Multicast Fundamentals

✓ Multicast communication is a technology that optimizes network bandwidth utilization and conserves system resources.
→ IP communication between network hosts typically uses one of the following transmission methods:
→ Unicast (one-to-one) → Broadcast (one-to-all) → Multicast (one-to-many)

The load on the server is reduced. (single Packet)

If each stream is 10 Mbps The server must maintain session The same video stream consumes only 10 Mbps
state information for all the sessions between the hosts. The network interface cards (NICs) of workstations must still
The bandwidth and load on the server increase as more process the broadcast packets , and send them on to the
receivers request the same video feed workstation’s CPU, which wastes processor resources.
The load on the server is reduced (single Packet)
The same video stream consumes only 10 Mbps
(PC-F) would drop the multicast traffic at the NIC level
(Why) it would not be programmed to accept the multicast traffic.
Multicast relies on:-
(PC-F) would not receive any multicast traffic if the switch enabled Internet Group Management Protocol (IGMP)
Internet Group Management Protocol (IGMP) snooping. its operation Between (Host to Router/switch Signaling)
Protocol Independent Multicast (PIM)
its operation Between Routers . (Router to Router Signaling)
✓ Multicast traffic provides one-to-many communication,
where only one data packet is sent on a link as needed and
then is replicated between links as the data forks (splits) on a
network device along the multicast distribution tree (MDT).
✓ The data packets are known as a stream

✓ uses a special destination IP address, known as a group address.

✓ Network devices request to receive the stream.

✓ Recipient devices of a multicast stream known as receivers.

✓ Common applications include real-time video, IPTV,

✓ Distance learning, video/audio conferencing, and gaming.

Multicast is UDP-based.
Best-effort delivery: (No acknowledgements)
No congestion avoidance
Out-of-sequence delivery
Duplicates (possible during network convergence)
 Multicast Addressing Class D Addressing Designation Multicast Address Range

Local network control block NOT routable 224.0.0.0 to 224.0.0.255

• Local network control block (224.0.0/24) - Addresses in the
local network control block are used for protocol control traffic Internetwork control block routable 224.0.1.0 to 224.0.1.255
that is not forwarded out a broadcast domain. Ad hoc block 224.0.2.0 to 224.0.255.255

• Internetwork control block (224.0.1.0/24) - Addresses in the Reserved 224.1.0.0 to 224.1.255.255

internetwork control block are used for protocol control traffic that SDP/SAP block 224.2.0.0 to 224.2.255.255
may be forwarded through the Internet.
Ad hoc block II 224.3.0.0 to 224.4.255.255
Well–Known Reserved Multicast Addresses Reserved 224.5.0.0 to 224.255.255.255
IP Multicast Address Description
Reserved 225.0.0.0 to 231.255.255.255
224.0.0.0 Base address (reserved)
Source Specific Multicast (SSM) block 232.0.0.0 to 232.255.255.255
224.0.0.1 All hosts in this subnet (all-hosts group)
224.0.0.2 All routers in this subnet GLOP block 233.0.0.0 to 233.251.255.255
224.0.0.5 All OSPF routers (AllSPFRouters) Ad hoc block III 233.252.0.0 to 233.255.255.255
224.0.0.6 All OSPF DRs (AllDRouters)
Reserved 234.0.0.0 to 238.255.255.255
224.0.0.9 All RIPv2 routers
Administratively scoped block 239.0.0.0 to 239.255.255.255
224.0.0.10 All EIGRP routers TTL 1
224.0.0.13 All PIM routers NOT routable Administratively scoped block (239.0.0.0/8) Private Range
224.0.0.18 VRRP
These addresses, are limited to a local group or organization.
224.0.0.22 IGMPv3
Like the reserved IP unicast ranges (such as 10.0.0.0/8)
224.0.0.102 HSRPv2 and GLBP
224.0.1.1 NTP
224.0.1.39 Cisco-RP-Announce (Auto-RP)
224.0.1.40 Cisco-RP-Discovery (Auto-RP) 224.1.1.1-226.1.1.1-239.1.1.1
 Layer 2 Multicast Addressing
Every multicast group address (IP address) is mapped to a special MAC address
that allows Ethernet interfaces to identify multicast packets to a specific group.
• The first 24 bits of a multicast MAC address always start with 01:00:5E.
• The low-order bit of the first byte is the individual/group bit (I/G) bit, also known as the unicast/multicast bit.
• When it is set to 1, it indicates that the frame is a multicast frame, and the 25th bit is always 0.

