Serial Line Address Resolution Protocol (SLARP)
by David Sudjiman ~ January 7th, 2007. Filed under: Cisco.When I was doing my lab, with two routers connected using serial DTE-DTC DTE-DCE. I configured Router1 with this configuration.
r1#sh run int s0 Building configuration... Current configuration : 99 bytes ! interface Serial0 ip address 131.108.3.1 255.255.255.252 no fair-queue clockrate 4000000 end
The next thing I want to do is to go to Router2 to make another configuration. Suddenly, my eyes captured these message.
Press RETURN to get started! 00:00:10: %LINK-3-UPDOWN: Interface Ethernet0, changed state to up 00:00:10: %LINK-3-UPDOWN: Interface Serial0, changed state to down 00:00:10: %LINK-3-UPDOWN: Interface Serial1, changed state to down 00:00:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up 00:00:29: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up 00:00:45: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down 00:03:39: %LINK-5-SLARP: Serial0 address 131.108.3.2, resolved by 131.108.3.1 00:04:09: %LINK-5-CHANGED: Interface BRI0, changed state to administratively down 00:04:10: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0, changed state to down 00:04:11: %LINK-5-CHANGED: Interface Ethernet0, changed state to administratively down 00:04:11: %LINK-5-CHANGED: Interface Serial1, changed state to administratively down 00:04:12: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to down 00:04:12: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down 00:04:13: %IP-5-WEBINST_KILL: Terminating DNS process 00:04:17: %LINK-3-UPDOWN: Interface Serial0, changed state to up 00:04:20: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to up 00:05:00: %SYS-5-RESTART: System restarted –
My Router2 Serial 0 got its own IP? I thought I just reloaded Router2 to clean up? Next thing I did.
Router>ena Router#sh ip int brie Interface IP-Address OK? Method Status Protocol BRI0 unassigned YES unset administratively down down BRI0:1 unassigned YES unset administratively down down BRI0:2 unassigned YES unset administratively down down Ethernet0 unassigned YES unset administratively down down Serial0 131.108.3.2 YES SLARP up up Serial1 unassigned YES unset administratively down down Router#sh run int s0 Building configuration… Current configuration : 80 bytes ! interface Serial0 ip address 131.108.3.2 255.255.255.252 no fair-queue end Router#
Ahh, that’s SLARP. 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.
The router will also send out Reverse Address Resolution Protocol (RARP) and 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.
BTW, It’s Serial and not Ethernet. I you try to test it using your Ethernet. It won’t work.
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