Prerequisite reading:
»Cisco Forum FAQ »Things to expect when setup network for home or small business
When you decide to have Broadband Internet access using xDSL (i.e. ADSL, SDSL) and Cable Internet, you most likely will deal with following aspects
* To use either a router or a firewall as the Internet gateway
* Layer-1: using either internal or external modem; Category 5/6 cable extension
* Layer-2: PPP (PPPoA, PPPoE); MAC address for DHCP
* Layer-3: auto-negotiate or static WAN IP address
Following are some details.
PPP
When you are using xDSL, ISDN, or T1/E1 circuits, you probably will be dealing with PPP technology. In a nutshell, PPP is Layer-2 technology providing connectivity to remote user (PPP client) to server (PPP server) using specific username and password. In this case, the PPP client is your Internet gateway (either router or firewall) and the PPP server is the ISP.
Typically you need a router as the PPP client. Specifically with PPPoE, you could use a firewall. However for PPPoA or legacy PPP, you need a router.
DHCP
When you are using either xDSL or Cable Internet, you probably will be dealing with DHCP technology. In a nutshell, DHCP is a mechanism that providing IP address and subnet mask dynamically to specific machine that needs one. In this case, the machine is your Internet gateway (either router or firewall) which will be the DHCP client and the DHCP server is at the ISP network.
Typically you could use either router or firewall as DHCP client. Unlike PPP that uses username and password to connect, DHCP process might require certain MAC address to connect to the ISP.
Following FAQ is some info on PPP, DHCP, and Static IP address assignment
»Cisco Forum FAQ »Between DHCP, PPP, Dynamic, and Static IP Address
Between Internal Modem and External Modem Usage
When you use an external modem, your Internet gateway might receive Ethernet hand-off. This is applicable when you use a firewall or a router without integrated modem. From practical perspective, you then only need to configure the Layer-2/Layer-3 aspect on the Internet gateway. For PPP, in general you only need to configure the username, password, and authentication method. For DHCP, in general you only need to verify that your Internet gateway MAC address is on the ISP database.
There are some things you need to confirm whether you use external or internal (integrated) modem. Some examples are your ISP DSL signaling type, bridge mode configuration, and VPI/VCI value settings when you use xDSL service. Fortunately, you may not need to worry about this when you use the "ISP-approved" external modem since those settings are pre-configured. Note that the keyword is "may".
When you use a router with integrated DSL modem for xDSL service, your integrated modem/router may not be the "ISP-approved" xDSL equipment. Note that even though the router is not "ISP-approved", doesn't necessarily mean that the router won't work.
In any case (either using integrated modem or external modem; "ISP-approved" or "ISP-non-approved"), you need to verify the Layer-1/Layer-2/Layer-3 aspects. As illustration, you need verify things like DSL signaling and ATM VPI/VCI value in addition of the username, password, and authentication method.
One good thing about using integrated modem within a router is that you can see Layer-1/Layer-2/Layer-3 aspects on one device which is the router itself. When you use an external modem, then you need to confirm two device configurations which are the external modem and the router.
Some Troubleshooting Insights
Following are some discussions of troubleshooting Layer-1/Layer-2/Layer-3 issues
»[HELP] How to read dsl interface
»What do these sh dsl int atm0/0 - atm0/1 mean ??
»[HELP] Cisco 1721 and WIC-1ADSL Slow, 320Kbit
»Frequent disconnects with 1801
»[HELP] Cisco 857W and Qwest
»[Config] Fun with Cisco 1720 WIC-1ADSL, WIC-1ENET and Cisco PIX
For more info on Layer-1 xDSL troubleshooting, you can always visit the DSL forum FAQ such as this
»SBC DSL FAQ
For more info on Layer-1 Cable Internet troubleshooting, you can always visit the Cable Internet forum FAQ such as this
Cable Modem General Info
Cable Modem Troubleshooting
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by aryoba 
last modified: 2008-06-14 05:22:00 |
