Route Summarization in Objective-C

Generation QR Code in Objective-C Route Summarization

FIGURE 8-7

Classless Interdomain Routing (CIDR), specified in RFC 2050, is an extension to VLSM and route summarization With VLSM, you can summarize subnets back to the Class A, B, or C network boundary For example, if you have a Class C network 19216810/24 and subnet it with a 26-bit mask, you have created four subnets Using VLSM and summarization, you can summarize these four subnets back to 19216810/24 CIDR takes this one step further and allows you to summarize a block of contiguous Class A, B, and/or C network numbers This practice is commonly referred to as supernetting Today s classless protocols support supernetting However, it is most commonly configured by ISPs on the Internet who use BGP as a routing protocol Figure 8-7 shows an example of CIDR In this example, a router is connected to four Class C networks: 19216800/24, 19216810/24, 19216820/24, and 19216830/24 The router is summarizing these routes into a single entry: 19216800/22 Table 8-1 illustrates the bits that are in common in this example In the first 2 bytes, all bits in the four networks match (192168) In the third octet, the first 6 bits match, totaling

TABLE 8-1

19216800 19216810 19216820 19216830 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 0 1

22 bits Notice the subnet mask for this summarization: 2552552520 This mask, along with the beginning network, 19216800, includes addresses from 19216800 to 1921683255, which are behind this router

To perform route summarization, you will need to set up your addressing in a hierarchical fashion Hierarchical addressing provides the following benefits:

It enables more efficient routing It uses route summarization to decrease the size of routing tables It decreases the amount of memory needed to store the smaller routing tables It decreases the impact on the router when needing to rebuild the routing

Figure 8-8 shows a simple example of hierarchical addressing In this example, the network is using 10000/8 This is summarized before being sent to another Understand the bene ts network This addressing space is broken up of hierarchical addressing into three campuses: 10100/16, 10200/16, and 1030016 Each of these sets of addresses is summarized when sharing routes between the campuses Within each campus, the addressing is further broken up for the two buildings: 10x10/24, 10x20/24, and so on To implement a hierarchical addressing design and to take advantage of route summarization, you ll need a routing protocol that supports VLSM: BGP, EIGRP,

FIGURE 8-8

Summarized route leaving AS 10000/8

Campus 1 10100/16

Campus 2 10200/16

Campus 3 10300/16

Building 1 10110/24

Building 2 10120/24

Building 1 10210/24

Building 2 10220/24

Building 1 10310/24

Building 2 10320/24

IS-IS, OSPF, or RIPv2 When implementing route summarization, you ll need to consider the following:

Routing decisions must be made on the entire destination IP address To summarize routing entries, they must have the same highest order

matching bits (see Table 8-1 as an example)

As mentioned in the preceding section, the routing protocol must carry the subnet mask Remember the with the corresponding network entries if you three bulleted points dealing with want to take advantage of route summarization implementation of route summarization Otherwise, if you had more than one subnet mask applied to a class network number, the router wouldn t know which mask to use when routing a packet to a destination A good example of this problem is apparent in classful protocols, such as RIPv1, and how you lay out your IP addresses in your network With classful protocols, routing updates are sent out with only network entries: no subnet masks are included in the routing updates The assumption is that the routers on other segments are connected to the same class network and thus know about the subnet mask since it is configured on their interfaces