Sparse mode logic (pull mode) is the opposite of Dense mode logic (push mode).
Dense mode it is supposed that in every network there is someone who is requesting the multicast traffic so PIM-DM routers begin by flooding the multicast traffic out of all their interfaces except those from where a prune message is received to eliminate the “leaf” from the multicasting tree (SPT), the Source-Based Tree (S, G); as opposed to Sparse mode that send the traffic only if someone explicitly requested it.
Not like Dense mode, which build a separated source-based tree (S, G) between the source and the requester of the traffic, Sparse mode mechanism is based on a fixed point in the network named Rendez-Vous point. All sources will have to register with the RP to which they send their traffic and thereby build a source-based tree (S, G) between them and the RP (not with the final multicast receiver like in PIM-DM) and all PIM-SM routers, “whatever” multicast traffic they are requesting, have to register with the RP and build a shared-tree (*. G)
Source Trees
A source tree is the simplest form of distribution tree. The source host of the multicast traffic is
located at the root of the tree, and the receivers are located at the ends of the branches. Multicast
traffic travels from the source host down the tree toward the receivers. The forwarding decision on
which interface a multicast packet should be transmitted out is based on the multicast forwarding
table. This table consists of a series of multicast state entries that are cached in the router.
State entries for a source tree use the notation (S, G) pronounced S comma G. The letterS represents the IP address of the source, and G represents the group address.
Shared Trees
Shared trees differ from source trees in that the root of the tree is a common point somewhere in
the network. This common point is referred to as the rendezvous point (RP). The RP is the point at which
receivers join to learn of active sources. Multicast sources must transmit their traffic to the RP. When receivers join a multicast group on a shared tree, the root of the tree is always the RP, and multicast traffic is transmitted from the RP down toward the receivers. Therefore, the RP acts as a go-between for the sources and receivers. An RP can be the root for all multicast groups in the network, or different ranges of multicast groups can be associated with different RPs.
Multicast forwarding entries for a shared tree use the notation (*, G), which is pronounced star comma G. This is because all sources for a particular group share the same tree. (The multicast groups go to the same RP.)
Therefore, the * or wildcard represents all sources.
RPF When troubleshooting multicast routing,t he primary concern is the source address. Multicast has a concept of Reverse Path Forwarding check (RPF check). When a multicast packet arrives on an interface, the RPF process checks to ensure that this incoming interface is the outgoing interface used by unicast routing to reach the source of the multicast packet. This RPF check process prevents loops. Multicast routing does not forward a packet unless the source of the packet passes a reverse path forwarding (RPF) check. Once a packet passes this RPF check, multicast routing forwards the packet based only upon the destination address.
IP IGMP Join-Group vs IP IGMP Static-Group
To join a router to a group, use the command ip igmp join-group—for instance, ip igmp join-group 227.0.0.1. This can be useful for troubleshooting the group. Since the router responds to ping, ICMP echo-requests sent to the group will result in a reply from the router. The multicast version of ping is called mping.
The ip igmp static-group command allows a router to be statically joined to a group. This is useful if you use a group to support a common application. Imagine that your company uses 227.0.0.1 as a multicast group for an internal application—using static joins on every router cuts out the processing of joins and leaves, removes the need for queries, and allows the router to fast switch the traffic.
Just to be clear—neither of these commands are commonly used. Joining a router to a group forces the router to process a lot of traffic that it cannot use, such as video captures. However, joining a group can be useful in troubleshooting a new multicast network. In a similar vein, static joins are useful if all traffic uses a well-known address. Multicast traffic tends to be fairly unpredictable, however, so this is not a solution for most networks.
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MSDP is a mechanism to connect multiple PIM-SM domains. MSDP enables multiple sources for a given group to be known to all RPs in the same or different domains.
MSDP messages are flooded across MSDP peers.However, to provide full resiliency, a full mesh peering must be established.
How does MSDP work?
When the source is initiated, the first hop router encapsulates the Mcast data in register messages and Unicasts the flow to the RP; RP de-encapsulates the register messages and forwards it down toward the last hop router; If MSDP is configured on the RP, the packet is also re-encapsulated in Source Active (SA) messages which are immediately forwarded to all MSDP peers.
The SA messages identify the source, the group address that the source is sending to and the oriinator-id, the originator-id is an optional and it is included only if it's configured.
