Classless routing and classless routing protocols

IP route selection protocols can be divided into classless routing and classless routing. Route Selection Updates sent by the classless Routing Protocol do not contain subnet mask information, such as r12001; Route Selection Updates sent by the classless routing protocol include subnet mask information, such as r00002, VPN, OSPF, IS-IS and BPG.

1. classless Routing Protocol

When a class protocol was initially developed, the network used was quite different from the current network. At that time, the maximum speed of the modem was bits/s, the maximum speed of the WAN line was 56 bits/s, and the vro memory was no more than kb, while the processor speed was measured at KHz, therefore, it is required that the route selection update be small enough, and the vro does not have enough resources to maintain the latest information about each subnet. This is the reason for the emergence of a similar routing protocol.

Because there is no subnet mask in the update of the classless routing protocol, when the classless router sends or receives a route selection update, it must make assumptions about the subnet mask used by the Network listed in the update, that is, the subnet mask used by each subnet. This assumption is based on the IP Address class.

When an update group is sent, if the subnet involved in the Update group and the IP address of the send interface are in the same classification network, the router will send the complete subnet information; if the subnet involved in the Update group and the IP address of the sending interface do not belong to the same classification network (that is, the subnet update is sent through the interface of the other network ), this router assumes that the remote router uses the corresponding default subnet mask based on the IP Address class. Therefore, the router does not send complete subnet information, it is the classification network address (Master network address) to which the subnet belongs, which is also called automatic summary at the network boundary.

When receiving updates, the router also assumes that the subnet mask of the subnet in the Update group is used. If the subnet involved in the Update group and the IP address of the receiving interface are in the same classification network, the router uses the subnet mask of the receiving interface as the subnet mask of the updated subnet; if it does not belong to the same classification network, the router applies the default subnet mask to the updated network.

In a non-continuous network, the classless routing protocol automatically summarizes routes at the primary network border. This means that the subnet will not be advertised to other primary networks, non-continuous subnets are invisible to each other. A non-continuous subnet refers to a subnet that is separated by another primary network and belongs to the same primary network. Because r12001 cannot subscribe to other subnets across the other primary network, when the subscribe to another subnet across the primary network, the router will automatically summarize. Therefore, in an discontinuous network, you can use the RIPv2, OSPF, IS-IS, or OSPF route selection protocol, and disable the automatic summary function to solve this problem, because these route selection protocols are classless routing selection protocols, the subnet mask information is included when an update is sent.

In the stub network using the classless routing protocol, the final Gateway Router (for example, only one router to the ISP network) you may need to use a default route or a supernetwork route to forward data packets from an unknown subnet. Other subnets that are located in the same primary network are known to routers, the ip classless command is required.

If the ip classless command is not applied, the route selection table itself performs the routing selection decision in a classless way by default, even if the routing selection protocol is not used. For example, if a vro does not run the routing selection Protocol but is configured with a static route, the default route will still be unable to reach other subnets of the known master network, unless the ip classless command is configured.

In cisco IOS 12.0 or later versions, the ip classless command is enabled by default.

2. classless Routing Protocol

The classless Routing Protocol should be considered as the 2nd generation route selection protocol. The design and development of this Protocol aims to overcome some shortcomings of the earlier classless routing protocol, such as: In the classless routing network environment, the subnet mask information is not carried in route updates. As a result, all subnets in the same primary network must have the same subnet mask, which limits the use of VLSM and leads to a serious waste of IP addresses.

Because subnet mask information is exchanged in the classless routing network environment, the router routing table may contain multiple subnet routing information of the same master network, therefore, the longest prefix matching principle is used to select a route when data packets are forwarded.

In a classless routing network environment, you need to automatically summarize the primary network boundaries, which also limits the applications of non-continuous subnets. In a classless routing selection environment, you can manually control the route aggregation mode, it can be summarized at any location.

By default, RIPv2 and VPN are automatically aggregated at the classification network boundary, just like the classless Inter-Domain Routing (classless Inter-Domain Routing) protocol. This primarily makes these routes backward compatible with their previous RIPv1 and IGRP protocols. However, r12002 and VPN allow the vro to configure the command no auto-summary to manually disable the automatic summary function so that the non-continuous subnet can run normally. However, you do not need to use this command when running OSPF or IS-IS, because they do not automatically aggregate networks by default.

Author: graynight