EIGRP has a default behavior called auto-summarizationand states that any router in an EIGRP process connected to two different major networks will not adver-tise subnets of either network into the other network. Only a summary address that encompasses all of the subnets will be advertised.To see what effect, if any, this rule has on our network, we need to look at an example of auto-summarization.
In Figure 2.22, the router sits on the boundary of both major networks 10.0.0.0 and 27.0.0.0. Following the rule of summarization, the networks are advertised as illustrated.
A positive impact that this behavior can have on the network is that if one of the networks such as 10.1.2.0 fails, that failure is not advertised to network 27.0.0.0.The result is what is referred to as a query boundary.When a diffusing computation begins in an attempt to discover a new route to the failed segment, routers in the 27.0.0.0 network are not queried.The network can reconverge in less time, and routers that do not need to waste processing cycles participating in the computation are left out.
The principal of boundaries needs to be considered as we begin to address our network. Another way to accomplish query boundaries is through address aggregation. Aggregation is similar in nature to address summarization, with the exception that you can work within the same major network. Aggregation is accomplished by shortening the mask of the advertised network as you move up through the hierarchy. As an example, in Figure 2.23 each progressive step yields a shorter address mask that aggregates all of the subnets below.
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Figure 2.22The EIGRP Auto-Summarization Process
10.1.2.0/24
To see how this works, we need to look at our addresses in binary. In Table 2.2, we have laid out our addresses and highlighted the most significant common bits.You then mask the common bits to derive the aggregate address.
Table 2.2Addresses and Significant Common Bits Summarization at Router B
00001010 00000001 00001100 00000000 10.1.12.0/24 00001010 00000001 00001101 00000000 10.1.13.0/24 11111111 11111111 11111110 00000000 10.1.12.0/23
Summarization at Router C
00001010 00000001 01110100 00000000 10.1.116.0/24 00001010 00000001 01110101 00000000 10.1.117.0/24 11111111 11111111 11111110 00000000 10.1.116.0/23
Summarization at Router A
00001010 00000001 00001100 00000000 10.1.12.0/23 00001010 00000001 01110100 00000000 10.1.116.0/23 11111111 11111111 10000000 00000000 10.1.0.0/17
We need to perform this exercise at each layer in our hierarchy to ensure aggregation.With our current network design, only the server farm and access segments require more than two IP addresses. All other segments are point to Figure 2.23Address Aggregation Can Limit Query Range
10.1.12.0/24
10.1.117.0/24 10.1.116.0/24 10.1.13.0/24
10.1.116.0/23 10.1.12.0/23
10.1.0.0/17
Router A
Router C Router B
point; therefore, our address conservation efforts will benefit greatly from the use of VLSM.We decide that we would like to reserve at least 50 addresses for the server farm segment, and 50 addresses for each access segment.We determine that we will have to reserve 6 bits in the last octet for host addresses.This is calculated using the formula
2x-2=usable host addresses
where X is the number of bits you will need to reserve. Since we selected 50 host addresses, our formula becomes 2x-2=50.This formula cannot be resolved without X becoming a fraction of a whole number.We can either round up or round down—since we are planning for growth, we will round up.Where 25 -2=30 and 26-2=62 we will use 6, resulting in 62 host addresses.
We only need two addresses for the point-to-point segments. Our formula becomes 2x-2=2, and X can be solved for directly resulting in X=2. Since we require a high number of host addresses, we are not going to be able to use a single octet to address all of the hosts.The total number of host addresses we need is nine segments of 62 addresses plus eight segments of 2, for a total of 574 host addresses. Plugging this total into our formula results in 2x-2=574.Where 29 -2=510 and 210-2=1022, we would need a minimum of 10 bits for host addresses.
However, we must take into account that this will only work in a flat address space. For each subnet we create, we lose two host addresses.When you are using VLSM, that value is compounded for each sub-subnet you create.We also need to consider that our address space will be hierarchical.When we apply addresses to our server farm, 192 host addresses will go unused because there are no other hosts at that level in the hierarchy. In addition, each of the access routers will need an individual network to divide between the two access segments that it serves, resulting in the reservation of another 128 hosts per access router. (This is not a loss of address space.The addresses can be used at any time in the future when additional network segments are added to the Access routers.) To allow room for these anomalies, we will use 11 bits for our address space.
