ICND2 – IP Routing
Static routing can only be configured for each route manually so it is more secure than dynamic routing which only needs to declare which networks to run -> A is correct.
Also static route does not use any complex algorithm to find out the best path so no routing updates need to be sent out -> reduce routing traffic load. Static routing is useful especially in stub network links.
Note: Stub network (or stub router) is used to describe a network (or router) that does not have any information about other networks except a default route. This type of network (or router) usually has only one connection to the outside.
By default a static route has the Administrative Distance (AD) of 1, which is always preferred to dynamic routing protocols. In some cases we may want to use dynamic routing protocols and set static routes as a backup route when the “dynamic” routes fail -> we can increase the AD of that static route to a higher value than the AD of the dynamic routing protocols.
Discontiguous networks are networks that have subnets of a major network separated by a different major network. Below is an example of discontiguous networks where subnets 10.10.1.0/24 and 10.10.2.0/24 are separated by a 18.104.22.168/8 network.
If we configure automatic summarization at classful boundaries, users on network 10.10.1.0/24 cannot communicate with users on network 10.10.2.0/24.
If you are not clear about automatic summarization please read the last part of this tutorial: http://www.9tut.com/eigrp-routing-protocol-tutorial.
Host1 can ping the Serial interface of R2 because R1 has the network of 192.168.1.4/30 as directly connected route. But R1 does not know how to route to the network of Host2 (192.168.1.128/26) so R1 will drop that ping without trying to send it out S0/0 interface. To make the ping work, we have to configure a route pointing to that network (for example: ip route 192.168.1.128 255.255.255.192 s0/0 on R1).
All the above networks can be summarized to 10.0.0.0 network but the question requires to “represent the LANs in Phoenix but no additional subnets” so we must summarized to 10.4.0.0 network. The Phoenix router has 4 subnets so we need to “move left” 2 bits of “/24”-> /22 is the best choice -> D is correct.
From the output we see a line “Gateway of last resort is 192.168.4.1 to network 0.0.0.0”. Gateway of last resort refers to the next-hop router of a router’s current default route. Therefore all the traffic through this router to destination networks not matching any other networks or subnets in the routing table will be sent to 192.168.4.1 (which is on Serial2) -> packet destined for 192.0.2.156 (or an unknown destination) will be forwarded via Serial2.
An weird thing in the output above is the missing of the asterisk mask (*) which represents for the candidate default route. To set the “Gateway of last resort is 192.168.4.1 to network 0.0.0.0” as the output above we can use these commands:
ip route 0.0.0.0 0.0.0.0 192.168.4.1
ip default-network 192.168.4.0
But these commands will create an static routing in the routing table with an asterisk mask. Maybe the output shown above is missing that route.
For more information about the command ip default-network please visit: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml.
Static routes remain in the routing table even if the specified gateway becomes unavailable. If the specified gateway becomes unavailable, you need to remove the static route from the routing table manually. However, static routes are removed from the routing table if the specified interface goes down, and are reinstated when the interface comes back up.
Therefore the static route will only be removed from the routing table if the S0/0 interface on RTA is shutdown.