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multicast drop-unknown

Created: Apr 18, 2020 01:53:32Latest reply: Apr 18, 2020 01:54:39 52 2 0 0
  Rewarded HiCoins: 0 (problem resolved)

Hello everyone,

Why are some protocol packets fail to be transparently transmitted after the multicast drop-unknown command is configured on a switch?

Please help me to solve this question. Thank you!

  • x
  • convention:

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Admin Created Apr 18, 2020 01:54:39 Helpful(1) Helpful(1)

Hi WDNJSQ,
After the multicast drop-unknown command is configured on a switch, the switch forwards packets with destination MAC and IP addresses as multicast addresses according to the multicast forwarding table. If a packet matches no multicast forwarding entry, the switch considers it as an unknown multicast packet and drops it.
Some protocol packets, such as OSPF packets, VRRP packets, and IPv6 RA messages, use reserved group addresses as destination MAC and IP addresses and match no multicast forwarding entry. If the switch is configured to drop unknown multicast packets using the multicast drop-known command, the switch drops these protocol packets.
Therefore, do not use the multicast drop-unknown command in a VLAN if the switch needs to transparently transmit protocol packets with reserved group addresses.
I hope it helps!

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  • x
  • convention:

Unicef Created Apr 18, 2020 02:21:41
Very good  
All Answers
DDSN Admin Created Apr 18, 2020 01:54:39 Helpful(1) Helpful(1)

Hi WDNJSQ,
After the multicast drop-unknown command is configured on a switch, the switch forwards packets with destination MAC and IP addresses as multicast addresses according to the multicast forwarding table. If a packet matches no multicast forwarding entry, the switch considers it as an unknown multicast packet and drops it.
Some protocol packets, such as OSPF packets, VRRP packets, and IPv6 RA messages, use reserved group addresses as destination MAC and IP addresses and match no multicast forwarding entry. If the switch is configured to drop unknown multicast packets using the multicast drop-known command, the switch drops these protocol packets.
Therefore, do not use the multicast drop-unknown command in a VLAN if the switch needs to transparently transmit protocol packets with reserved group addresses.
I hope it helps!

View more
  • x
  • convention:

Unicef Created Apr 18, 2020 02:21:41
Very good  

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