It will serve us well to work with our addresses at the bit level now to deter-mine our hierarchy. If you try to start applying addresses at the top or at the bottom before you have done the work to ensure summarization, you will meet with severe difficulty.
Since we know that we will need 11 bits for our address space, we can safely say that at the 21stbit will be our highest level of summarization.We will make this bit a 1 in all of our addresses to set a delimiter for our network.We also know that the first two octets will always be 172.16, and we can begin a chart for
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our bit-level addressing. In Table 2.3, with the 21stbit set to 1 and the remaining bits reserved for hosts, we see that we can begin our addressing with network 172.16.8.0/21. As long as we do not change any bits below 22 (1–21), our net-work can be summarized at Core Router A with this address.
Table 2.3Bit-Level Addressing Table for Core Router A Summarized Network
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
172 16 8 X
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 X X X X X X X X X X X
At this point, it would be easiest to split the network in half at Core Router A. On the left side, we have Distribution Router A and Access Routers A and B.
On the right, we have Distribution Router B and Access Routers C and D.We can distinguish between the two sides by toggling the 22nd bit. For all routes through Distribution Router A, bit 22 will be 0. For all routes through Distribution Router B, bit 22 will be 1.Table 2.4 shows this addition.
Table 2.4Addition of Distribution Router B
Routes through Distribution Router A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
172 16 8 X
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 X X X X X X X X X X
Routes through Distribution Router B
172 16 12 X
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 X X X X X X X X X X
Looking at the left side of our network, the total number of hosts needed is four segments of 62 addresses and three segments of 2, for a total of 254 host addresses. Again, due to the hierarchical structure of our network, we must account for the unused address space, 128 hosts per access router, plus the loss of 2 hosts per subnet.The result is approximately 524 host addresses. Inserting this number into our formula, we get 2X=524.Where 29=512 and 210=1024, we will use 10 bits for our address space on this segment. As this is the same number of host addresses that we will need on the right side of our network, we can make
the decision to set the 23rdbit to 1 as the delimiter for host addresses below the Distribution layer. Again, it will be useful to split the network into halves, this time below the Distribution routers. Focusing on the two halves below
Distribution Router A, we can distinguish between the two sides by toggling bit 24. For all routes through Access Router A, bit 24 will be 0; for routes through Access Router B, bit 24 will be 1.Table 2.5 establishes the summary addressing for the two sides of the network following this same rule for the routes through Access Routers C and D.
Table 2.5 Summary Addressing for Two Sides of the Network
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Below Distribution Router A Routes through Access Router A
172 16 10 X
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 1 0 X X X X X X X X
Routes through Access Router B
172 16 11 X
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 1 1 X X X X X X X X
Below Distribution Router B Routes through Access Router C
172 16 14 X
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 0 X X X X X X X X
Routes through Access Router D
172 16 15 X
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 X X X X X X X X
Moving down to Access Router A, the number of host addresses needed is two segments of 62 addresses for each half plus the unused address space of 128 hosts, and an additional two hosts for the subnet space, for a total of 254 addresses.
Inserting this number into our formula, we get 2X=254.Where 28=254, we will use the 25th bit to delimit below the Access routers, and we can distinguish
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between the two sides by toggling bit 26.Table 2.6 establishes the summary addressing at the Access layer.
Table 2.6Summary Addressing at the Access Layer
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Below Access Router A Segment One
172 16 10 128
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 1 0 1 0 X X X X X X
Segment Two
172 16 10 192
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 1 0 1 1 X X X X X X
Below Access Router B Segment One
172 16 11 128
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 1 1 1 0 X X X X X X Segment Two
172 16 11 192
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 1 1 1 1 X X X X X X
Below Access Router C Segment One
172 16 14 128
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 0 1 0 X X X X X X Segment Two
172 16 14 192
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 0 1 1 X X X X X X
Continued
Below Access Router D Segment One
172 16 15 128
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 1 0 X X X X X X Segment Two
172 16 15 192
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 X X X X X X
To see how this summarization will work, we need to group our addresses and find the common bits.Table 2.7 shows the summarization from the bottom to the top.
