Auto-RP works in sparse mode -- it shouldn't!

[borek]

Without "ip pim autorp listener" Auto-RP should not work in sparse mode as it cannot properly send the multicast packets it needs to operate. But in my lab it seems to work no matter what I do. Clearing mroute table, clearing RP mappings... nothing, still works.

How is that possible?

[Harold Ritter]

Borek,

This is normal behavior if all routers are directly connected to the RP. This is because the directly connected routers receive the announce and discovery messages regardless.

These messages though are not propagated beyond the directly connected routers without configuring the interfaces as sparse-dense-mode or "ip pim autorp listener" on these same routers.

Try having one router not directly connected to the RP and you will see that it will not automatically learn the address of the RP.

Hope this helps,

[borek]

The multicast source was not directly connected (in fact, there were two hops to RP). I tried to set-up another source-receiver pair and that did not work... so maybe there was some state retained in my network for some time?