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Request for Help Reverse Telnet from router to switch

Discussion in 'General Cisco Certifications' started by NetEyeBall, Nov 16, 2006.

  1. NetEyeBall

    NetEyeBall Kilobyte Poster

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    I might not understand this totally since I don't use it on a daily basis but have seen my network support people use it. They telnet to a router then use the console cable to telnet to the a switch that normally isn't available due to a switching loop or other issue. They have to use a particular port and such.

    Can someone give me a run down on this? Please and thank you. :eek:
     
    Certifications: CCNA, A+, N+, MCSE 4.0, CCA
    WIP: CCDA, CCNP, Cisco Firewall
  2. NetEyeBall

    NetEyeBall Kilobyte Poster

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    Am I not explaining it well enough? Or doesn't anyone do this?
     
    Certifications: CCNA, A+, N+, MCSE 4.0, CCA
    WIP: CCDA, CCNP, Cisco Firewall
  3. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    I can't answer your question even though I get the gist of what you are asking.

    It is a specific question which I am sure will be addressed if you give more details about the router manufacturer, model, firmware/software and what you are trying to achieve or understand etc.

    There is a Cisco article here that might help but I am not the Cisco kid :biggrin

    http://ezinearticles.com/?Cisco-CCN...Configuring-PortFast-And-BPDU-Guard&id=173609
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  4. simongrahamuk
    Honorary Member

    simongrahamuk Hmmmmmmm?

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    Let me see if I understand this correctly, They Telnet into a router (Using RJ45 connection I assume?). What they also do is to Use the console cable to connect to a switch and configure that? Does the console cable come from the PC that is initiating the session with the switch, or does the console cable go from the router that was telnet(ed) into to the switch?

    If it goes the way I think it does (PC - ROUTER - SWITCH) then I've done it before in a Lab and it is fairly straight forward simply telnet or HyperTerminal (or other) into the Router then when connected to that router enter telnet and the IP Address of the switch. This will work provided that the switch is configured to accept telnet connections.

    Am I way off track with this?:rolleyes:
     
  5. NetEyeBall

    NetEyeBall Kilobyte Poster

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    PC -->Router-->Switch. PC to Router via telnet and then by telnet but from a particular port across the aux cable to the Switch. Like I said I am not familiar with it. They had to enable the aux port and then find the port to use in the telnet sesssion.

    Damn I wish I new more because it is something they use quite a bit, but I don't in my current position. When I build my lab up I plan on doing it.
     
    Certifications: CCNA, A+, N+, MCSE 4.0, CCA
    WIP: CCDA, CCNP, Cisco Firewall
  6. Spice_Weasel

    Spice_Weasel Kilobyte Poster

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    Reverse telnet allows you to telnet from a router or switch port to the console port of another router/switch. For example, I have a cable running from the aux port on a 2611 router to the console port on a 2960G switch. That way, if the switch is unavailable due to network problems via the usual ssh, I can connect to the 2960G's console port from the router. BTW you can also reverse ssh if you want.

    If you have an access server router you can get an octal cable and connect to eight devices per port.

    It is fairly easy to set up:

    First, connect the aux port on your main device to the console port on the remote device using a rollover cable.

    Next, check your lines to see the line number on the aux port, e.g.:

    router#sh line
    Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
    0 CTY - - - - - 0 0 0/0 -
    65 AUX 9600/9600 - - - - - 0 0 0/0 -
    * 66 VTY - - - - 12 97 0 0/0 -
    67 VTY - - - - 12 28 0 0/0 -
    68 VTY - - - - 12 1 0 0/0 -
    69 VTY - - - - 12 0 0 0/0 -
    70 VTY - - - - 12 0 0 0/0 -

    Note that in above my aux line is 65 and is set for 9600 (the default).
    Next, make sure your aux line allows telnet in, e.g.:

    #line aux 0
    #transport input telnet

    Again, all this is on the main device, the one you are using to reach the remote device.

    All you need to do now is to telnet to the aux port and a connection will be opened to the console on the remote device, e.g.:

    #telnet 192.168.1.1 2065

    For the ip address, use a valid address on the main device, not the remote device! Also, the reserved port range is 2000 - 2999, where tty 0 = 2000, tty 1 = 2001, etc. So in the above example, tty 65 (the aux port) = 2065.

    If you want, you can create a host entry to make it even easier, for example:

    config#ip host rem-sw 2065 192.168.1.1

    Now, all you have to do is type:

    #rem-sw

    - and you will be connected to the switch. All the usual login details apply, of course, and you can use a modem on the aux port as well. you can also use a modem on a console port, but that is more complex, for a simple reverse telnet all you need is a rollover cable.

    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  7. NetEyeBall

    NetEyeBall Kilobyte Poster

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    Thats it!!!!! Thank you!!!!!!!!!!!!!!!! :D

    Come to Nashville and I will buy you a drink! That helps me tons!
     
    Certifications: CCNA, A+, N+, MCSE 4.0, CCA
    WIP: CCDA, CCNP, Cisco Firewall
  8. NetEyeBall

    NetEyeBall Kilobyte Poster

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    Everyone studying CCNA should copy this answer into a notepad document of usefull info and stick it on their thumb-drive!
     
    Certifications: CCNA, A+, N+, MCSE 4.0, CCA
    WIP: CCDA, CCNP, Cisco Firewall
  9. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Interesting - this looks like a Cisco specific thing.

    It looks to me as if there is a sort of terminal server being run here - is this correct?

    Harry (a Cisco newbie)
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  10. Spice_Weasel

    Spice_Weasel Kilobyte Poster

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    Thanks, glad it was useful. Sometime I'll make my way down to Nashville for that drink! :)

    BTW, to switch back to the host device, use ctrl+shift+6, then x

    Handy to know when you have multiple sessions running. To view sessions, type:

    router#show sessions

    You will see the open sessions. To resume the connection type the connection number, e.g.:

    router#1

    You will also resume a connection if you just hit "enter" which can be suprising the first time ;)

    To cancel a session, type:

    router#disconnect 2

    Where the 2, above, is the number of the connection from the show connection output.


    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  11. Spice_Weasel

    Spice_Weasel Kilobyte Poster

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    Harry, actually virtually any Cisco device, that has a console or aux port, can use reverse telnet. The access server router I mentioned would be something like an old 2509, 2511 or 2512 router with asynch serial interfaces. You can get serial to rj45 cables (one serial to 8 or 16 rj45) and reverse telnet into plenty of devices.

    BTW, those old routers are nice, able to connect to modems, ISDN TA's, DEC VAX, terminals, mainframes, slot machines :) as well as reverse telnet.

    Spice_Weasel
     
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
    WIP: CCIE
  12. NetEyeBall

    NetEyeBall Kilobyte Poster

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    And it worked great. I bought a 2511 and an octal cable and tried it out. Fantastic! Thanks again!!!!!!
     
    Certifications: CCNA, A+, N+, MCSE 4.0, CCA
    WIP: CCDA, CCNP, Cisco Firewall

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