How RTP Works with Conditional Receive Flag.

[Ravi_916]

Could you please explain How RTP actually works in EIGRP? and when Conditional Receive flag Set, and Why?

[Georg Pauwen]

Hello,

as I understand it, the Conditional Receive flag is set and used by RTP when a message is supposed to be received only by a subset of receivers, that is, when interfaces are set as 'passive-interface', they would not get the message.

As to how RTP works, there are numerous good explanations to be found, have a look at the link below:

https://packetlife.net/blog/2009/jan/17/rtp-eigrp/

[Giuseppe Larosa · Accepted solution]

Hello @Ravi_916 ,

EIGRP loop free is based on the assumption that all the EIGPR neighbors are reiceving and acknowledeged each update in an ordered manner.

Actually RTP uses multicast to send the updates. if one or more EIGRP neighbors on a LAN interface do not acknowledge the update the RTP is in charge of resending the update with the purpose of receiving the ack from the affected neighbors only.

This is the scenario where the conditonal receive flag is set to indicate that the update is not meant for all neighbors on the LAN but only for those that have not processed the previous update packet.

By doing so RTP allows even to go on sending updates with no flag for updated neighbors and to try to recover those neighbors with issues.

The specific details of when RTP switches to send unicast updates to a slow neighbor are not explained in public documents. For each neighbor a parameter flow timer is calculated based on a sort of average of the delays on their acks to updates sent by local node.

This value might be used as a trigger to switch to unicast transmission for a specific neighbor that is really too slow in answering back.

Hope to help

Giuseppe

[Georg Pauwen]

Hello Guiseppe,

would passive interfaces 'trigger' the conditional receive flag ? I kind of assumed that setting an interface as passive would exclude that interface from the subset that receives the update...

[Giuseppe Larosa]

Hello Georg,

over a passive interface no EIGRP hellos can be sent or received so no EIGRP neighborships can be formed.

At this point a device with its own interface in passive interface does not process any EIGRP packet on the LAN segment so I would say that it should not trigger the conditional receive flag in RTP because RTP is used between established EIGRP neighbors. And for each neighbor a Q count is kept to list how many updates it has acknwoledged.

What happens if you change an interface that was active in EIGRP with setting passive-interface suddenly I don't know it should tear down all the EIGRP neighorships this might be done by simply stopping to send EIGRP hellos or in a softer way by sending a sort of goodbye message like in graceful restart scenarios.

Hope to help

Giuseppe