Solved: Why do serial Interface using HDLC and PPP need an address ?

Reprovoid · ‎10-31-2011

Hi.I'm curious why the serial Interfaces need an address when there's only one place the data can go to ? Excuse my naivety.

Peter Paluch · ‎10-31-2011

Hello,

In order for an interface to be allowed to process IP packets (i.e. send and receive them), the IP protocol must be enabled on it. On Cisco devices, an IP protocol processing is allowed on an interface after you assign it an IP address. Alternatively, you could go with the IP Unnumbered but I would not complicate things using that feature for now.

So on Cisco devices, you actually can not enable IP on an interface without assigning an IP address to it. In addition, the interface can be a source of IP packets itself - consider, for example, routing protocols running over the serial link. Any IP-based interior routing protocol (OSPF, EIGRP, RIP) will try to send its packets with the source IP address set to the interface's address. If there is no IP address configured on the serial interface, the routing protocol can not work over it (not even mentioning the fact that if there is no IP address on the interface, no network command will apply to it).

For all these, and many more, reasons, yes, you have to configure an IP address even on a serial interface running PPP or HDLC.

Best regards,

Peter

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paolo bevilacqua · ‎10-31-2011

In fact they don't - ip unnumbered.

However, it's more complex than the way you put it.

Reprovoid · ‎10-31-2011

Hi.Thanks for replying.I'm sure It Is more complex.I'm new to this.

Why ,In a lab environment, must I give the two serial Interfaces an Ip address.For example 192.168.6.1 and 192.168.6.2 respectively?

cadet alain · ‎10-31-2011

Hi,

if you want to communicate at L3 you need an IP address.

Alain.

Don't forget to rate helpful posts.

Peter Paluch · ‎10-31-2011

Hello,

In order for an interface to be allowed to process IP packets (i.e. send and receive them), the IP protocol must be enabled on it. On Cisco devices, an IP protocol processing is allowed on an interface after you assign it an IP address. Alternatively, you could go with the IP Unnumbered but I would not complicate things using that feature for now.

So on Cisco devices, you actually can not enable IP on an interface without assigning an IP address to it. In addition, the interface can be a source of IP packets itself - consider, for example, routing protocols running over the serial link. Any IP-based interior routing protocol (OSPF, EIGRP, RIP) will try to send its packets with the source IP address set to the interface's address. If there is no IP address configured on the serial interface, the routing protocol can not work over it (not even mentioning the fact that if there is no IP address on the interface, no network command will apply to it).

For all these, and many more, reasons, yes, you have to configure an IP address even on a serial interface running PPP or HDLC.

Best regards,

Peter

Reprovoid · ‎10-31-2011

I guess I'm confused In that PPP and HDLC are layer 2 protocols from what I've read.

Peter Paluch · ‎10-31-2011

Hi,

Yes, PPP and HDLC indeed are Layer 2 protocols, in the same way as Ethernet framing is. In the same way you assign IP addresses to Ethernet interfaces, you need to assign IP addresses to HDLC/PPP interfaces as well. IP packets are carried inside Ethernet/PPP/HDLC frames.

Best regards,

Peter

Reprovoid · ‎10-31-2011

Ok ,Got It! Thanks