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New Member

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

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.

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Cisco Employee

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

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

7 REPLIES
Hall of Fame Super Gold

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

In fact they don't - ip unnumbered.

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

New Member

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

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?

Purple

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

Hi,

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

Alain.

Don't forget to rate helpful posts.
Cisco Employee

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

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

New Member

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

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

Cisco Employee

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

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

New Member

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

Ok ,Got It! Thanks

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