Routing
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PIM Sparse Mode
PIM sparse mode was designed for networks where multicast receivers are sparsely located across subnets on the network. PIM spare mode also works quite well in dense multicast receiver environments too. PIM sparse mode uses the unicast routing table to carry out reverse path forwarding checks, and can utilise any routing protocol that populates the…
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PIM Shortest Path Tree Switchover
PIM sparse mode allows the last hop router to switch from the shared tree to a shortest path tree for a specific source. This is a default behaviour in Cisco routers and happens after the first multicast packet is received from the rendezvous point from the shared tree, even if the shortest path is through…
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Multicast Source Sparse Mode Registration
When the source for the group sends a packet, the first hop router attached to that source is required to register the source with the rendezvous point. The rendezvous point is also requested to build a tree back towards the first hop router. The first hop route encapsulates multicast data from the source into a…
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PIM Dense Mode
PIM dense mode constructs the multicast tree by flooding traffic out of every interface from the source to every dense mode router across the network. PIM dense mode is a suitable configuration where receivers of the multicast group are located across every subnet on the network. It is applicable to small networks, but not recommended…
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PIM Operating Modes
There are five operating PIM modes: PIM Dense Mode (PIM-DM) PIM Spare Mode (PIM-SM) PIM Spare Dense Mode PIM Source Specific Multicast (PIM-SSM) PIM Bidirectional Mode (Bidir-PIM) PIM Control messages are sent using IP Protocol 103. Register and register stop messages can be sent using unicast, or multicast with a time-to-live of 1 to all…
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PIM Terminlogy
Reverse Path Forwarding (RPF) Interface The reverse path forwarding interface is the interface with the lowest cost path to the IP address of the root of the shortest path tree (the source of the multicast stream). The lowest cost is based on the factors of administrative distance and metric. If there are multiple interfaces with…
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PIM Distribution Trees
A multicast router will create a distribution tree that will define the path multicast traffic follows to reach receiver devices. There are two types of multicast distribution trees known as source trees. They are also known as shortest path trees, SPTs and shared trees. Source Tree A source tree is a multicast distribution tree where…
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PIM – Protocol Independent Multicast
Hosts utilise IGMP to join a multicast group, this works perfectly fine when the router that the membership join request gets sent to also hosts the subnet that sends out the multicast traffic flows. If the receiving host and the multicast source router are several hops apart, routing between the routers will need to be…
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IGMP Snooping
IGMP snooping is a method that allows switches to only send traffic to interested receivers of multicast packet streams. IGMP snooping helps switches to optimise forwarding and minimise flooding of multicast streams to all connected ports on their device. Typically with unicast traffic, switches learn about MAC addresses and where they belong to by inspecting…
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IGMP Version 3
When a host in IGMPv2 sends a membership report to join a multicast group, it does not specify the source that it would like to receive the multicast traffic from. IGMPv3 is an extension on top of IGMPv2 that adds support for the multicast source filtering, giving the host a choice to pick the source…