Cisco T1 Internet Config


I am pretty new to cisco router config, but I have looked at various router configuration from our office existing router. I am just trying to understand the config done by our previous employee in our remote office, so please help me out. This is a T1 internet line by MCI and use unnumbered in their serial interface, here is what I don't understand as most of the line in our main office have ip on their serial interface.

The line under Serial0/0 encapsulation frame-relay IETF -- Do I need this line? This is not a frame relay but an internet line only.

Also, the interface Serial0/0.1 part interface Serial0/0.1 point-to-point -- Do I need this 4 lines at all? bandwidth 1536 ip unnumbered FastEthernet0/0 frame-relay interface-dlci 500 IETF

I just need a bare minimum for internet access only, and here is the complete config (removed lines with password and line) Please help, thank you so much.

C2620-R2-R23#sh running-config Building configuration...

Current configuration : 1528 bytes ! ! No configuration change since last restart ! version 12.3 service timestamps debug datetime localtime service timestamps log datetime localtime service password-encryption ! hostname C2620-R2-R23 ! boot-start-marker boot-end-marker ! logging buffered 8196 debugging ! memory-size iomem 15 clock timezone PST -8 clock summer-time PDT recurring no aaa new-model ip subnet-zero ip cef ! ip audit po max-events 100 ! interface FastEthernet0/0 description To Office Ethernet R23 ip address duplex auto speed auto ! interface Serial0/0 bandwidth 1536 no ip address encapsulation frame-relay IETF no fair-queue service-module t1 timeslots 1-24 frame-relay lmi-type ansi ! interface Serial0/0.1 point-to-point bandwidth 1536 ip unnumbered FastEthernet0/0 frame-relay interface-dlci 500 IETF ! no ip http server no ip http secure-server ip classless ip route Serial0/0.1 ! ntp clock-period 17180386 ntp server prefer ! end

Reply to
Alan Siu
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That is pretty close to bare minimum there. First, don't confuse your layer

2 protocol with the services that are provided over the line. Meaning, just because this is internet, doesn't mean that the connection is not frame relay. If the line is functioning properly with this config, then it is certainly frame relay. That's like saying you don't have DSL/Cable, you have internet. Well, DSL/Cable just gets you to the service provider. In this case, frame relay is just getting you to the service provider.

I personally would not have used a sub interface in this case as you only have one pvc and frame relay would most likely have just auto discovered it through lmi - no other config for this would have been needed. Except the ip unnumbered command would have moved to the serial interface. But, it is not a bad config as is.

So, if it ain't broke....

But, you may want to consider hardening this a little bit. Check out the SDM utility - free download from Cisco. Great place to get started on this stuff.


Reply to

MCI uses frame-relay layer-2 encapsulation for most of their delivery for their own management policies. Doesn't mean its a frame-relay network. You do need this line, as you have to match what MCI is doing. If MCI requires frame-relay layer-2 encapsulation, you have to match what they want.

Its best to put frame-relay entries in a sub-interface, and you do need the DLCI definition in order to talk to MCI across the link.

Technically, you aren't doing anything that references the bandwidth line, but why remove it? You may do something in the future that does.

You appear to be running unnumbered across the serial link, and thats what 'ip unnumbered' does. You do need it, its required for your setup again with MCI.

This config is pretty bare-bones basic. There isn't much you can remove and still have it function to match what MCI is doing.

Why are you considering removing anything?

There isn't anything you could make "simpler" about this config.

Reply to
Doug McIntyre

Yes correct, that's MCI line. Does it mean the config may vary between different ISP? When I look at the config in the remote office compared to the one I have in my local office, it just look a lot more complicated than the config on my T1 router. Compared to the T1 I have with Verizon with an IP in the serial interface, it looks a lot simpler.

interface Serial0

ip address

no ip redirects

no fair-queue


interface FastEthernet0

ip address

no ip redirects

speed auto



ip classless

ip route

no ip http server

Last question, does the config varies between 1700 or 2600 series router? Why there is a sub-interface Serial 0/0 and 0/0.1 in 2600 series but in 1700 series, you only have interface Serial 0 or 1?

Thanks for all the people who answered.

Reply to
Alan Siu


Verizon are using HDLC encapsulation, which is the default on Cisco routers, what you get if you don't do anything else, so it does look simpler. I guess HDLC is also a simpler protocol then Frame Relay.

Yes and No, more no than yes. The same technology, a Frame Relay interface for instance, is the same on both routers.

Interface numbering differs across platforms, depending on the number and type of available slots on the router. Subinterfaces are used with frame-relay encapsulation but not with HDLC.

Reply to
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