From: Glenn (ggoldie2@lineone.net)
Date: Fri Oct 10 2003 - 08:08:07 GMT-3
Hello,
It looks like the router aaa is being used as a terminal server with 3
modems (or other devices) hanging off tty lines (async) 2,3 and 4. The
hostnames are mapped like this :
bbb = 1.1.1.1 port 2002
ccc = 2.2.2.2 port 2003
ddd = 3.3.3.3 port 2004
Read it as: From aaa: To get to bbb, telnet to ip address 1.1.1.1 on
port 2002.
Because the destination ip address is the routers own loopback address,
when you type "telnet bbb" you will be telnetting to the loopback
address 1.1.1.1, that is telnetting to yourself and opening a connection
to yourself on port 2002. This is what cisco refers to as reverse
telnet. This config uses a seperate loopback interface and address for
each device (bbb, ccc, ddd), but you could also just do it with a single
address like this :
interface loopback 0
ip address 1.1.1.1 255.255.255.255
ip host bbb 2002 1.1.1.1
ip host ccc 2003 1.1.1.1
ip host ddd 2004 1.1.1.1
It depends on your router platform, but in this case (probably a 2500):
port 2002 = tty line 2 = device bbb
port 2003 = tty line 3 = device ccc
port 2004 = tty line 4 = device ddd
When you 'reverse telnet' to these tty lines on router aaa you will be
given a connection to the end devices bbb, ccc, or ddd. This could be a
router console port or perhaps a modem (so that you can issue AT
commands directly to the modem).
If you telnet to any of router aaa's ip addresses on these async port
numbers (from anywhere in your network, provided you have routes) you
can connect to the end devices directly from your windows box or any
other telnet capable device. You don't have to do it from router aaa.
---The "ip routing" command is normally enabled by default so unless you have disabled IP routing with the "no ip routing" command then you wouldn't normally have to use it. You would use it to re-enable IP routing on the router. It's probably just shown here for completeness.
Regards, Glenn.
Tan Chai Heng wrote:
>Hi, > >I am trying to understand of the use of loopback interface in the config >below. There's an ip to name mapping using the loopback interface. How can >this work (well it does I just need to understand why and how!) :-) Also, >when is it necessary to use "ip routing"? > >Thanks in advance! > >aaa---------bbb > | |_____ ccc > |_______ddd >bbb,ccc and ddd is connected to aaa async-ly > >hostname aaa >service password-encryption >enable password sitamoht >username michel password sitamoht >ip host bbb 2002 1.1.1.1 >ip host ccc 2003 2.2.2.2 >ip host ddd 2004 3.3.3.3 >ip routing >ip subnet-zero >interface ethernet 0 >ip address 172.16.80.1 255.255.240.0 >no shutdown >interface loopback 0 >ip address 1.1.1.1 255.255.255.255 >no shutdown >interface loopback 1 >ip address 2.2.2.2 255.255.255.255 >no shutdown >interface loopback 2 >ip address 3.3.3.3 255.255.255.255 >no shutdown >line 2 4 >transport input all >flowcontrol hardware >speed 9600 >login local >databits 8 >modem inout >line con 0 >login local >line aux 0 >line vty 0 4 >login local > > >Tan Chai Heng >Schlumberger NIS > >***Get your CCIE and a FREE vacation: Shop.GroupStudy.com*** >_______________________________________________________________________ >Please help support GroupStudy by purchasing your study materials from: >shop.groupstudy.com > >Subscription information may be found at: >http://www.groupstudy.com/list/CCIELab.html
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