Table 2.7Address Summarization from Bottom to Top Summarization at Access Router A
172 16 10 128
10101100 00010000 00001010 10000000
172 16 10 192
10101100 00010000 00001010 11000000
11111111 11111111 11111111 10000000=172.16.10.128/25
(Advertised to Distribution Router A) Summarization at Access Router B
172 16 11 128
10101100 00010000 00001011 10000000
172 16 11 192
10101100 00010000 00001011 11000000
11111111 11111111 11111111 10000000=172.16.11.128/25
(Advertised to Distribution Router A) Summarization at Distribution Router A
172 16 10 128/25
10101100 00010000 00001010 10000000
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Continued
172 16 11 128/25 10101100 00010000 00001011 10000000
11111111 11111111 11111110 00000000=172.16.10.0/23 (Advertised to Core Router A) Summarization at Access Router C
172 16 14 128
10101100 00010000 00001110 10000000
172 16 14 192
10101100 00010000 00001110 11000000
11111111 11111111 11111111 10000000=172.16.14.128/25
(Advertised to Distribution Router B) Summarization at Access Router D
172 16 15 128
10101100 00010000 00001111 10000000
172 16 15 192
10101100 00010000 00001111 11000000
11111111 11111111 11111111 10000000=172.16.15.128/25
(Advertised to Distribution Router B) Summarization at Distribution Router B
172 16 14 128/25
10101100 00010000 00001110 10000000
172 16 15 128/25
10101100 00010000 00001111 10000000
11111111 11111111 11111110 00000000=172.16.14.0/23 (Advertised to Core Router A) Summarization at Core Router A
172 16 10 0/23
10101100 00010000 00001010 00000000
172 16 14 0/23
10101100 00010000 00001110 00000000
11111111 11111111 11111000 00000000=172.16.8.0/21
(Advertised to Corporate Network) Table 2.7Continued
Figure 2.24 graphically illustrates this aggregation.
Now we have to implement our IP addressing scheme and address summa-rization. EIGRP does not perform this type of summarization automatically.
Auto-summary only occurs at major network boundaries.We must manually configure summarization within a major network address space.This is done with the command ip summary-address eigrp <process id> <network> <mask>.
This command is an interface configuration level command that tells EIGRP to suppress advertisements of more specific routes.
To configure Access Router A to advertise 172.16.10.128/25, use the fol-lowing interface configuration where Ethernet 0 connects to Distribution Router A:
A-Router A
interface ethernet0
ip summary-address eigrp 100 172.16.10.128 255.255.255.128
The remaining configurations are as follows, where the interface specified is connected to the upstream neighbor:
A-Router B
interface ethernet0
ip summary-address eigrp 100 172.16.11.128 255.255.255.128
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Figure 2.24The Aggregation of Routes in a Sample Network
C-Router A
A-Router B A-Router C
A-Router A A-Router D
D-Router A Server Farm D-Router B
172.16.10.128/26
A-Router C
interface ethernet0
ip summary-address eigrp 100 172.16.14.128 255.255.255.128
A-Router D
interface ethernet0
ip summary-address eigrp 100 172.16.15.128 255.255.255.128
D-Router A
interface ethernet1
ip summary-address eigrp 100 172.16.10.0 255.255.254.0
D-Router B
interface ethernet1
ip summary-address eigrp 100 172.16.14.0 255.255.254.0
C-Router A
interface ethernet1
ip summary-address eigrp 100 172.16.8.0 255.255.248.0
Configuring ip summary-address on the Core router assumes that the corpo-rate office is also using 172.16.0.0. If this were not the case, it would not be nec-essary to manually configure summarization at Core Router A because it would automatically advertise 172.16.0.0/16 following the rules of auto-summarization.