239.255.1.1 >>>>> 01:00:5E:7F:01:01

Last 23 bit

Out of the 9 bits from the multicast IP address that are not copied into the multicast MAC address
High-order bits 1110 are fixed, We miss 5 bits of mapping information: 25 This means we will map 32 multicast IP addresses to
1 multicast MAC address ( 224.1.1.1- 224.129.1.1 >>> 225.1.1.1- 225.129.1.1 >>>>> 238.1.1.1- 238.129.1.1>>> 239.1.1.1
 Internet Group Management Protocol (IGMP)
Sender
✓ communication protocol that enable receivers to Join a multicast group
✓ When a receiver wants to receive multicast traffic from a source,
✓ it sends a membership report (an IGMP join) to its router.
✓ The router then adds the receiver to the appropriate group.
✓ If the router does not have IGMP enabled on the interface, the request is ignored.

IGMP
PIM
✓ Enabling Protocol Independent Multicast (PIM) on a router’s interface enables IGMP Receiver Receiver
operation on that interface.

IGMP snooping on a switch:- listens on IGMP messages exchanged between devices

and the router. By analyzing these messages
Smart Packet Forwarding: Based on the IGMP messages, the switch builds a table
that tracks which devices belong to which multicast groups.
This allows the switch to intelligently forward multicast traffic only to the ports
connected to interested devices.
Prevents Traffic Floods

❖ IGMP versions:
1. IGMP Version 1
2. IGMP Version 2 (Default)
3. IGMP Version 3
IGMPv1 uses two specific message structures
✓ Report Messages:
• Used by the client to join the group
✓ Query Messages:
• Used by router to see if the members of the group still exist
• Always sent to 224.0.0.1 by multicast routers

✓ R1 sends periodic IGMP queries to the 224.0.0.1 address. (All-hosts group)

✓ If Only one member per group per subnet sends the IGMP report (join) message to the router
✓ In this case, H2 is the reporting host.
✓ While hosts H1 and H3 suppress theirs
✓ IGMPv1 uses a timer-based approach (query interval is 60 seconds and 180 delay timer)
✓ After 180 seconds if R1 doesn't hear any reports (join) from any devices,
✓ it assumes there are no longer any interested devices in the multicast group and stops forwarding for that group.
✓ Once you enable PIM on a layer 3 interface ✓ When a client want to leave the group
✓ it will automatically enable IGMPv2 on that interface. ✓ they will send a “IGMP Leave Msg” to 224.0.0.2
✓ Then router will query “IGMP Query Msg” to 234.1.1.1
✓ If a host want to join for a particular multicast group,
✓ (group Specific query)
✓ they will send an IGMP Membership report to 224.0.0.1 ✓ As long as at least one client is in this group,
✓ (all-hosts in local subnet) first hop router will get it. ✓ will forward IGMP Membership report back to router.

MRT (Maximum Response Time):

Querier election process: ✓ It helps to manage responses to queries sent by a router
▪ when there are two routers in the same subnet then only ✓ Timers set by receivers after receiving the membership query,
▪ one of them should send query messages. determining when they respond with a report indicating their interest.
▪ only one router becomes the active querier. ✓ It prevents devices from all responding at the same time, which could
▪ The router with the lowest IP address becomes the active querier. overload the network.
✓ For example, if the MRT is set to 10, this means that hosts have up to 1 second (10 * 0.1
seconds) to respond to the IGMP query.
✓ IGMPv3 is an extension of IGMPv2
✓ Hosts can join one group and leave another in the same transaction
✓ IGMPv2 requires separate report/leave messages.
✓ Allows us to do source specific multicast
✓ v1/v2 only support group specific multicast (*, G) join
✓ (any source/specific Group)
✓ V3 support Source specific multicast (S, G) join.
✓ which gives the receivers the capability to pick the source
✓ In IGMPv3 all host membership reports send to 224.0.0.22
✓ (IGMPv3 routers multicast address).
 Multicast Routing Protocols

