Understanding the network.pdf - Back to Home

outer in the process. The idea behind the default-network is to send external traffic
not to a specific gateway, but rather to a default network where the gateway can be
determined (in the case of multiple network exit points) by the locally attached
router. It should be noted that a default gateway setting using the may be required on the actual outbound
gateway that is attached to the default network.
EIGRP Specific Configuration Options
EIGRP's support of IP summary-address announcements has an impact on CIDR
supernets. Although it is recommended that you disable classful summarizations or
avoid classful summarization altogether, if you use CIDR supernetting with a class C
address space (not uncommon with sites that use small regional ISPs for Internet
access), you will need to use summary announcements in order to have EIGRP
announce the superset correctly. IP summarizations are configured using the
interface configuration subcommand . Here is an
example: The ISP for the Albany network (refer to Figure 10.1) has provided four
class C addresses to support a network expansion that will be attached to asbr-2a.
The addresses were assigned as a single /22:
192.168.4.0 /22 = 192.168.4.0, 192.168.5.0, 192.168.6.0, 192.168.7.0
Moreover, we want to maintain this address provisioning. Under normal operation,
EIGRP would announce each class C network as a separate route entry, even though
the address block is being used as a contiguous CIDR superblock. By using an
aggregate summary address, the 192.168.4.0 /22 network is announced correctly
as a supernet:
asbr-a2(config)#int e2/0
asbr-a2(config-if)#ip address 192.168.4.1 255.255.252.0
asbr-a2(config-if)#ip summary-address eigrp 99 192.168.4.1 255.255.252.0
Using the command we can see the summarization listed
(notice that auto-summarization has been disabled):
Routing Protocol is "eigrp 99"
Outgoing update filter list for all interfaces is not set