ICND2 – Chapter 4 – Static and Connected Routes

Chapter 4 was a pretty short chapter and most of it covered material that I was already very familiar with. However, I had never heard of or seen the use of the ip default-network command before. Here are the notes from my review:

Any subnets assigned to interfaces with an up/up status are placed in the routing table as a CONNECTED route.

You can add a second IP address to an interface using the following command:

Router(config-if)#ip address 192.168.1.2 255.255.255.0 secondary

IOS can restrict the use of the Zero-Subnet. The default setting allows the use of the all 0s subnet. To disallow the use of the Zero-Subnet use this command:

Switch(Config)#no ip subnet-zero

A router can route between VLANS by using ISL or 802.1Q on an interface. This is normally achieved using sub-interfaces. To configure use one of the following commands:

Router(config)#interface FA0/0.1
Router(config-if)#encapsulation ISL 1
Router(config)#interface FA0/0.2
Router(config-if)#encapsulation ISL 2

or

Router(config)#interface FA0/0.1
Router(config-if)#encapsulation dot1q 1
Router(config)#interface FA0/0.2
Router(config-if)#encapsulation dot1q 2

Note: When a router’s only purpose is to route between VLANs and it has a single connection to the LAN, it is often referred to as “Router on a Stick” because of how it looks on a network diagram.

Use the ip route s.s.s.s m.m.m.m nh.nh.nh.nh command to configure a Static Route.

Configure a static default route (Gateway of Last Resort) as follows:

Router(config)#ip route 0.0.0.0 0.0.0.0 nh.nh.nh.nh (where nh is the next hop address)

You can also use the ip default-network command that works only on classful networks. It takes the route for that classful network and uses it as the default route. For example, you use ip default network 10.0.0.0 and router uses the next-hop for that network as the next-hop for the default route.

If a router learn multiple default routes (static of via protocol) the router notes each with an * in the list. It then chooses the best and installs it as the gateway of last resort.

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