➢ Distance Vector Multicast Routing Protocol (DVMRP) (legacy)

➢ Multicast OSPF (MOSPF) (legacy)
➢ Protocol Independent Multicast (PIM)

There are currently five PIM operating modes:

✓ ■ PIM Dense Mode (PIM-DM)
✓ ■ PIM Sparse Mode (PIM-SM) ➢ PIM used as router-router signaling for multicast traffic
✓ ■ PIM Sparse Dense Mode ➢ IGMP used as host-router multicast signaling protocol
✓ ■ PIM Source Specific Multicast (PIM-SSM)
✓ ■ PIM Bidirectional Mode (Bidir-PIM)

PIM Modes :-Tell us How the multicast tree is built from Sender to Receiver
✓ Flood & Prune Process repeats every 3 minutes
✓ State refresh message (This control message refreshes the prune
state on the outgoing interfaces of each router in the distribution tree.
bandwidth is saved by reducing unnecessary reflooding (configured)
✓ Graft message (not wait prune time )
✓ Hello message 224.0.0.13 All pim Routers 30 sec

Dense modes Source based Tree

Use shortest path from the sender to the Receiver
1.Reverse Path Forwarding (RPF)
✓ check is used to prevent multicast routing loops
✓ The source IP address of incoming multicast packets are checked
against a unicast routing table.
✓ If the datagram arrived on the interface specified in the routing
table for the source address; then the RPF check succeeds.
✓ Otherwise, the RPF Check fails.
✓ PIM-SM works in the opposite manner to PIM-DM (But same RPF , Neighbor/ADJ , Hello , DR)
✓ Source isn't forwarded multicast traffic unless some members requests it.
✓ When a member wants to receive traffic for a group, it sends a join request to its nearest router.
✓ The join request is propagated up the multicast tree to the source router.
✓ Upon the source receiving the join request
✓ the source router begins forwarding multicast traffic for the group out the appropriate interface
✓ PIM-SM works via the use of an RP (Rendezvous Point).

Shared Tree
(*, G)
1
 Shared Tree (RPT to Receiver)
• Uses shortest path from Sender to RP
• Shortest path from RP to Receiver
• Used to eliminate flooding & pruning &
makes Routing more scalable
• Sparse-mode only

STEPS
1-Shared Tree (RPT to Receiver)
The receiver sends an IGMP Join message to the first hop router (FHR), its direct neighboring
This router will then send a PIM Join to the RP.
A shared tree is then built from the receiver to RP based upon (*, G).
 Unicast

PIM-SM: Data Transmission:-

✓ The source starts to send multicast traffic.
✓ The FHR DR (R-A) encapsulates multicast packet into a PIM register message & sends unicast to the RP tells about a new source.
✓ The RP decapsulates the packet and checks the multicast group to see if it has any receivers. (with S, G)
✓ If so, the RP sends a PIM Join message back to the source, to build an SPT (Shortest Path Tree) to the source.
✓ The RP receives the packet (with S,G), and in turn, Sends a register-stop to stop receiving the register messages unicast
✓ NOTE: RP learn about Source using PIM Register message & learn about Receivers using Join message
 What if there is no RP
Here we have 2 tree 1-Source can't Register
✓ Shared Tree RPT (RPT to Receiver) 2-Join can’t be Processed
✓ Source Tree SPT (receiver shortest path to source)
✓ Merge 2 Trees together
✓ Joining shortest path to source & Prune toward RP
❑ RP (Rendezvous Point) is the reference point for the root of the shared multicast tree.

