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New Member

Puzzled by this troubleshooting question

Here is a challenge question that I can't solve:

Anyone has an idea what else one could do to establish IP

communications to RouterB ? I don't have more detailed info about this question because I've seen similar question like the one below from an exam from a certain organization, and I don't have and can't disclose exact details. Do you have at least an idea on how to approach such problem below:

Note that IOS 'config t' commands

are not available in ANY router involved:

RouterA(DCE) s0--------s0RouterB(DCE) s1-------s0 RouterC

I have privileged access to RouterA and RouterC.

I do 'show int' on RouterA and RouterA and s0/0

interfaces are up/up.

I do show ip route and I see no routes learned from neighbor routers.

I ping RouterB s0 and s1 and interfaces;it times out.

From RouterA and RouterC, I do telnet <s0_IP address_RouterB> but

that is not successful either. Telnet <s1_IP Address_RouterB> is

not successful as well.

'configuration mode' is not available for any router involved.

1 ACCEPTED SOLUTION

Accepted Solutions
Silver

Re: Puzzled by this troubleshooting question

If you cannot telnet to the routers, try to connect to console to check the status. I have no idea how to remote login to the router.

Moreover, do you mean you can access the config. now ?

The CDP can provide the negibhor's info., e.g. hardware, connecting interface, etc. but not the config.

Did you try to trace the IP in all routers ?

And, if you configure "no keepalive", the interface will remain up. So, the status show up may not actually up at both side.

Can you provide the "show interface" for reference ?

Hope this helps.

12 REPLIES
New Member

Re: Puzzled by this troubleshooting question

ip details are missing

New Member

Re: Puzzled by this troubleshooting question

The only thing I can remember is that RouterA---RouterB Interfaces was a 172.16.0.0/24 network. On RouterA I remember 'router rip version 2'.

On RouterB s1---s0RouterC network was 192.168.0.0/24. Also router rip r2 on RouterC.

Sorry I don't have detailed IP address.

My question is, can you throw some ideas on what one could do to make the communication work if you cannot telnet to routerB and also you are unable to use configuration terminal to make changes ?? Any ideas ?

Again, my apologize for the lack of detailed information.

New Member

Re: Puzzled by this troubleshooting question

It's realy hard to guess the configs.probable solutions would be.

0.as u mention no route to destination u may start trying by using a static route

1.Run a routing protocol and redistribute connected on both end routers ( a& C)

2.If connectivity is there between A&C u may create a tunnel.

HTH

New Member

Re: Puzzled by this troubleshooting question

Rajvrajan1, but if I have no access to config terminal, I think I couldn't do the 'ip route' because that would require the global configuration mode as far as I know.

Same it goes for option 1.

How about your proposal 2; in this case how could you create a tunnel ? Are you sayng an IPSec tunnel ? If so, again you would still need the configuration mode...

Yeah, I may be missing something here. Thank you for your willing to help me, even with my lack of information.

Silver

Re: Puzzled by this troubleshooting question

I believe you cannot solve the problem and only be able to troubleshoot the problem,right ?

I suggest below steps :

1) Ping router A,B,C all interfaces to check whihc one is active.

2) Check the status of the interface that cannot be ping by "show" command.

3) Check any physical or signalling issue.

4) Check any IP address mismatch.

5) Check running routing protocol by "show ip route" & "show ip interface".

6) According to above results, you should able to identify the problem, e.g. physical connection ? IP addressing ? DTE/DCE (clock rate ? or mismatch of male/female type of DTE/DCE cable is using) ? Routing protocol ? Cable problem (can swap another one ?) ?

Or try to troublshoot from the physcial layer to upper layers.

Hope this helps.

New Member

Re: Puzzled by this troubleshooting question

since the so/o of the routerA is up, the connectivity must be fine. In that case check the foll

1. there might be a ping block applied on the serial interfaces

2. the telnet block acl might also be applied on the interface

3. the interface s1 of router B incase hadnt any ping or telnet block acl, to be accessed from the router A should have a route (atleast a static one) for its ip pointing to the so interface.

4. Similar routing is needed for the so ip address in the router C

New Member

Re: Puzzled by this troubleshooting question

I recall I checked the show run config for ACLs on both RouterA and RouterC but no ACLs were configured. Even if there were ACL's there, I should use configuration terminal commands (correct me I am overlooking something) to be able to get rid of them and that was definitely not an option.

Tell me something, is there any other creative way to get to RouterB not using Telnet ? Perhaps the ACL was applied on RouterB.

Could I use ssh or another utility perhaps to get to RouterB instead of using telnet ?

New Member

Re: Puzzled by this troubleshooting question

One more thing that I did not do:

cdp neigh detail; I guess that should let me see more info about RouterB. That may be a way to see how RouterB is actually configured.

Silver

Re: Puzzled by this troubleshooting question

If you cannot telnet to the routers, try to connect to console to check the status. I have no idea how to remote login to the router.

Moreover, do you mean you can access the config. now ?

The CDP can provide the negibhor's info., e.g. hardware, connecting interface, etc. but not the config.

Did you try to trace the IP in all routers ?

And, if you configure "no keepalive", the interface will remain up. So, the status show up may not actually up at both side.

Can you provide the "show interface" for reference ?

Hope this helps.

New Member

Re: Puzzled by this troubleshooting question

I will get back to you all. Next week I should be able to get back to this. Thanks all so far.

Green

Re: Puzzled by this troubleshooting question

Cisco also supports the Unix "R" commands (RSH=Remote Shell, RCP = Remote Copy, RLOGIN ....)

CDP neighbor shold permit you to verify that the masks for the interfaces are correct. I'm sure the RIP **V2** would be a good clue that it's perhaps mask oriented ... like Router B is configured only for RIP V1 and the masks are all being converted to classful boundries (watch for a class A or B address with a 24 bit mask going through a router with a different class of address)

IF you can see the configs, look at the network statement to make sure the "Network" statements have been properly configured so the routes are propagated.

You might also pay attention to the INTERFACE configs to see if any are set to broadcast or receive a specific version of RIP (like listen to V2, transmit V1) and check to see if the right version is set for the right function.

You can do a DEBUG (it's an exec-level command) to see what routing updates are being sent/received (debug protocol rip ....)

You can check the MTU size of the frame. IF the MTU is less than 512, it would cause routing updates to be fragged, except routing protocols won't tolerate being fragmented and they are disposed. I used to use this in classs ... tell the students that the customer reduced the MTU to make his real-time (i.e., VoIP) traffic not have to wait behind the very large Database frames (VoIP from Cisco uses ~262 byte frames).

Anyway, think about some DEBUG commands that'll give you some interesting information. Review what happens when a classless packet passes through a classful router. Dust off your details on RIP v1 versus v2 ...

Good Luck

Scott

New Member

Re: Puzzled by this troubleshooting question

You guys are rocking. I agree, the RIP v2 perhaps is a good clue on what is going on.

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