31.3.2 IPv6 multicast packet forwarding

(1) Hardware forwarding

Hardware-based IPv6 multicast packet forwarding consists of the following four steps:

1. Searching of the IPv6 multicast forwarding entries

   When a packet bound for an IPv6 multicast group is received, the hardware multicast forwarding entries are searched for the corresponding entries.

2. Checking the validity of the packet reception interface

   If the search in step 1 returns entries, whether the IPv6 packet was received from a valid interface is checked.

3. Filtering

   The information registered in the IPv6 filtering table is checked to determine whether the packet is to be forwarded.

4. Determining whether to forward the packet based on the hop limit, and decrementing the TTL value

   The hop limit value in the packet is checked to determine whether the packet is to be forwarded. If the packet is to be forwarded, the hop limit value in the packet is decremented.

(2) Software forwarding

The processing for software-based IPv6 multicast packet forwarding differs as described for the following conditions:

• If no corresponding entries are found by the search performed in hardware-based forwarding

  When a packet from a given sender bound for a given IPv6 multicast group is first received, it is forwarded by software based on a forwarding entry generated from the IPv6 multicast routing information. At the same time, an IPv6 multicast forwarding entry is registered in hardware.

• When IPv6 encapsulation processing is performed

  Forwarding is performed with temporary IPv6 encapsulation up to the rendezvous point, and then decapsulation is performed at the rendezvous point for each forwarding destination.

(3) Retrieval of IPv6 multicast routing information or IPv6 multicast forwarding entries

When an IPv6 multicast packet is received, the multicast routing information or IPv6 multicast forwarding entries are searched for the entries that match both the DA (destination group address) and SA (source address) of the packet. The following figure shows how IPv6 multicast routing information or IPv6 multicast forwarding entries are searched.

(4) Negative cash

Negative caching is functionality that uses hardware to discard multicast packets that cannot be forwarded. A negative cache is a forwarding entry for which no forwarding destination interface exists. When a negative cache receives a multicast packet that cannot be forwarded, it registers it in hardware. Then, when a multicast packet is received with the same address as the registered multicast packet, the packet is discarded by using hardware. This prevents undue load when many multicast packets that cannot be forwarded are received.

(5) VRF function [SL-L3A]

When an IPv6 multicast is performed over multiple VRFs, IPv6 multicast forwarding entries can be set for each individual VRF. For different VRFs, IPv6 multicast forwarding entries that have the same IP address can be created. If an IPv6 multicast extranet is used, multicast communication can be performed between different VRFs.