This KB article describes a multicast flood issue observed after power maintenance on EX4600 devices, outlines the suspected root cause, and provides the recommended solution.
During a power maintenance and had to power down all the network gear. They noticed the cross links between two switches taking bursts of multicast traffic all of a sudden after the maintenance.
root@switch> show interfaces et-0/0/24 extensive | match multicast
Multicast packets 235596270792 1422388
root@switch> show interfaces et-0/0/25 extensive | match Multicast
Multicast packets 1425631 235761230685
Issue state -
root@switch> show igmp snooping statistics vlan VLANTEST_1
Vlan: VLANTEST_1
IGMP Message type Received Sent Rx errors
Membership Query 8485 134990 0
V1 Membership Report 0 0 0
DVMRP 0 0 0
PIM V1 0 0 0
Cisco Trace 0 0 0
V2 Membership Report 7144594 7167540 0
Group Leave 533 572 0
Mtrace Response 0 0 0
Mtrace Request 0 0 0
Domain Wide Report 0 0 0
V3 Membership Report 1379930 1381521 0
Other Unknown types 0
root@switch> show igmp snooping statistics vlan VLANTEST_2
Vlan: VLANTEST_2
Membership Query 8485 134994 0
V2 Membership Report 97579 99341 0
Group Leave 3443 3766 0
V3 Membership Report 0 0 0
The incident occurred due to a timing‑specific condition where both core switches restarted simultaneously, temporarily removing multicast coordination information.
Both switches rely on IGMP L2 snooping with L2 querier fallback (no L3 IGMP querier present). When both switches rebooted at the same time, all IGMP snooping state was lost and both self‑elected as active L2 IGMP queriers.
Querier election failed to converge, leading to excessive IGMP queries and millions of membership reports as seen in the below output.
As a result, IGMP snooping could not stabilize, and the switches entered failsafe multicast flooding mode, forwarding multicast traffic out all VLAN ports indefinitely.
Evidence:
Resolution:
Rebooting one switch removed the dual‑querier condition, allowing IGMP to converge and multicast snooping state to rebuild.
In the latest outputs we do not see any membership queries received on switch 1 since the querier is selected.
Membership Query 0 1418 0
V2 Membership Report 38783 0 0
Group Leave 0 0 0
V3 Membership Report 7752 0 0
Membership Query 0 1420 0
V2 Membership Report 501 0 0
Recommendation:
Introduce a stable L3 IGMP querier (IRB or upstream router) for multicast VLANs.
Put a router or IRB in each multicast VLAN.
Effect:
Querier never disappears
Snooping rebuilds immediately after L2 reboot
Flood window shrinks to seconds
Dual‑EX reboot becomes safe