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Chapter 10 - Route Summarization and CIDR

Cisco & IP Addressing
Louis D. Rossi, Louis R. Rossi and Thomas Rossi
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Chapter 10: Route Summarization and Classless Interdomain Routing (CIDR)
Overview
Route Summarization is a technique that allows a router to have a single entry in the routing table that points to several different networks.
There are two main advantages of route summarization:
  1. Reduces the size of routing tables
  2. Reduces routing processing
Reducing the size of a routing table can have a positive effect on the router. If a router has fewer entries in a routing table, less memory will be required to keep track of those routes.
If a route that is being summarized goes down, that topology change will remain localized; routers that are not in the local area will not have to create new routing tables.
In Figure 10.1, without route summarization Router_B would have 32 separate entries in the routing table that would identify the individual networks of Routers A1, A2, C1, and C2. Router_A and Router_C each would require 16 unique router entries.
Figure 10.1  Route Summarization Topology
Let us also assume that if network 10.1.25.0 should go down, Router_C2  would notify Router_C, which in turn would notify Router_B that this topology change has taken place. Router_B would then communicate this change to Router_A and so on. All routers would create a new routing table based upon this new information.
With route summarization configured, Router_B would have two entries in its routing table that would, in effect, point to these 16 networks. One entry would point to Router_A for networks 10.1.0.0 through 10.1.15.0 and the other entry would point to Router_B for networks 10.1.16.0 through 10.1.31.0.
If network 10.1.25.0 should go down Router_C2 would not need to notify Router_B or Router_C that this topology change has taken place.
Route summarization can be compared to the way in which ZIP codes work.
The first digit of a ZIP code identifies a national area, a 3 for example is the southeast area, while a 9 in the first place identifies the West Coast, Alaska, and Hawaii.
So if I mail a letter to Honolulu, HI from Tallahassee, FL, Tallahassee post office employees do not care about the last four digits of the ZIP code. They know that this letter is going to the West Coast. At some point down the road a post office will look at the next two digits, which identify a sectional center. Finally the remaining two digits identify a single post office.
If you are doing a mass mailing and you bag your envelopes so that all the ZIP codes share the same first three digits you save money because this reduces the processing required by the post office to forward your letter.
In the same way Router_B does not care what specific network a packet is destined for. Router_B just needs to know what “range” the network falls into.
If Router_B receives a packet with a destination network address of 10.10.4.0, Router_B would send this packet to Router_A because the destination address falls into the range of 10.10.0.0 – 10.10.15.0.
At this point do not be worried by how the routers will be configured; we will do that in a later chapter. The objective here is to learn the concept.
Now think back to the early 60s, when the U.S. Postal Service designed the ZIP code scheme.
If they had assigned ZIP codes at random, computers and reading devices would have had to look at all 5 digits before making a forwarding decision.
For instance, suppose 10001 identified NYC and 10002 identified LA. With this type of random scheme we could not make any conclusions concerning location by looking at the first few digits.
This is the same situation if we have IP addresses that have been randomly assigned. No conclusions can be drawn, therefore the routers would be required to have each unique entry in the routing table. A router would be forced to look at all the network bits before it could make a forwarding decision.
If you design your IP addressing scheme in such a way that the first part of the address defines a region you save processing time at the router.
In the example above we have a contiguous addressing scheme. When we have addresses that are contiguous we can summarize. These networks were assigned planning to take advantage of route summarization.
If addresses are assigned haphazardly, summarization is impossible.
What makes a series of IP addresses contiguous?
When IP addresses share the same high order bits the addresses can be summarized.
