Lab Objective
Configure PIM Sparse Mode on two routers running HSRP for the same LAN segment, verify PIM's own Designated Router election aligns with the HSRP active router by default, and confirm multicast traffic is forwarded consistently through the active router rather than being duplicated or dropped due to disagreement between the two redundancy mechanisms.
Lab Purpose
HSRP, covered extensively earlier in this series, and PIM each run their own independent election process on a shared segment — HSRP elects an active router for unicast default gateway purposes, while PIM independently elects a Designated Router (DR) for multicast forwarding. Without deliberate alignment, these two elections could disagree, causing multicast traffic to take an unexpected path relative to unicast traffic on the same LAN.
Lab Topology
R1 (HSRP active, higher priority) ---- Gi0/1 ---- Switch1
R2 (HSRP standby) ---- Gi0/1 ---- Switch1
Virtual IP: 192.168.190.1/24
Receiver connected via Switch1, in the
same VLAN
PIM sparse-mode enabled on both R1 and
R2's Gi0/1, plus their WAN-facing interfaces
toward a source/RP elsewhereTask 1: Verify HSRP's Active/Standby State
Confirm R1 is HSRP active and R2 is standby, as configured in the earlier HSRP lab.
Task 2: Verify PIM's Independent DR Election on the Same Segment
Check which router PIM elected as Designated Router on the shared Gi0/1 segment.
Task 3: Confirm Default PIM DR Election Criteria
Understand that PIM's DR election is based on highest IP address by default, independent of HSRP state entirely.
Task 4: Verify Multicast Forwarding Follows the PIM DR
Generate multicast traffic and confirm it is forwarded onto the segment specifically by the PIM DR, checking whether this aligns with or diverges from the HSRP active router.
Task 5: Align PIM DR Priority with HSRP if a Mismatch Exists
If PIM's DR differs from the HSRP active router, adjust PIM DR priority on the interface to bring the two into alignment.
Solution and Verification
R1# show standby brief
Interface Grp Pri P State Active Standby Virtual IP
Gi0/1 1 150 P Active local 192.168.190.3 192.168.190.1
-- R1 confirmed as HSRP active, consistent
-- with the earlier HSRP labR1# show ip pim interface gigabitethernet0/1
Address Interface PIM Nbr Query DR
Count Intvl
192.168.190.2 Gi0/1 on 2 30 192.168.190.3
-- The DR IP shown is 192.168.190.3 --
-- R2's actual interface address, NOT R1's
-- (192.168.190.2) -- PIM elected R2 as DR
-- purely because it has the higher raw IP
-- address on this segment, completely
-- unaware of HSRP's active/standby roles-- This confirms PIM's default election
-- criterion: highest IP address wins DR,
-- with no consideration of HSRP state,
-- priority, or any unicast forwarding
-- role whatsoeverSource> [sends multicast traffic]
R2# show ip mroute 239.1.1.1
(*, 239.1.1.1), ...
Outgoing interface list:
GigabitEthernet0/1, Forward
-- R2, the PIM DR (but HSRP standby), is
-- the one actually forwarding multicast
-- traffic onto the shared segment -- a
-- potential inconsistency worth noting,
-- though not necessarily broken, since
-- multicast and unicast forwarding roles
-- are logically independent-- To deliberately align PIM's DR election
-- with the HSRP active router (R1),
-- increase R1's PIM DR priority above R2's:
R1(config)# interface gigabitethernet0/1
R1(config-if)# ip pim dr-priority 200
R2(config)# interface gigabitethernet0/1
R2(config-if)# ip pim dr-priority 100R1# show ip pim interface gigabitethernet0/1
Address Interface PIM Nbr Query DR
Count Intvl
192.168.190.2 Gi0/1 on 2 30 192.168.190.2
-- R1 (192.168.190.2) is now the PIM DR,
-- matching its role as HSRP active --
-- dr-priority, when configured, always
-- takes precedence over the default
-- highest-IP tiebreakerKey Takeaway
PIM's DR election and HSRP's active/standby election are entirely independent processes by default, and nothing automatically synchronizes them — a network relying on both should deliberately configure ip pim dr-priority to match the intended HSRP active router, since leaving both at default settings can result in multicast traffic taking an unexpected path relative to unicast traffic on the same segment, a subtle design detail easy to overlook when HSRP and PIM are configured by different engineers or at different times.