Newbee here! need help for a common interview question !

karthikheb

Hi guys,

This is my first post here. I have been asked this question a couple of times in interviews(didnt clear) , i am not sure whether this is the right place to post this but here it goes,

Question:
Assume the following topology: Router1 - SwitchA - SwitchB. Switch A has a web server connected to it, SwitchB has a workstation. The workstation cannot load a web page from the server. Tell me every possible cause you can think of and how you would go about identifying each.

So how exactly would you debug this. I know there can be many different reasons , but what common approach should one take when tackling such a issue ?

My answer would be something like this

1) I would check the interfaces to see whether they are up
2) I would ping the website and check whether the DNS resolves the website. If it does not resolve then I would assume the issue is with the DNS server.
3)If ping is working fine , then i would use traceroute to see if any device in the path is failing .
4) I will check to see whether there are any ACLs configured that is not allowing me to visit the particular website.

Any help is appreciated !!!

OfWolfAndMan

It depends on how the topology is setup. If they're in different VLANs, there would have to be some interVLAN routing, whether it be via SVIs or sub interfaces on the router. Don't forget about troubleshooting the physical as well.

BlackBeret

I'd put in a help desk ticket and see if that resolves it....

But on a serious note; Ping doesn't test for DNS, pinging an IP address purposely doesn't use DNS. I'm assuming you know this, but based on your explanation it might not be clear. You can test DNS resolution with NS Lookup. You could try asking other questions as well. What type of host is it? Check the hosts file on the system and see if the wrong IP is hard coded in.

Really it would depend on the job I'm applying for and what systems I'd be working with. I don't work directly in networking but I'd start by asking a LOT more questions about all of the devices in the path. What information is being returned by various methods. As with anything else I would start from the beginning and work my way to the end. Are you diagnosing the server or the host? Is the webserver reachable from other hosts? Will the host reach other web sites? Then one device at a time: Is it plugged in, is it powered on, is it attached to the network, is the network device properly configured, can you ping the webserver, can you ping your own gateway, can you ping something outside of the gateway, can you ping the target switch, can you ping anything else inside of the target switch, can you resolve other host names, can you resolve the server host name from another host, are there known outages, when you run out of options do NOT forget to escalate. No one knows everything, you can always ask.

karthikheb

BlackBeret wrote: »

I'd put in a help desk ticket and see if that resolves it....

But on a serious note; Ping doesn't test for DNS, pinging an IP address purposely doesn't use DNS. I'm assuming you know this, but based on your explanation it might not be clear. You can test DNS resolution with NS Lookup. You could try asking other questions as well. What type of host is it? Check the hosts file on the system and see if the wrong IP is hard coded in.

Really it would depend on the job I'm applying for and what systems I'd be working with. I don't work directly in networking but I'd start by asking a LOT more questions about all of the devices in the path. What information is being returned by various methods. As with anything else I would start from the beginning and work my way to the end. Are you diagnosing the server or the host? Is the webserver reachable from other hosts? Will the host reach other web sites? Then one device at a time: Is it plugged in, is it powered on, is it attached to the network, is the network device properly configured, can you ping the webserver, can you ping your own gateway, can you ping something outside of the gateway, can you ping the target switch, can you ping anything else inside of the target switch, can you resolve other host names, can you resolve the server host name from another host, are there known outages, when you run out of options do NOT forget to escalate. No one knows everything, you can always ask.

Thanks !

I do know that ping doesnt test for DNS. What i was getting at is that when i ping with ping www.yahoo.com if it does not work and if i directly give ping 72.82.32.45( assume this to be yahoos ip) and it works. My conclusion would be that there might be a issue with the DNS. And yes i agree that nslookup would be better for this purpose.