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General Questions - DS3 Router Config


Hello Everyone,

I have three general questions regarding our new T3 router configuration.

Question 1:

The I received the first bit of config info from the ISP:

interface Serial1/0

ip address

encapsulation ppp

load-interval 30

dsu bandwidth 44210


interface GigabitEthernet0/0

ip address

duplex full

speed 100

media-type rj45

I'm used to configuring T1 connections. To me this configuration looks odd. I'm used to the Serial interface having a public routable IP address that routes to the ISP. In this case it's a private IP address. Is this correct for DS3 connections?

Question 2:

The second bit of config info was this:

ip route Serial1/0

Is there a difference in doing a route via the interface versus using an IP address?

Question 3:

The third bit of config info was this:

controller T3 1/0

clock source line

cablelength 10

I'm not familiar with the cablelength command. How is this determined? Is this just a basic setting or is the ISP specifying this for our connection?

Thanks in advance.

5 Replies 5

Rick Morris
Frequent Contributor
Frequent Contributor

Question 1.

Yes typically if you are connecting to the ISP for public internet feed you will use a public IP, typically a /30

The gig interface would have applied your public routable network.

Question 2.

this command is saying your default route is using serial1/0 as the outbound interface. Essentually this is the same as ip route

Question 3.

yes this is just a basic config.

You will not even probably need to adjust anything in the cablelength section. However if you do:

Here is a section that describes it, not much detail but here it is just in case.


To configure the cable length for a channelized DS-3 (CDS3) Frame Relay port adapter, use the cablelength controller configuration command. To restore the default cable length, use the no form of this command.

cablelength value

no cablelength value

Syntax Description


Cable length of 0 to 450 feet.


224 feet

Command Modes

Controller configuration

Command History

Release Modification


New command

Usage Guidelines

Although you can specify a cable length from 0 to 450 feet, the hardware only recognizes two ranges: 0 to 224, and 225 to 450.

For example, if you enter 150 feet, the 0 to 224 range is used. If you later change the cable length to 200 feet, there is no change because 200 is within the 0 to 224 range. However, if you change the cable length to 250, the 225 to 450 range is used. The actual number you enter is stored in the configuration file.


The following example configures the cable length on controller t3 to 450 feet.

Switch(config)# controller t3 4/0/0

Switch(config-controller)# cablelength 450

Switch# show running-config

controller T3 4/0/0

clock source reference

cablelength 450

Giuseppe Larosa
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Hello Frankie,

1) it is not general of DS3 or specific of DS3 but the provider has sold you a public range that you use on the LAN interface.

None will be able to ping your serial interface but if everything works no problem.

This is permitted and nowdays with the shortage of public IPv4 addresses can be reasonable.


this config is appropriate out a serial interface, out a LAN interface I wouldn't use it because it would rely on proxy-arp enabled on gateway and would cause an high ARP activity ( one ARP request for each new destination instead of a single ARP request for the MAC of the gateway)


these commands are DS3 specific and provides information on how much to compensate for phase shift in recovering clock.

The command is appropriate if the length of bnc cables is in the order of 10 feet.

Hope to help



Thank you everyone for the responses.

The Serial interface is our NM-1T3/E3 card. I guess I was surprised and confused to see a IP address on a Serial interface. Normally the Serial interface links back to the ISP and has a routable IP which links up the connection.

I understand our F/E ports have our public IPs which is normal. The F/E ports will connect to our servers, firewall, etc.

But how does our router talk to the ISP over a IP address?

i guess you are connecting to your isp's mpls cloud, that's would explain why you are provided with private ip on the wan interface.

if they connect you directly to internet, you would get a public ip.

they route your public range through an internet gateway if you purchase this service.


To close out the the thread:

1. I simply requested a /30 IP be assigned to the WAN interface from the ISP. I replaced the with the IP they provided. This worked.

2. I did a little more digging and found that some suggested routing via an interface is not as efficient as routing through an IP. Once a /30 was assigned to the WAN interface, I was able to use that instead.

3. Thanks for the cablelength info. It gave a little more insight into the purpose of this setting.

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