I am able telnet even though line protocol was down on interface

Unanswered Question
Aug 31st, 2010

Hi All,

Recently I was able to telnet to our backbone router even though the line protocol was down on interface from our LAN. This IP is not assigned to any other interface on this router/network.

Could anyone tell me how is this possible?

Interface config details are as below.

Router Model: Cisco 3650

System image file is "flash:c3640-i-mz.121-1.T.bin"

BBBB#sh int Serial2/1:13
Serial2/1:13 is up, line protocol is down
Hardware is DSX1
Description: XXXXXXX 

Internet address is 10.1.1.1/30
MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec,
Reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
Keepalive set (10 sec)
Last input 3w0d, output 9w0d, output hang never
Last clearing of "show interface" counters 07:12:05
Input queue: 0/75/2/0 (size/max/drops/flushes); Total output drops: 3452
Queueing strategy: weighted fair

CCCCC#10.1.1.1

Trying 10.1.1.1 ... Open

*****************************************************
*          ! ! ! ! !   WARNING ! ! ! ! !            *
* Unauthorized access and use of this system is     *
* not permitted and is strictly prohibited by       *
* security policies, regulations, state and federal *
* laws.                                             *
*                                                   *
* UNAUTHORIZED USERS ARE SUBJECT TO CRIMINAL AND    *
* CIVIL PENALTIES AS WELL AS COMPANY-INITIATED      *
* DISCIPLINARY PROCEEDINGS.                         *
*****************************************************

User Access Verification

Password:
BBBB#

Thanks,

Prathin Kumar

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Federico Coto F... Tue, 08/31/2010 - 09:02

Hi,

Telnet is a Layer 7 application protocol.

Line protocol down is a Layer 2 problem.

No way you can have anything above layer 2 working if the line protocol is down (i.e. you cannot PING to that IP either).

I think that either you were seeing something incorrect or somehow the telnet is going into another interface (eventhough you mentioned the IP is not assigned nowhere else).

Federico.

prathinkumarshetty Tue, 08/31/2010 - 22:01

Dear Federico,

I mean that, particular IP is assigned only on single interface and the line protoccol of that interface is down (interface details given earlier). I try to ping that IP, its not happening(as you said Layer2 down), but whenevr I try to telnet the same IP I am able to telnet without any problem.

One more thing I noticed in same router, some other IP (which is assigned in different interface) also able to telnet when line protocol is down.

I want to know that how telnet is happening in this condition.

Prathin.

Richard Burts Tue, 08/31/2010 - 22:29

Prathin

I believe that there is a simple explanation for what you are experiencing. It took me a couple of times reading your post to realize that the address that you are attempting to telnet to is the address of the router itself. When you telnet to yourself the telnet packet does not go out any physical interface (and if it tried it would fail if the interface protocol is down).

So basically what is happening is that you issue the telnet command, the router prepares a telnet connection request (essentially it is preparing a TCP SYN to port 23), the router looks for how to forward this request, the router realizes that it is the destination of the request and processes it, without need to physically transmit anything.

The results would certainly be different if you were attempting telnet to the device on the other end of that serial link.

HTH

Rick

prathinkumarshetty Wed, 09/01/2010 - 05:06

Hi Rick,

Thanks for your response.

Whats actually happening is I am able to telnet with the same IP address over a WAN link as well thats the puzzling part!!!!

Prathin.

Richard Burts Wed, 09/01/2010 - 05:39

Prathin

The example given in your original post was clearly telnet to the local router itself. If you can also telnet to a remote router that way perhaps you could post an example of that.

I would also suggest that if you have an unusual telnet such as you describe that it could be that the router has an alternate route to that destination. This happened to me recently. I was working on a router which had an interface with protocol down state. Yet I was able to access the address of the router on the other end of the point to point connection. When I looked into the routing table I found that there was a route for this subnet in the routing table using another interface which connected to a router which had connectivity to both routers. Think of it as router A and B and C. The link between A and B is down. But A can get to C and C can get to B and this gives the ability of A to access the interface on B of the problem interface.

So before you do the telnet I would suggest that you do show ip route and see if there is a valid path to that destination.

HTH

Rick

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