Kevin Dorrell, CCIE #20765

26 Apr 2008

NMC Lab 17.9.2 – IRDP

Filed under: IRDP — dorreke @ 14:44

Lab 17 involves IRDP.  IDRP is a protocol that allows a host (R7 in this case) to discover the gateway address.  It is a bit of a corner-case, but since it is in this lab, I had better take the opportunity to get to know it.

Where is the docuemtnation for IRDP?  This is not the first time I have been unable to find the Conf Guide for First-Hop routing protocols.  It is, in fact, in the IP Application Services Guide.  Who would have guessed?  But once you are there, (12.4) you find GLBP, HSRP, and VRRP, but no IRDP.  They seem to have forgotten to include it.  It is, in fact, in the 12.4T IP Application Services Guide.  Huh?

Even once you have found the documentation, it is not very good.  It does not tell you how to set up a router-host to get its gateway address from IDRP.  For reference, the command is ip gdp idrp.  GDP stands for “Gateway Discovery Protocol”.  This command also allows the router-host to discover the gateway address from RIP.  I cannot find this command anywhere in the IOS version 12 command references, and only very few glancing references to it in the IOS version 11 command references.

The IRDP is enabled on the routers (R3 and R6) with the interface comnmand ip irdp.  At the most basic level, that is all there is to it.

This particular scenario wants us to prefer R  over R3, and this can be achieved simply with ip idrp preference.  (The debug refers to “preference” as “priority”.)  Also, apparently there has been some confusion in the past over whether high wins or low wins, but the concensus seems to be that high wins.

Note that this priority mechanism is not pre-emptive.  At least, not with the 12.2(15)T17 I am using in my 2520 R7 client.

There is then a whole load of other options to do with the timing an format of the advertisments.  These parameters are each configured as a seperate command line, but behave in a strange way.  If you do no ip idrp, then they all disappear from the config.  If you then re-instate IRDP with ip irdp, then these former parameters automagically reappear.

One parameter deserves a mention: ip irdp multicast.  This sends out the IRDP hellos on a special multicast address so that Sun workstations can understand them.  However, it also means that a router-host such as R7 cannot understand them.


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