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Running auto-RP with router interfaces in sparse mode

Sam Brynes

I'm running this lab in EVE-NG:




All RTR# devices have their interfaces configured with "ip pim sparse-mode", and all RTR# devices have "ip multicast-routing" enabled. Also, OSPF is enabled on all RTR# interfaces in area 0. RTR# devices are OSPF neighbors on their directly connected interfaces.


RTR3's loopback0 interface ( is configured to be the RP: RTR3 is also the configured mapping agent and an RP candidate:


interface Loopback0
ip address
ip pim sparse-mode

ip pim rp-address
ip pim send-rp-announce Loopback0 scope 16
ip pim send-rp-discovery Loopback0 scope 16



I did a "show ip pim rp mapping" on RTR1 and expected to see it empty (because all interfaces are configured to be in sparse mode). RTR2 is getting the RP discovery messages sent to from RTR3 since it's directly connected to RTR3, but RTR1 is some how still learning the RP to be from R2. I did a debug ip pim auto-rp on RTR1 to confirm this.



RTR1#sh ip pim rp mapping
PIM Group-to-RP Mappings

RP (?), v2v1
Info source: (?), elected via Auto-RP
Uptime: 00:17:49, expires: 00:01:58


*Feb 14 16:03:41.565: Auto-RP(0): Received RP-discovery packet of length 48, from, RP_cnt 1, ht 181
*Feb 14 16:03:41.565: (0): pim_add_prm::, rp=, repl = 1, ver =3, is_neg =0, bidir = 0, crp = 0
*Feb 14 16:03:41.565: Auto-RP(0): Update
*Feb 14 16:03:41.565: prm_rp->bidir_mode = 0 vs bidir = 0 (, RP:, PIMv2 v1


RTR2's "show ip mroute" shows that it is sending the auto-RP discovery messages over an SPT tree to RTR1 via its Ethernet0/1 interface. I'm not sure why that is, though.


RTR2#show ip mroute

<output omitted>

(,, 00:21:35/00:02:12, flags: LT
Incoming interface: Ethernet0/2, RPF nbr
Outgoing interface list:
Ethernet0/1, Forward/Sparse, 00:21:35/00:02:31


Meanwhile at the other end of the topology, RTR4 does not appear to be forwarding the RP-discovery messages it receives on Et0/3 over to RTR5 on its Et0/0 interface:


RTR4#show ip mroute

<output omitted>

(,, 00:49:08/00:02:25, flags: PLT
Incoming interface: Ethernet0/3, RPF nbr
Outgoing interface list: Null


RTR5 doesn't seem to know about the RP:


RTR5#sh ip pim rp mapping
PIM Group-to-RP Mappings


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