• The route with the most number of equivalent left-most The route with the most number of equivalent left-most bits, or the longest match, is always the preferred route.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 1Longest Match: Level 1 Parent Parent and Level 2 child routes (1)and Level 2 child routes (1)


• The first match that occurs is with the level 1 The first match that occurs is with the level 1 parent route, 172.16.0.0. parent route, 172.16.0.0.

• Remember, with non-VLSM subnets the Remember, with non-VLSM subnets the classful mask of the parent is now displayed. classful mask of the parent is now displayed.

• Before any child routes (subnets) are Before any child routes (subnets) are examined for a match, there must first be a examined for a match, there must first be a match with the classful address of the parent match with the classful address of the parent route. route.

• Because the first route entry is a level 1 Because the first route entry is a level 1 parent route that matches the destination parent route that matches the destination address (Step 1b of the route lookup process), address (Step 1b of the route lookup process), the route lookup process moves to Step 2.the route lookup process moves to Step 2.


• Because there is a match with the Because there is a match with the parent route, the level 2 child routes parent route, the level 2 child routes will be examined for a match. will be examined for a match.

• However, this time the actual subnet However, this time the actual subnet mask of /24 is used for the minimum mask of /24 is used for the minimum number of left-most bits that must number of left-most bits that must match. match.

Route look up with VLSM (1)Route look up with VLSM (1)

Route look up with VLSM (2)Route look up with VLSM (2)

• As with non-VLSM networks, if there is a match As with non-VLSM networks, if there is a match between the packet's destination IP address and between the packet's destination IP address and the classful mask of the level 1 parent route, the the classful mask of the level 1 parent route, the level 2 child routes will be searched.level 2 child routes will be searched.

• The only difference with VLSM is that child routes The only difference with VLSM is that child routes display their own specific subnet masks. display their own specific subnet masks.

• These subnet masks are used to determine the These subnet masks are used to determine the number of left-most bits that must match the number of left-most bits that must match the packet's destination IP address. packet's destination IP address.

• For example, for there to be a match with the For example, for there to be a match with the 172.16.1.4 child route, a minimum of 30 left-most 172.16.1.4 child route, a minimum of 30 left-most bits must match because the subnet mask is /30. bits must match because the subnet mask is /30.

Routing behaviorRouting behavior (1)(1)

• Routing behavior influences the Routing behavior influences the process of searching for the preferred process of searching for the preferred route using the route using the no ip classless or ip no ip classless or ip classless commandsclassless commands. .

• Classless and classful routing Classless and classful routing behaviors are not the same as behaviors are not the same as classless and classful routing classless and classful routing protocolsprotocols. .

Routing behaviorRouting behavior (2)(2)

• Classful and classless routing protocols Classful and classless routing protocols affect how the routing table is populated. affect how the routing table is populated.

• Classful and classless routing behaviors Classful and classless routing behaviors determine how the routing table is determine how the routing table is searched after it is populated. searched after it is populated.

• In the figure, the routing sources In the figure, the routing sources (including classful and classless routing (including classful and classless routing protocols) are the inputs used to protocols) are the inputs used to populate the routing table.populate the routing table.

Classful Routing Behavior Classful Routing Behavior (1)(1)

Classful Routing Behavior Classful Routing Behavior (5)(5)• Because router R2 is using classful routing Because router R2 is using classful routing

behavior, behavior, no ip classlessno ip classless, the router will not , the router will not search beyond the child routes for a lesser match.search beyond the child routes for a lesser match.


• The routing table process will not use the The routing table process will not use the default route, 0.0.0.0/0, or any other route. default route, 0.0.0.0/0, or any other route.

• A common error is to assume that a default A common error is to assume that a default route will always be used if the router does route will always be used if the router does not have a better route. not have a better route.

• In the example, R2's default route is not In the example, R2's default route is not examined nor used, although it is a match. examined nor used, although it is a match.

• This is often a very surprising result when a This is often a very surprising result when a network administrator does not understand network administrator does not understand the difference between classful and the difference between classful and classless routing behavior.classless routing behavior.

Classless Routing Behavior Classless Routing Behavior (1) (1)

Classless Routing Behavior Classless Routing Behavior (2)(2)

Is the router implementing classful or classless routing behavior?


Classful routing behavior: If classful routing behavior is in effect, terminate the lookup process and drop the packet.


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.


If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.


If there is not a match with any route in the routing table, the router drops the packet.

Classless Routing Behavior Classless Routing Behavior (12)(12)• Because we are using classless routing Because we are using classless routing

behavior (ip classless), the router continues behavior (ip classless), the router continues searching the routing table, beyond this searching the routing table, beyond this parent route and its child routes.parent route and its child routes.

Classless Routing Behavior Classless Routing Behavior (13)(13)• S* 0.0.0.0/0 is directly connected, Serial0/0/1S* 0.0.0.0/0 is directly connected, Serial0/0/1

• The mask is /0, which means that zero or no bits need to The mask is /0, which means that zero or no bits need to match. A default route will be the lowest-bit match. In match. A default route will be the lowest-bit match. In classless routing behavior, if no other route matches, the classless routing behavior, if no other route matches, the default route will match. default route will match.


• In this case the router will use the default In this case the router will use the default route, because it is the best match. route, because it is the best match.

• The packet will be forwarded out the Serial The packet will be forwarded out the Serial 0/0/1 interface.0/0/1 interface.

Classful - Classless behavior Vs Classful - Classless behavior Vs Classful – Classless routing Classful – Classless routing protocols protocols

• Remember that classful and classless Remember that classful and classless routing behaviors are independent from routing behaviors are independent from classful and classless routing protocols. classful and classless routing protocols.

• A router could be configured with classful A router could be configured with classful routing behavior (no ip classless) and a routing behavior (no ip classless) and a classless routing protocol, such as RIPv2. classless routing protocol, such as RIPv2.

• A router could also be configured with A router could also be configured with classless routing behavior (ip classless) and classless routing behavior (ip classless) and a classful routing protocol, such as RIPv1. a classful routing protocol, such as RIPv1.

• In today's networks, it is recommended to In today's networks, it is recommended to use classless routing behavior so that use classless routing behavior so that supernet and default routes can be used supernet and default routes can be used whenever needed.whenever needed.

routing table : closer look w.lilakiatsakun. sample routing table static route dynamic routing...

Documents

network route

ultimate route

default route

supernet route

classful network

network address

networks classful mask

subnet mask equal