qr code c# sample 19: Basic Routing in Objective-C

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19: Basic Routing
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You might want to use the permanent parameter if you never want packets to use another path to a destination, perhaps because of security reasons
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Remember the syntax for creating a static IP route with the ip route command If you omit the
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administrative distance, it defaults to 0 or 1, depending on how you con gured the static route
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A default route is a special type of static route Where a static route specifies a path a router should use to reach a specific destination, a default route specifies a path the router should use if it doesn t know how to reach the destination Note that if a router does not have any path in its routing table telling it how to reach a destination, and the router receives a packet destined for this network, the router will drop the packet This is different from a switch, which will flood unknown destinations Therefore, a default route can serve as a catch-all: if no path to the destination is specified, the router will use the default route to reach it To set up a default route, use the following syntax for a static route:
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Router(config)# ip route 0000 0000 IP_address_of_next_hop_neighbor [administrative_distance] [permanent]
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Router(config)# ip route 0000 0000 interface_to_exit [administrative_distance] [permanent]
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The network number of 0000/0 at first appears a bit strange Recall from 7, however, that network 0000 represents all networks, and a mask of all 0s in the bit position represents all hosts in the specified network
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Use an address and subnet mask of 0000 0000 when creating a default routeThis is used when no other
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speci c route is available to a destination within the routing table
Static Routes
Default Routes and Distance Vector Protocols
A default route sometimes causes problems for certain routing protocols A routing protocol can fall under two additional categories: classful and classless Examples of classful protocols include RIPv1 and IGRP (no longer supported by Cisco) Examples of classless protocols include RIPv2, Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), Intermediate System-Intermediate System (IS-IS), and Border Gateway Protocol (BGP) A classful routing protocol understands only class subnets For instance, if you have 19216810/23 in a routing update, a classful routing protocol wouldn t understand it, since a Class C network requires 24 bits of network numbers This can create problems with a default route, which has a /0 mask Also, when a classful router advertises a route out its interface, it does not include the subnet mask For example, you might have 19216811/26 configured on your router s interface, and the router receives a routing updated with 19216810 With a classful routing protocol, the router will comprehend subnet masks only for network numbers configured on its interfaces In this example, the router assumes that for 19216810, the only valid mask is /26 Therefore, if the routers sees the 19216810/26 as the network number, but the network is really 19216810/27, a lot of routing confusion results Classless protocols, on the other hand, do not have any issues accepting routing updates with any bit value for a subnet mask However, for classful protocols, you must configure the following command to accept nonconforming subnet masks, such as a default route:
Router(config)# ip classless
This command is also used to deal with discontiguous subnets in a network that is using a classful protocol: subnets separated by a different class network For example, assume that you have networks 1721610/24, 1721620/24, and 1721630/24 However, a different class network, 19216810/24, sits between the first two Class B subnets and 1721630/24 In this situation, the router connected to 1721610/24 and 1721620/24, when it receives 1721600 from the side of the network connected to the discontiguous subnet, will ignore this routing entry Remember that when routes cross a class boundary in a classful protocol, the network number is sent as its classful number Therefore, the router connected to 19216810/24 and 1721630/24, when it advertises updates across the 19216810/24 subnet, will advertise 1721600 not the actual subnet number
19: Basic Routing
Since the router connected to 1721610/24 and 1721620/24 ignores the 1721600 routing information, it will not be able to reach 1721630 On top of this problem, even if you have a default route configured, since the router is connected to the 1721600 subnets, it assumes that 1721630 must also be connected; and if it isn t in the routing table, then the route cannot be reached This topic was discussed in 8 By using the ip classless command, you are overriding this behavior; you re allowing your classful router to use a default route to reach discontiguous subnets Not that this is a recommended design practice, but it does allow you to solve reachability problems for discontiguous subnets
Classful protocols, such as IP RIPv1, understand only classful subnets you can apply only one subnet mask to a class address Classless protocols, such as RIPv2, EIGRP, OSPF, and IS-IS, do not have this restriction
The ip classless command allows a classful protocol to use a default route; omitting this command will cause the router to drop packets that don t match a speci c destination network entry in the routing table
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