Learning by Doing: CISCO Certified Network ... - SCN Research

5. Setup the workstations with IP address, subnet masks, and gateways addresses.
You will need to reboot the workstations. If they ask for a password for network
connectivity just put anything in and you should see a message something like “no
domain server is available, you may not have some networking functions.” It’s ok
if you see it, but you probably will not be able to ping outside of your workstation
without seeing that error message.
6. Test connectivity from workstation A to workstation C.
7. Turn on debug ip rip on each router.
8. Now let’s add in that other serial line between Prophet and Lightning. This will
create a routing loop in our network. By default CISCO routers are prepared for
routing loops. To create a problem with a routing loop use this command:
prophet(config)#interface s0/0
prophet(config-if)#no ip split-horizon
9. You should see lots of debug messages about routing loops now. To stop those
routing loop problems type “ip split-horizon” again on the serial interface or just
disconnect that serial line. This problem is known as “counting to infinity.”
Supplemental Lab or Challenge Activity:
1. You can also solve the problem of routing loops by changing the metrics for the
routing protocol. Having just completed the lab on RIP metrics, try this lab again
changing the metrics for RIP from 16 hops to 3 and see what happens.
2. Define and differentiate between split-horizon, poison reverse update and count to
infinity. More than likely you will see a question about these on your test.
So What Have I Learned Here?
In this lab you learned that problems with routing loops are automatically taken care of
by the ip split-horizon command in your router. Why did we bother learning about it?
Well no network is pure and chances are you will have routers from other vendors in your
network. No all of them automatically eliminate routing loop problems so you need to be
aware of them.
Guest Router Name Derivation
In September 1998 a hacker known as “Prophet” (Robert Riggs) cracked the BellSouth
network and downloaded copies of operating manuals to his own computer and copied
them to a BBS. He also sent them to another hacker “Knight Lightning” (Craig Neidorf)
who published the information in his underground electronic magazine “Phrack.”
Prophet pled guilty to wire fraud. Knight Lightning fought his case because he had only
taken a copy of the document and “didn’t hurt anything.” It turned out the document was
also available for sale from Bellsouth, but Knight Lightning was still left with a six-figure
legal bill for a document he could have purchased legally for $13.00 and Prophet has a
criminal record.
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