The Cisco HDLC extension also includes the definition of Serial Line Address Resolution Protocol (SLARP). SLARP provides dynamic address assignment between 2 endpoints across a serial link, as well as a keepalive mechanism for ensuring the availability of a serial link.
The keepalive mechanism works by carrying the sequence numbers in the SLARP frames. The sequence numbers are incremented for each keepalive sent. The health of the serial link can be determined by each endpoint by keeping track of the sequence. The serial link goes down (line protocol down) after a certain number of sequence numbers is missed (keepalive lost).
Below shows how SLARP dynamically assign an IP address for a router interface resides on a /30 point-to-point serial link:
RT2#sh run int s0/0 Building configuration... Current configuration : 52 bytes ! interface Serial0/0 no ip address shutdown end RT2# RT2#conf t Enter configuration commands, one per line. End with CNTL/Z. RT2(config)#int s0/0 RT2(config-if)#ip address ? A.B.C.D IP address negotiated IP Address negotiated over PPP pool IP Address autoconfigured from a local DHCP pool slarp Set IP address through SLARP RT2(config-if)#ip address slarp ? retry Define retry time interval RT2(config-if)#ip address slarp retry ? <1-60> Retry timer interval, from 1 to 60 seconds RT2(config-if)#ip address slarp retry 10 RT2(config-if)#no shut RT2(config-if)#^Z RT2# 00:22:47.799: %SYS-5-CONFIG_I: Configured from console by console 00:22:49.363: %LINK-3-UPDOWN: Interface Serial0/0, changed state to up 00:22:50.367: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0, changed state to up 00:23:05.099: %LINK-5-SLARP: Serial0/0 address 10.10.10.2, resolved by 10.10.10.1 RT2# RT2#sh ip int brief Interface IP-Address OK? Method Status Protocol Serial0/0 10.10.10.2 YES SLARP up up Serial0/1 unassigned YES unset administratively down down Serial0/2 unassigned YES unset administratively down down Serial0/3 unassigned YES unset administratively down down FastEthernet1/0 unassigned YES unset administratively down down RT2# RT2#ping 10.10.10.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 16/31/44 ms RT2# RT2#sh cdp neighbors detail | in Device ID|IP address|Interface ------------------------- Device ID: RT1 IP address: 10.10.10.1 Interface: Serial0/0, Port ID (outgoing port): Serial0/0 RT2# RT2#sh run int s0/0 Building configuration... Current configuration : 93 bytes ! interface Serial0/0 ip address 10.10.10.2 255.255.255.252 ip address slarp retry 10 end RT2# RT2#sh ip int s0/0 Serial0/0 is up, line protocol is up Internet address is 10.10.10.2/30 Broadcast address is 255.255.255.255 Address determined by Reverse ARP from host 10.10.10.1 --- output omitted ---