Now go back to our ZIP code analogy; if we have ZIP codes 1xxxx and 9xxxx, can these envelopes be placed in the same bag en route to their destination? No, because they do not share the same high-order bits. Two envelopes with 1xxxx and 1xxxx can be put in the same bag because they do share the same high-order bits.
One can easily see the advantage of having IP addresses that share the same high-order bits. To illustrate what is meant by high-order bits we will use the 32 addresses from our example above.
There is no question that in our example the 16 high-order bits ( first two octets) are the same for all 32 addresses; but do they have more than just these 16 bits in common? For a closer look at the 3rd octet, refer to Table 10.1.
Table 10.1  Summarization Bits for Router A and Router C
128
64
32
16
8
4
2
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
0
0
0
0
0
0
1
0
2
0
0
0
0
0
0
1
1
3
0
0
0
0
0
1
0
0
4
0
0
0
0
0
1
0
1
5
0
0
0
0
0
1
1
0
6
0
0
0
0
0
1
1
1
7
0
0
0
0
1
0
0
0
8
0
0
0
0
1
0
0
1
9
0
0
0
0
1
0
1
0
10
0
0
0
0
1
0
1
1
11
0
0
0
0
1
1
0
0
12
0
0
0
0
1
1
0
1
13
0
0
0
0
1
1
1
0
14
0
0
0
0
1
1
1
1
15
0
0
0
1
0
0
0
0
16
0
0
0
1
0
0
0
1
17
0
0
0
1
0
0
1
0
18
0
0
0
1
0
0
1
1
19
0
0
0
1
0
1
0
0
20
0
0
0
1
0
1
0
1
21
0
0
0
1
0
1
1
0
22
0
0
0
1
0
1
1
1
23
0
0
0
1
1
0
0
0
24
0
0
0
1
1
0
0
1
25
0
0
0
1
1
0
1
0
26
0
0
0
1
1
0
1
1
27
0
0
0
1
1
1
0
0
28
0
0
0
1
1
1
0
1
29
0
0
0
1
1
1
1
0
30
0
0
0
1
1
1
1
1
31
0
0
1
0
0
0
0
0
32
Notice that the first 16 addresses share the same 4 high-order bits of 0000; these are the networks connected to Routers A1 and A2.
The next 16 addresses share the same 4 high order bits of 0001; these are the networks connected to Routers C1 and C2.
Simply put, Router_A will “tell” Router_B:
“If the bit pattern of the first 20 bits is 00001010 00001010 0000 send the packet to me.”
Router_C will “tell” Router_B:
“If the bit pattern of the first 20 bits is 00001010 00001010 0001 send the packet to me.”
It is not necessary for Router_B to look at the remaining 12 bits, just as it is not necessary for the post office to look at the entire ZIP code.
Notice that Router_C cannot summarize the “32” address because the bit pattern changes.
In the same way that Router_A and Router_C can summarize their networks to Router_B, Routers A1 and A2 can summarize their networks to Router_A.
And of course Routers C1 and C2 can summarize their networks to Router_C (Table 10.2).
Table 10.2  Summarization Bits for Routers A1, A2, C1, and C2
128
64
32
16
8
4
2
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
0
0
0
0
0
0
1
0
2
0
0
0
0
0
0
1
1
3
0
0
0
0
0
1
0
0
4
0
0
0
0
0
1
0
1
5
0
0
0
0
0
1
1
0
6
0
0
0
0
0
1
1
1
7
0
0
0
0
1
0
0
0
8
0
0
0
0
1
0
0
1
9
0
0
0
0
1
0
1
0
10
0
0
0
0
1
0
1
1
11
0
0
0
0
1
1
0
0
12
0
0
0
0
1
1
0
1
13
0
0
0
0
1
1
1
0
14
0
0
0
0
1
1
1
1
15
0
0
0
1
0
0
0
0
16
0
0
0
1
0
0
0
1
17
0
0
0
1
0
0
1
0
18
0
0
0
1
0
0
1
1
19
0
0
0
1
0
1
0
0
20
0
0
0
1
0
1
0
1
21
0
0
0
1
0
1
1
0
22
0
0
0
1
0
1
1
1
23
0
0
0
1
1
0
0
0
24
0
0
0
1
1
0
0
1
25
0
0
0
1
1
0
1
0
26
0
0
0
1
1
0
1
1
27
0
0
0
1
1
1
0
0
28
0
0
0
1
1
1
0
1
29
0
0
0
1
1
1
1
0
30
0
0
0
1
1
1
1
1
31
0
0
1
0
0
0
0
0
32
With route summarization configured:
Router_A1 will be the destination router when the first 21 bits are 00001010 00001010 00000.
Router_A2 will be the destination router when the first 21 bits are 00001010 00001010 00001.
Router_C1 will be the destination router when the first 21 bits are 00001010 00001010 00010.
Router_C2 will be the destination router when the first 21 bits are 00001010 00001010 00011.
When using private addresses we can easily design our addressing scheme to use contiguous addressing to take advantage of summarization.
Classless Interdomain Routing (CIDR)
CIDR is a technique to provide multiple IP addresses in a contiguous block.
You may have heard the term “CIDR block”. This refers to a set of contiguous addresses.
195.10.12.0 255.255.252.0 identifies 4 Class C addresses:
195.10.12.0
195.10.13.0
195.10.14.0
195.10.15.0
Remember what we learned earlier, that a mask identifies bits that cannot be changed. In this example if we can NOT change the most significant 6 bits of the 3rd octet that means we can change the least significant 2 bits.
Table 10.3 shows all the bit combinations the above address and mask identify:
Table 10.3  Combinations When the Least Significant 2 bits can be Changed
3rd Octet
128
64
32
16
8
4
2
1
0
0
0
0
1
1
0
0
12
0
0
0
0
1
1
0
1
13
0
0
0
0
1
1
1
0
14
0
0
0
0
1
1
1
1
15
0
0
0
1
0
0
0
0
16
I have also included the “16” address to show how the bit pattern changes and therefore is not included.
Keep the big picture in mind; if we have a CIDR block we can summarize addresses and router summarization is a good thing.

 


 
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