Troubleshooting IP Routing
Case 1:- PC1 is able to ping 10.1.1.1 but not able to ping any other IP. Reason?
Ans:- For any machine to ping outside its subnet, it should have correct Gateway and subnet mask configured. If it is not the case, ping or traffic won’t go outside. Router doesn’t forward broadcast by default and it will drop the traffic immediately.
Case 2:- PC1 is able to ping PC2 but PC2 is unable to ping PC1.
Ans:- Since PC1 can ping PC2 which means that Default Gateway configuration is correct on Both PC1 and PC2 and Static routing or routing protocol is advertising all of the subnets well. Also PC2 is replying to echo requests of PC1 makes me it clear that reverse routing path is correct too. Now, we need to look for any blockage or denial of ping packets. We need to start by checking the configuration at Fa0/0 of R2 and observe for any Access-groups attached. Then move to R2 S0/0 , R1 S0/0 and R1 Fa0/0 configuration.
If ping is not working – doesn’t mean that other traffic won’t work. So if there is any attached ACL- we need to check for deny statement or permit statements since there is implicit deny at the end of each acl. Also check for any possible transparent firewall which you can’t see by checking static routes on routers. Some time, there is local firewall settings which prevent application level traffic- apps such as Mcafee HIPS( Host intrusion Preventing system).
Case 3:- PC1 is able to ping 10.1.3.2 s0/0 of R2 But PC2 is unable to ping 10.1.3.2. Why?
Ans:- If PC1 can reach 10.1.3.2 which is outside the boundary or segment of PC1 i.e 10.1.1.0/24. It means default gateway setting is correct. It also means that Link between R1 and R2 is up and working. There is no need of any additional static route because subnet 10.1.3.0/30 is directly connected. Now, PC2 with IP 10.1.2.5/24 can’t ping 10.1.3.2- so we need to check its next hop i.e. its own gateway 10.1.3.1, if it is pinging means layer 1 connectivity is ok . Now, check default gateway- If it is set wrongly then it could be the reason for the problem in IP Routing. Also GW is correct but still not working , then restriction of access-list level can be the problem.
Case 4:- From R1 console, I can ping 10.1.2.5 but from R2 console I can not ping 10.1.1.5.
Ans :- R1 can ping 10.1.2.5 means that static route for 10.1.2.0/24 is okay and pointing towards 10.1.3.2/30. Now, since R2 has problems reaching 10.1.1.5- means that either static route for 10.1.1.0/24 is not present or it is set incorrectly. Incorrect route could be wrong subnet mask or wrong next hop address or wrong exit interface.
Case 5:- What will happen if I configure Default Gateway IP as 10.1.1.2 instead of 10.1.1.1 on PC1?
Ans:- Default Gateway should be set correctly else traffic meant for outside segment would not work. In this case it is set to wrong IP which does not exist, no ARP response or response or any unicast destined for 10.1.1.2. Even if 10.1.1.2 exists ( or being used by any other user in that LAN) then all unicast will go to that user and drop there because of no capability of routing on user machine.
Troubleshooting IP routing has often been described as one of the most important criteria to be a good network technician . So if you have your basics correct then it will be way to go for you.