SLARP (Serial Line Address Resolution Protocol)

SLARP stands for Serial Line Address Resolution Protocol. 

Basically when the router reloaded, it is then sending DHCP discover packets looking for DHCP server. If no DHCP server available, it will look for BOOTP server. The next thing if it can’t find BOOTP server, it will send RARP and SLARP, which is  used in serial lines for assigning an IP address to a interface.

Serial Line Address Resolution Protocol (SLARP) requests. AutoInstall will use the first available method (DHCP, BOOTP, RARP, or SLARP) for configuration. If all LAN interface configuration options fail, AutoInstall will attempt to configure an available serial interface using SLARP. Though DHCP is the preferred method for AutoInstall over LAN interfaces, these other options remain enabled to ensure backward compatibility with older network topologies.

SLARP is an extension of Cisco HDLC, where if the remote router of the serial link does not yet have a config saved in NVRAM, the router will SLARP on the connected serial link to obtain a valid IP address.  This feature benefits the Cisco autoinstall feature where an admin can connect a fresh router to the network and have limited connectivity with little effort.

The following table describes the make up of a cHDLC frame on the wire.

Address	Control	Protocol Code	Information	Frame Check Sequence (FCS)	Flag
8 bits	8 bits	16 bits	Variable length, 0 or more bits, in multiples of 8	16 bits	8 bits

• The Address field is used to specify the type of packet contained in the cHDLC frame; 0x0F for Unicast and 0x8F for Broadcast packets.
• The Control field is always set to zero (0x00).
• The Protocol Code field is used to specify the protocol type encapsulated within the cHDLC frame (e.g. 0x0800 for Internet Protocol).

SLARP stands for: Serial Line Address Resolution Protocol

SLARP address request–response frame structure

A SLARP frame is designated by a specific cHDLC Protocol Code field value of 0x8035.

Currently only three types of SLARP frame are defined: address requests (0x00), address replies (0x01), and keep-alive frames (0x02).

The following table shows the makeup of a SLARP cHDLC address request–response frame.

Address	Control	Protocol Code	SLARP Op-Code	Address	Mask	Reserved	Frame Check Sequence (FCS)	Flag
8 bits	8 bits	16 bits (0x8035)	32 bits	32 bits	32 bits	16 bits	16 bits	8 bits

• The op-code will be 0x00 for address requests and 0x01 for address responses.
• The Address and Mask fields are used to contain a four-octet IP address and mask. These are 0 for address requests.
• The two-byte Reserved field is currently unused and undefined.

SLARP Keep-Alive frame structure

The following table shows the makeup of a SLARP cHDLC keep-alive frame.

Address	Control	Protocol Code	SLARP Op-Code	Sequence Number (Sender)	Sequence Number (Last Received)	Reliability	Frame Check Sequence (FCS)	Flag
8 bits	8 bits	16 bits (0x8035)	32 bits	32 bits	32 bits	16 bits	16 bits	8 bits

• The op-code is 0x02 for keep-alives.
• The sender sequence number increments with each keep-alive sent by this sender.
• The received sequence number is the last sequence number received by this sender.
• The two-byte Reliability field is required to be set to 0xFFFF.