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Webcast-Catalyst9k
New Member

Serial Int config

Hi All,

A quick question on serial interfaces. Is it really necessary to have sub interfaces on point-to-point connections? I understand the need when running ATM or frame or something of the sort, but for a standard ppp config with one ip? why would you need to confuse things any farther? i keep seeing configs with sub interfaces and just wonder if i am doing something wrong or what.

Thanks

7 REPLIES
New Member

Re: Serial Int config

Hi,

Perhaps someone else may correct me, but to the best of my knowledge you can't configure subinterfaces on a ppp link. Can you post the config?

I agree with you, I am all for keeping things simple..-)

Rgds

E.

New Member

Re: Serial Int config

The following is from an ATT router config guide:

interface serial x/y:z

ip address

encapsulation ppp

no peer neighbor-route

no cdp enable

framing c-bit

dsu mode 0

dsu bandwidth

Now by no means am I saying that the guide is correct, but I just wanted to make sure as this is supposed to help me, not confuse things! ;)

Cisco Employee

Re: Serial Int config

Hi,

Sure, there's nothing wrong with configuring a PPP serial connection with an IP.

example:

interface Serial0

ip address 10.1.1.1 255.255.255.0

encapsulation ppp

HTH,

Bobby

New Member

Re: Serial Int config

So I suppose its really up to the person doing the configuring? lol

I just wanted to make sure I was not going against some age old cisco "best practice". the way i see it, i just want to keep things as simple as possible!

thanks

New Member

Re: Serial Int config

Hello,

For a basic "non-channelized" point-to-point ckt connection you do not need to have subinterfaces. If you have say a T1 controller or external CSU/DSU you can add an IP to the repsective Serial interface an add whatever encapsulation you like.

However, if you have a circuit that has channelized connections off of it you will need the subinterfaces (e.g you have a T3 that is channellized into 28 T1s (all point-to-point) you would need to have that many sub-interfaces).

HDLC is default encap type for Serial interfaces. If the enccap matches both ends you should be okay. You don't need a subinterface for PPP to work either.

Hope this helps.

Regards,

James

New Member

Re: Serial Int config

Hi James,

I thought those were classed as logical interfaces and not subinterfaces.

Rgds

E.

Purple

Re: Serial Int config

Hi,

I think your point of confusion with the config you supplied is that you are thinking that the 'z' refers to a sub-interface number. That is not the case. The 'z' refers to a channel-group neighbor.

interface serial x/y:z

ip address

encapsulation ppp ...

When you have interfaces such as a T1 or an E1, you can configure multiple channels on them, as follows:

controller e1 0/1

channel-group 1 timeslots 1-10

channel-group 2 timeslots 11-20

Each of the channel-groups will result in the creation of a serial interface. For example, for the above case, the serial interfaces would be:

serial0/1:1 ! for channel-group 1

serial0/1:2 ! for channel-group 2

Each of the above is a physical (main) interface, not a sub-interface.

Note that when using T1 cards with builtin CSU/DSUs you don't get the ability to configure the T1 controller - all the config appears under the serial interface.

Hope that helps - pls rate the post if it does.

Paresh

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