Advanced Cisco Routing: Summary Addresses

So this probably isn't really an advanced routing topic, but it's not something I ran into while working on my CCNA, either...

Anyway.

Consider the following network:


On the left, we have our server farm (okay, it's a rather small data center). On the right, we have our desktop users (see previous comment, lol). Because we actually engineer our network, we have divided our network up into logical units, each on its own subnet. R2, R3 and R4 are department routers, each hosting a subnet -- say, 192.168.2.0/24 for R2, 192.168.3.0/24 for R3 and 192.168.4.0/24 for R4. R5 is the data center router, and all of the hosts in the data center are on the 172.16.0.0/24 subnet. R1 is our dist router, and uses 192.168.1.x/30 for each of its links to R2, R3 and R4, as well as using 10.0.0.0/30 for the link to R5. IRL, we should probably be using another 192.168.1.x/30 subnet for the R1 -- R5 link, but I'm actually treating R5 as both an edge and dist router here. I've enabled EIGRP on all five routers.

By default, EIGRP will try to summarize all of the routes by class; i.e., networks from 0.0.0.0 to 127.255.255.255 will be Class-A networks, networks from 128.0.0.0 through 191.255.255.255 will be Class-B networks, and networks from 192.0.0.0 through 223.255.255.255 will be Class-C network, no matter how you've actually subnetted them.

Modern best-practice, however, is not to do that, as free IPv4 address space is becoming increasingly rare, and therefore, you tend to see Class-A, Class-B and even Class-C networks subnetted into smaller blocks of IP space, as we've done on R5's LAN, on the link between R1 and R5, and on each of the links between R1 and R2, R3 and R4 here. Consequently, when using EIGRP in modern production networks, you will typically see the command "no auto-summary" somewhere in the EIGRP configuration.

However, on a large production network, this can be really ugly, as you see /30 network after /30 network listed in the "sho ip route" ouptut:
R5#sho ip route
<...snip...>
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.0.0 is directly connected, Vlan10
D EX 192.168.4.0/24 [170/30976] via 10.0.0.1, 00:00:04, FastEthernet1/0
10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
D EX 10.254.254.2/32 [170/158720] via 10.0.0.1, 00:00:04, FastEthernet1/0
D EX 10.254.254.3/32 [170/158720] via 10.0.0.1, 00:00:04, FastEthernet1/0
C 10.0.0.0/30 is directly connected, FastEthernet1/0
D EX 10.254.254.1/32 [170/156160] via 10.0.0.1, 00:00:04, FastEthernet1/0
D EX 10.254.254.4/32 [170/158720] via 10.0.0.1, 00:00:04, FastEthernet1/0
C 10.254.254.5/32 is directly connected, Loopback0
192.168.1.0/30 is subnetted, 3 subnets
D 192.168.1.8 [90/30720] via 10.0.0.1, 00:00:04, FastEthernet1/0
D 192.168.1.0 [90/30720] via 10.0.0.1, 00:00:04, FastEthernet1/0
D 192.168.1.4 [90/30720] via 10.0.0.1, 00:00:04, FastEthernet1/0
D EX 192.168.2.0/24 [170/30976] via 10.0.0.1, 00:00:04, FastEthernet1/0
D EX 192.168.3.0/24 [170/30976] via 10.0.0.1, 00:00:04, FastEthernet1/0
R5#

Notice that there are six 192.168.x.x subnets shown in the output of the "sho ip route" command on this router, and this is a SMALL network, so imagine how it would look on the service-provider network where I work! Not only does it make it more difficult to parse the output of the "sho ip route" command, but EIGRP on R1 has to advertise each of these networks separately to R5, making the update larger than it has to be.

Fortunately, this isn't an all-or-nothing choice. It is possible to have EIGRP summarize some routes, but advertise individual subnets for others. Furthermore, you don't have to summarize into a classful network -- you can use any valid IP subnet to summarize the routes. In the example network shown in this lab, we have 192.168.1.0/30, 192.168.1.4/30, 192.168.1.8/30, 192.168.2.0/24, 192.168.3.0/24 and 192.168.4.0/24 in use. The smallest valid subnet that contains all of these networks would be 192.168.0.0/21 (192.168.0.0 -- 192.168.7.255). If we assume that that IP space is reserved for use by R1 and edge routers hanging off of R1, then we can have R1 summarize the 192.168.x.x subnets into one single route advertisement for this entire range:
R1(config)#int fa0/0
R1(config-if)#ip summary-address eigrp 10 192.168.0.0 255.255.248.0
R1(config-if)#
01:44:20: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 10: Neighbor 10.0.0.2 (FastEthernet0/0) is down: summary configured
R1(config-if)#
01:44:23: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 10: Neighbor 10.0.0.2 (FastEthernet0/0) is up: new adjacency
R1(config-if)#

Now, if we look at the routes on R5, we will see that, rather than advertising six 192.168.x.x subnets, R1 is only advertising a single /21 that contains each of the previous six subnets:
R5#sho ip route
<...snip...>
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.0.0 is directly connected, Vlan10
10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
D EX 10.254.254.2/32 [170/158720] via 10.0.0.1, 00:03:55, FastEthernet1/0
D EX 10.254.254.3/32 [170/158720] via 10.0.0.1, 00:03:55, FastEthernet1/0
C 10.0.0.0/30 is directly connected, FastEthernet1/0
D EX 10.254.254.1/32 [170/156160] via 10.0.0.1, 00:03:56, FastEthernet1/0
D EX 10.254.254.4/32 [170/158720] via 10.0.0.1, 00:03:55, FastEthernet1/0
C 10.254.254.5/32 is directly connected, Loopback0
D 192.168.0.0/21 [90/30720] via 10.0.0.1, 00:03:55, FastEthernet1/0  <---
R5#

Cool!