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Dense mode it is supposed that in every network there is someone who is requesting the multicast traffic so PIM-DM routers begin by flooding the multicast traffic out of all their interfaces except those from where a prune message is received to eliminate the “leaf” from the multicasting tree (SPT), the Source-Based Tree (S, G); as opposed to Sparse mode that send the traffic only if someone explicitly requested it.
Not like Dense mode, which build a separated source-based tree (S, G) between the source and the requester of the traffic, Sparse mode mechanism is based on a fixed point in the network named Rendez-Vous point. All sources will have to register with the RP to which they send their traffic and thereby build a source-based tree (S, G) between them and the RP (not with the final multicast receiver like in PIM-DM) and all PIM-SM routers, “whatever” multicast traffic they are requesting, have to register with the RP and build a shared-tree (*. G)
Source Trees
A source tree is the simplest form of distribution tree. The source host of the multicast traffic is
located at the root of the tree, and the receivers are located at the ends of the branches. Multicast
traffic travels from the source host down the tree toward the receivers. The forwarding decision on
which interface a multicast packet should be transmitted out is based on the multicast forwarding
table. This table consists of a series of multicast state entries that are cached in the router.
State entries for a source tree use the notation (S, G) pronounced S comma G. The letterS represents the IP address of the source, and G represents the group address.
Shared Trees
Shared trees differ from source trees in that the root of the tree is a common point somewhere in
the network. This common point is referred to as the rendezvous point (RP). The RP is the point at which
receivers join to learn of active sources. Multicast sources must transmit their traffic to the RP. When receivers join a multicast group on a shared tree, the root of the tree is always the RP, and multicast traffic is transmitted from the RP down toward the receivers. Therefore, the RP acts as a go-between for the sources and receivers. An RP can be the root for all multicast groups in the network, or different ranges of multicast groups can be associated with different RPs.
Multicast forwarding entries for a shared tree use the notation (*, G), which is pronounced star comma G. This is because all sources for a particular group share the same tree. (The multicast groups go to the same RP.)
Therefore, the * or wildcard represents all sources.
RPF When troubleshooting multicast routing,t he primary concern is the source address. Multicast has a concept of Reverse Path Forwarding check (RPF check). When a multicast packet arrives on an interface, the RPF process checks to ensure that this incoming interface is the outgoing interface used by unicast routing to reach the source of the multicast packet. This RPF check process prevents loops. Multicast routing does not forward a packet unless the source of the packet passes a reverse path forwarding (RPF) check. Once a packet passes this RPF check, multicast routing forwards the packet based only upon the destination address.
IP IGMP Join-Group vs IP IGMP Static-Group
To join a router to a group, use the command ip igmp join-group—for instance, ip igmp join-group 227.0.0.1. This can be useful for troubleshooting the group. Since the router responds to ping, ICMP echo-requests sent to the group will result in a reply from the router. The multicast version of ping is called mping.
The ip igmp static-group command allows a router to be statically joined to a group. This is useful if you use a group to support a common application. Imagine that your company uses 227.0.0.1 as a multicast group for an internal application—using static joins on every router cuts out the processing of joins and leaves, removes the need for queries, and allows the router to fast switch the traffic.
Just to be clear—neither of these commands are commonly used. Joining a router to a group forces the router to process a lot of traffic that it cannot use, such as video captures. However, joining a group can be useful in troubleshooting a new multicast network. In a similar vein, static joins are useful if all traffic uses a well-known address. Multicast traffic tends to be fairly unpredictable, however, so this is not a solution for most networks.
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MSDP is a mechanism to connect multiple PIM-SM domains. MSDP enables multiple sources for a given group to be known to all RPs in the same or different domains.
MSDP messages are flooded across MSDP peers.However, to provide full resiliency, a full mesh peering must be established.
How does MSDP work?
When the source is initiated, the first hop router encapsulates the Mcast data in register messages and Unicasts the flow to the RP; RP de-encapsulates the register messages and forwards it down toward the last hop router; If MSDP is configured on the RP, the packet is also re-encapsulated in Source Active (SA) messages which are immediately forwarded to all MSDP peers.
The SA messages identify the source, the group address that the source is sending to and the oriinator-id, the originator-id is an optional and it is included only if it's configured.
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