Q&A: Router Troubleshooting
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Hey, Scott,
I'm a student in Cisco's CCNA program out of Mt. Hood
Community College. My study partner and I have a few Cisco routers -- 1605R, 2611, and 2501 -- and Catalyst 1900
switches. We're trying to connect one of the routers to an external modem to get onto the Internet. We're able to ping the Default gateway on Verizon's DSL, but we can't ping anything else. What are we doing wrong? What kind of
information do we need to get and where would we look for
it? Any help would be appreciated.
-- Boggled in Gresham, Oregon
Dear Boggled,
Well, troubleshooting is a much larger process than looking at the "small picture"! You need to look at many pieces beyond what's listed here.
Let's go through what you have already. One of your routers is connected to a DSL modem and attempting to reach the Internet.
Router -------- DSL Modem -------- DSLAM/Verizon Gateway
First, your DSL modem itself likely doesn't have an IP
address, but merely creates the layer 2 "circuit" to move
information.
Much of the necessary information will come from Verizon, and that may affect what you are attempting to do. How did you place an IP on the router? Was it given to you? Did you use DHCP (ip address negotiated)?
The fact that you can ping the Verizon Gateway is a start,but is that whom you are supposed to ping? Many DSL providers in different parts of the country are utilizing PPPoE (Point to Point Protocol over Ethernet) in order to finalize the connections. It may be that this is required in order to form a link with the SSG (Service Selection Gateway) on Verizon's network.
If your 2611 router is being used for the external router,
you may have the PPPoE capability (depending on IOS
version).
Vpdn enable
Vpdn group (#)
Request dialin
Protocol pppoe
Interface Ethernet 1/0 (or whatever interface)
Pppoe enable
Pppoe-client dial-pool-number 1
Interface dialer 1
Ip address negotiated
Ip mtu 1492
Ppp authentication (protocol)
Dialer pool 1
Dialer-group 1
Dialer-list 1 protocol ip permit
That gives you an example of the configuration necessary
for PPPoE. Check with Verizon though to find out specifics
of what is needed on their network. It may simply be that things are not as straightforward as you would like them to be!
The vpdn configuration (virtual private dial network) specifies you're requesting dial-in using PPPoE. The configuration for your physical interface, Ethernet 1/0, is specifying to use protocol PPPoE and is a member of dialer pool 1.
The virtual dialer interface will be triggered through any
IP traffic (dialer-list is referenced with the dialer-group command). It specifies learning an address and using
whichever ppp authentication protocol you choose(typically
chap).
Note the MTU size change! Typically, Ethernet is 1,500
bytes, but we're specifying 1492 as an MTU within the
dialer. That's because the PPP header is eight bytes long, and we don't want to mess anything else up!
For more information on PPPoE on Cisco devices, check out
http://www.cisco.com/univercd/cc/td/doc/product/software/io
s120/120newft/120limit/120dc/120dc3/pppoe.htm or the TAC
Technology pages.
This brings up my wonderful missive about network design
and troubleshooting! DOCUMENTATION! Without it,
troubleshooting is a pain in the rear! Do it, do it often
and do it well! Document your network!
Hope that helps.
--Scott
CCIE Scott Morris, independent uber-geek, does Cisco training and
technology consulting around the world. He's also a speaker at many industry events. Learn more about him at
http://smorris.uber-geek.net.
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