10G Core Routing Switch User Manual
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A multicast routing protocol runs between multicast routers to create and maintain multicast routes for correct and
efficient forwarding of multicast packet. The multicast routing creates a loop-free data transmission path from one
source to multiple receivers.The task of multicast routing protocols is to build up the distribution tree architecture. A
multicast router can use multiple methods to build up a path for data transmission, that is, a distribution tree.
As in unicast routing, the multicast routing can also be intra-domain or inter-domain. Intra-domain multicast routing is
rather mature and protocol independent multicast (PIM) is the most wildly used intra-domain protocol, which can
work in collaboration with unicast routing protocols. The inter-domain routing first needs to solve how to transfer
routing information between ASs. Since the ASs may belong to different telecom carriers, the inter-domain routing
information must contain carriers’ policies, in addition to distance information. Currently, inter-domain routing
protocols include multicast source discovery protocol (MSDP) and MBGP multicast extension.
IP Multicast Packet Forwarding
To ensure that multicast packets reach a router along the shortest path, the multicast router must check the receiving
interface of multicast packets depending on the unicast routing table or a unicast routing table independently provided
for multicast. This check mechanism is the basis for most multicast routing protocols to perform multicast forwarding,
and is known as Reverse Path Forwarding (RPF) check. A multicast router uses the source address of a received
multicast packet to query the unicast routing table or the independent multicast routing table to determine that the
receiving interface is on the shortest path from the receiving station to the source. If a source tree is used, the source
address is the address of the source host sending the multicast packet. If a shared tree is used, the source address is
the RP address of the shared tree. A multicast packet arriving at the router will be forwarded according to the
multicast forwarding entry if it passes the RPF check, or else, it will be discarded.
IGMP Snooping Configuration
IGMP Snooping Overview
IGMP Snooping Principle
IGMP Snooping (Internet Group Management Protocol Snooping) is a multicast control mechanism running on the
Layer 2 Ethernet switch and it is used for multicast group management and control.
IGMP Snooping runs on the link layer. When receiving the IGMP messages transmitted between the host and router,
the Layer 2 Ethernet switch uses IGMP Snooping to analyze the information carried in the IGMP messages. If the switch
hears IGMP host report message from an IGMP host, it will add the host to the corresponding multicast table. If the
switch hears IGMP leave message from an IGMP host, it will remove the host from the corresponding multicast table.
The switch continuously listens to the IGMP messages to create and maintain MAC multicast address table on Layer 2.
And then it can forward the multicast packets transmitted from the upstream router according to the MAC multicast
address table. When IGMP Snooping is disabled, the packets are multicast on Layer 2.
See the following figure:
