IRDP - ICMP Router Discovery Protocol

IRDP enables Routers to automatically discover the IP of their potential Default Gateway. It uses ICMP and Solicitation Messages.

Potential GW Routers periodically announce the IP address of their IRDP configured interface to a roadcast destination. IRDP Preference value is advertised with these messages, along with the IP Address.

Step 1:
The configuration is pretty straight-forward. First you MUST turn the Routing off on the router that you want to discover it's own GW:
(config)#no ip routing

Step 2:
IRDP Needs to be enabled on the Router:
(config)#ip gdp ?
  eigrp  Discover routers transmitting EIGRP router updates
  irdp   Discover routers transmitting IRDP router updates <- THIS ONE is the one we want here
  rip    Discover routers transmitting RIP router updates

Step 3:
Here is what needs to be defined on the interface:
 (config-if)#ip irdp <- ENABLE IRDP ON THE INTERFACE
 (config-if)#ip irdp maxadvertinterval 5 <- DEFINE THE ADVERTISING TIMERS
 (config-if)#ip irdp minadvertinterval 3
 (config-if)#ip irdp holdtime 15
 (config-if)#ip irdp preference 600 <- DEFINE THE ROUTER PREFERENCE

Step 4:
TEST by pinging the IP behind the routers that are supposedly advertising the GW
PING will work ONLY if Proxy-ARP is enabled on the IP Interface
 #sh ip inter fa0/0 | i ARP
  Proxy ARP is enabled <- THIS ONE MATTERS
  Local Proxy ARP is disabled
 #show ip route
 Gateway         Using  Interval  Priority   Interface
 10.187.117.2      IRDP        4       600     FastEthernet0/0
 10.187.117.1      IRDP        4       200     FastEthernet0/0

When you do a DEBUG of ICMP, you see that IRDP is using the ICMP Type 9 Code 0 messages to advertise the GW:
ES-MAT-AES-SR03#debug ip icmp
ICMP packet debugging is on
*Nov 14 16:03:08.288: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.2
*Nov 14 16:03:09.340: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.1
*Nov 14 16:03:12.288: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.2
*Nov 14 16:03:12.340: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.1
*Nov 14 16:03:16.288: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.2
*Nov 14 16:03:16.340: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.1
*Nov 14 16:03:19.340: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.1
*Nov 14 16:03:20.288: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.2
*Nov 14 16:03:23.288: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.2
*Nov 14 16:03:23.340: ICMP: rdp advert rcvd type 9, code 0, from 10.187.117.1