A: Which IPv4 Static Routes Could be Useful?
The previous sample question gave us lots of example static IPv4 routes to think about. The answer and explanation are below the fold, as usual. Ask’em if you’ve got’em!
Answer Letter
C
Explanation
The first two parameters on the ip route command are the subnet number and subnet mask, which together define the range of addresses defined by the route. All four answers to this question list a subnet ID and mask that refer to some subnet that exists in this network:
10.1.1.0, 255.255.255.0 – R1’s LAN subnet
10.1.3.0, 255.255.255.0 – R3’s LAN subnet
172.16.2.0, 255.255.255.128 – R2’s LAN subnet
10.1.14.0, 255.255.255.0 – the serial link subnet between R1-R4
First consider subnet 10.1.14.0 / 255.255.255.0. The question asked for routes that the routers would need for the purpose of forwarding packets to hosts A, B, C, and D, which are hosts on Ethernet LANs. Subnet 10.1.14.0 is not a LAN-based subnet. As it turns out, only routers R1 and R4 need a connected route for this subnet. So you can rule out answer D.
Before considering the other three answers, recall that the third parameter of the ip route command can either be the IP address of the neighboring router, or the local router’s outgoing interface. If an IP address is listed, it should be the IP address of a neighboring router – that is, a router on a subnet connected to that same router.
Answer A Analysis
Answer A lists the subnet number for R1’s LAN subnet, with next-hop route IP address of 10.1.12.2, which is R2’s S0/0/0 interface IP address. This answer is syntactically correct.
The question asks that we think about whether the routes are useful on either R2 or R3, so starting with R2… t is useless on R2. R2 could not make use of a route with it’s own address as the next-hop address. The next-hop address would need to be neighboring router R1’s IP address on their common link, namely 10.1.12.1. Changing this command to use a next-hop IP address of 10.1.12.1 would make it useful to router R2.
Similarly, this route is useless to R3, because the next-hop IP address listed, 10.1.12.2, is not on a subnet connected to R3.
Answer B Analysis
Answer B lists the correct subnet number for R3’s LAN subnet.
Thinking about this potential static route from R2’s perspective, R3 already has a connected route for this subnet, so R3 does not need a static route for this same subnet.
From R2’s perspective, the command happens to be syntactically correct, but it is not useful. From R2’s perspective, the next-hop router IP address is R2’s own IP address, rather than the neighboring router’s (R3’s) IP address on a connected subnet.
To be useful as a command on router R2, change this route to use a next-hop address of 10.1.23.3, R3’s serial link IP address on the link from R2 to R3.
Answer C Analysis
Answer C, the one correct answer, would be useful to router R3. This route lists the subnet number and mask of router R2’s LAN subnet, with next-hop address 10.1.23.2. From R3’s perspective, the next-hop IP address is a neighboring router (R2) on a connected subnet, on the serial link between R3 and R2.
Loving this site for additional practice Wendell thank you for that.
I got the correct answer but fell in to the old trap of not reading and remembering the question fully. Due to that I chose D as also being correct, forgetting that the question was asking for host subnet routes only 🙂 Good practice to fully read and especially understand what is being asked.
Which part of question says for host subnet routes only ?
Host D’s IP address is not within the range of useful addresses if we calculate the subnet using R4’s LAN interface (IP address 172.30.4.4)
Correct, Host D should have an IP address between 172.30.4.1/26 and 172.30.4.62/26 excluding R4 IP of course.
I am not sure how the route from c is useful it R2 itself, it already knows about that LAN since it is directly connected. R3 yes it is helpful for.
Hi David,
If I understand your words correctly, I agree with your logic. That is, the route is useful on R3, and not on R2. But then answer C is correct because the question asks which command would be useful to “either” R2 or R3. Does that complete the connection?
Wendell
D. ip route 10.1.14.0 255.255.255.0 S0/0/1
This route isn’t useful for R3 as the outgoing physical interface S0/0/1 goes to R2 which does not have a connected route to 10.1.14.0/24
For R2 outgoing S0/0/1 connects to R1 which does have 10.1.14.0/24 in its routing table as a connected route.