CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- With classless routing, the default route is used whenever a packet does not match a more
specific route in the routing table (unknown subnets of known a classful network).
- If the supernet or default route is learned via a classless routing protocol (eg: OSPF, IS-IS),
the classful nature (the no ip classless global configuration command is configured) of a router
is ignored – a classful router may use a default route to reach networks that are not listed in the
routing table regardless of the ip classless command.
Secondary IP Addressing
- Secondary addressing provides a solution to the problem of running out of addresses in a subnet
(due to poor network design) by allowing multiple networks (or subnets) to reside on the same
data link media. An interface may have an unlimited number of secondary addresses.
RT1
S0/0
S0/0
RT2
Fa1/0
10.1.1.1
10.1.2.0/24
Fa1/0
Figure 16-4: Sample Secondary IP Addressing Network
- Secondary IP Addressing configuration on RT2:
RT2(config)#int fa1/0
RT2(config-if)#ip add 10.1.3.1 255.255.255.0
RT2(config-if)#ip add 10.1.4.1 255.255.255.0 secondary
RT2(config-if)#no shut
RT2(config-if)#^Z
RT2#
RT2#sh ip route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 4 subnets
R 10.1.1.0 [120/1] via 10.1.2.1, 00:00:10, Serial0/0
C 10.1.2.0 is directly connected, Serial0/0
C 10.1.3.0 is directly connected, FastEthernet1/0
C 10.1.4.0 is directly connected, FastEthernet1/0
RT2#
- By implementing secondary IP addressing on RT2, all PCs are able to ping each other.
PC1
10.1.1.2
10.1.3.1
10.1.4.1 secondary
- Note: RT2 would use its primary IP address (10.1.3.1) as the source IP address when it
communicates with hosts in the secondary network (10.1.4.0/24).
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Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com
PC2
10.1.3.2
PC3
10.1.4.2