Considering the lowest three layers of the OSI reference model on router Ethernet interfaces for a moment, there are no required configuration commands related to Layers 1 and 2 for the interface to be up and working, forwarding IP traffic. The Layer 1 details occur by default once the cabling has been installed correctly. Router IOS defaults to use Ethernet as the data link protocol on all types of Ethernet interfaces, so no Layer 2 commands are required. To make the interface operational for forwarding IP packets, the router needs one command to configure an IP address on the interface, and possibly a no shutdown command if the interface is in an “administratively down” state.
Similarly, serial interfaces on Cisco routers that use HDLC typically need no specific Layer 1 or 2 configuration commands. The cabling needs to be completed as described in Chapters 4 and 16, but there are no required configuration commands related to Layer 1.
IOS defaults to use HDLC as the data link protocol, so there are no required commands that relate to Layer 2. As on Ethernet interfaces, the only required command to get IP working on the interface is the ip address command and possibly the no shutdown command.
Similarly, serial interfaces on Cisco routers that use HDLC typically need no specific Layer 1 or 2 configuration commands. The cabling needs to be completed as described in Chapters 4 and 16, but there are no required configuration commands related to Layer 1.
IOS defaults to use HDLC as the data link protocol, so there are no required commands that relate to Layer 2. As on Ethernet interfaces, the only required command to get IP working on the interface is the ip address command and possibly the no shutdown command.
Step 1 Configure the interface IP address using the ip address interface subcommand.
Step 2 The following tasks are required only when the specifically listed conditions are true:
a. If an encapsulation protocol interface subcommand that lists a protocol besides HDLC already exists on the interface, use the encapsulation hdlc interface subcommand to enable HDLC.
b. If the interface line status is administratively down, enable the interface using the no shutdown interface subcommand.
c. If the serial link is a back-to-back serial link in a lab (or a simulator), configure
the clocking rate using the clock rate speed interface subcommand, but only on the one router with the DCE cable (per the show controllers serial number command).
Step 3 The following steps are always optional, and have no impact on whether the link works and passes IP traffic:
a. Configure the link’s speed using the bandwidth speed-in-kbps interface subcommand.
b. For documentation purposes, configure a description of the purpose of the interface using the description text interface subcommand.
Step 2 The following tasks are required only when the specifically listed conditions are true:
a. If an encapsulation protocol interface subcommand that lists a protocol besides HDLC already exists on the interface, use the encapsulation hdlc interface subcommand to enable HDLC.
b. If the interface line status is administratively down, enable the interface using the no shutdown interface subcommand.
c. If the serial link is a back-to-back serial link in a lab (or a simulator), configure
the clocking rate using the clock rate speed interface subcommand, but only on the one router with the DCE cable (per the show controllers serial number command).
Step 3 The following steps are always optional, and have no impact on whether the link works and passes IP traffic:
a. Configure the link’s speed using the bandwidth speed-in-kbps interface subcommand.
b. For documentation purposes, configure a description of the purpose of the interface using the description text interface subcommand.