Problems telnetting across frame relay

Unanswered Question
Jul 26th, 2010

Trying to telnet from router to router across frame relay (details below). Can ping OK but when I telnet it gets as far as telling me the session is open but I never get prompted for a password. I hit enter and every key combination I can think of. I have tried entering the telnet password despite not being prompted. Nothing. The connection times out anywhere from a few seconds to several minutes after connecting or after hitting enter a few times. A field tech said he could telnet to the router from the ethernet side (when he was on site before) but no one has actually demonstrated this to me.

This is what happens when I ping:

router#ping vrf ABC 192.168.0.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.0.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 84/85/88 ms

This is what happens when I telnet:

router#telnet 192.168.0.2 /vrf ABC
Trying 192.168.0.2 ... Open

...

<some time later>

[Connection to 192.168.0.2 closed by foreign host]


Below are some configuration details.


Telnetting from: Cisco 3845 Version 12.4(16a), RELEASE SOFTWARE (fc2)

Telnetting to: Cisco 2821 Version 12.4 (sorry, that's all the details I have on this one -- from a show run printout)


Destination (FAR end) router vty config:
line vty 0 4
password 7 xxxxxxxxxxx
login
transport input telnet
!


Destination (FAR end) router interface config:
interface Serial0/0/0.181 point-to-point
description PVC for Monitoring
bandwidth 56
ip address 192.168.0.2 255.255.255.252
frame-relay interface-dlci 181  
  class 56ps_16cir_Shaping


NEAR end interface config:
interface Serial1/0.180 point-to-point
description ABC group2 (ABC)
bandwidth 56
ip vrf forwarding ABC
ip address 192.168.0.1 255.255.255.252
no cdp enable
frame-relay interface-dlci 180


Thanks in advance.

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David Aicher Thu, 08/12/2010 - 10:32

Ok interesting issue but will take a bit to understand where the problem is.

Telnet is a layer 7 protocol it is a really great test since you will test the entire protocol stack.   Ping on the other hand is layer 3 and below.  It is not uncommon to be able to ping but not telnet or SSH.  There are to many things that could cause this issue to list.  The most common are ACLs either on the interface or on the vty.  Check the VTY configs to make sure.

The best way to find the problem is to run a few debugs if you have access to the console.  Please be careful as these can be chatty and increase the cpu utilization.

debug telnet

debug ip tcp transactions.

With these two debugs you can confirm layer 4 and above.

If TCP finishes the 3 way handshake it should look something like this.

01:47:45: TCP0: state was LISTEN -> SYNRCVD [23 -> 10.1.1.2(11004)]
01:47:45: TCP0: Connection to 10.1.1.2:11004, received MSS 1460, MSS is 516
01:47:45: TCP: sending SYN, seq 2128839637, ack 1556820750
01:47:45: TCP0: Connection to 10.1.1.2:11004, advertising MSS 1460
01:47:45: TCP0: state was SYNRCVD -> ESTAB [23 -> 10.1.1.2(11004)]
01:47:45: TCB17C3A530 setting property TCP_TOS (1) 120C7A08

After this you should start to see the telnet negotiation.

01:47:45: Telnet1: 1 1 251 1
01:47:45: TCP1: Telnet sent WILL ECHO (1)
01:47:45: Telnet1: 2 2 251 3
01:47:45: TCP1: Telnet sent WILL SUPPRESS-GA (3)
01:47:45: Telnet1: 80000 80000 253 24
01:47:45: TCP1: Telnet sent DO TTY-TYPE (24)
01:47:45: Telnet1: 10000000 10000000 253 31
01:47:45: TCP1: Telnet sent DO WINDOW-SIZE (31)
01:47:45: TCP1: Telnet received DO SUPPRESS-GA (3)
01:47:45: TCP1: Telnet received WILL TTY-SPEED (32) (refused)
01:47:45: TCP1: Telnet sent DONT TTY-SPEED (32)
01:47:45: TCP1: Telnet received WILL WINDOW-SIZE (31)
01:47:45: TCP1: Telnet received WILL LOCAL-FLOW (33) (refused)
01:47:45: TCP1: Telnet sent DONT LOCAL-FLOW (33)
01:47:45: TCP1: Telnet received DO ECHO (1)
01:47:45: TCP1: Telnet received WONT TTY-TYPE (24)
01:47:45: TCP1: Telnet sent DONT TTY-TYPE (24)
01:47:45: Telnet1: recv SB NAWS 80 24
01:47:45: TCP1: Telnet received WONT TTY-SPEED (32)
01:47:45: TCP1: Telnet received WONT LOCAL-FLOW (33)

Hope this helps.

Regards

Dave

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