1-Source Registration:
✓ Sources send data to the RP using PIM Register messages.
✓ RP acknowledges with a Register-Stop message and adds the source to the tree (S,G) entry

2-Receiver Join:
1. Receivers send PIM Join messages toward the RP to express interest in multicast traffic.
3-Tree Merging:
1. The RP merges the source tree (from the source) and the shared tree (from receivers).
4-Shortest Path Transition:
1. Routers switch to the shortest path tree (SPT) directly to the source.
2. Once SPT is established, routers prune the path toward the RP

❑Impact of Missing Rendezvous Point (RP) in PIM-SM:-

✓ Sources cannot send PIM Register messages
✓ Receivers Join messages cannot be processed
✓ No Tree Formation: The shared tree (*,G) and source tree (S,G) cannot be established without the RP
1-Shared Tree (RP to Receiver)
The receiver sends an IGMP Join message to the first hop router (FHR), its direct neighboring
This router will then send a PIM Join to the RP.
A shared tree is then built from the receiver to RP based upon (*, G).
PIM-SM: Data Transmission:-
✓ Source sends multicast packet to RP
✓ Packet is attached to an RP Register message
✓ When packet reaches RP, it is forwarded in the tree
✓ Also: RP sends a Join message on reverse path to S1
✓ When Join messages reaches R1, it sends a native multicast packet to the RP
(in addition to the packet attached to the register message)
✓ When RP receives a native multicast packet, it sends a register stop message to R1.
This message stops the transmission of register messages from R1.
Shared Tree:
• Uses shortest path from Sender to RP
• Shortest path from RP to Receiver
• Used to eliminate flooding & pruning and makes Routing more scalable
• Sparse-mode only

1-Shared Tree (RP to Receiver)

The receiver sends an IGMP Join message to the first hop router (FHR), its direct neighboring
This router will then send a PIM Join to the RP.
A shared tree is then built from the receiver to RP based upon (*, G).

2. Shortest Path Tree (Source to RP) FHR Know about Source

LHR Know about Receivers
✓ Next, the source starts to send multicast traffic.
✓ The FHR encapsulates the multicast packet into a PIM register message & sends unicast to the RP
✓ The RP decapsulates the packet and checks the multicast group to see if it has any receivers.
✓ If so, the RP sends a PIM Join message back to the source, to build an SPT (Shortest Path Tree) to the source.
✓ The source sends another multicast packet, however now there is a SPT from the RP to the FHR.
✓ Therefore, the packet is now sent across the distribution tree to the RP.
✓ The RP receives the packet nativity (with S, G), and in turn, sends a register-stop message back
✓ to the sources FHR to stop receiving the register messages (via unicast).

✓ NOTE In the event of multicast sources being present. Multiple SPT distribution trees
✓ would be formed from the RP back to the FHRs.
 ❑ Finding Rendezvous Point (RP) Methods:-
➢ RP Selection Methods:
1. Static RP:
✓ Manually configured on all routers in the domain.
✓ Simple but lacks redundancy and Loadbalance (for small networks).
▪ Rx(config)#ip pim rp-address x.x.x.x

➢ Dynamic:
1-Auto-RP (Cisco Proprietary):
• Uses mapping agent to distribute RP information dynamically.
• Supports multiple RPs for redundancy.

2-Bootstrap Router (BSR) (Standards-Based):

• Elects BSR router to advertise RP-set information.
• Scalable works in multi-vendor networks.
 ❑Auto-RP in Cisco PIM-SM (Cisco Proprietary)
➢ Auto-RP involves two roles:
1-Candidate RP: Router that announces itself as RP.
✓ Sends RP Announcement packets to multicast address 224.0.1.39.
2-Mapping Agent (MA):
❑ Listens to RP announcements (224.0.1.39).
❑ Elects the active RP (Lower priority is better) if not highest IP).
❑ Advertises the RP-to-group mappings to 224.0.1.40 for all routers.

➢ How Auto-RP Works

•Step 1: Candidate RPs send announcements (224.0.1.39).
•Step 2: Mapping Agent collects announcements and selects the best RP.
•Step 3: MA sends final RP mapping to 224.0.1.40.
•Step 4: All routers update their RP tables based on MA’s advertisement.

➢ 4. Benefits
✓ Automatic RP Discovery
✓ Redundancy: Multiple C-RPs can be configured for failover.
✓ Load Balancing: Group ranges can be split among RPs.
✓ Cannot use two C-RPs at the same time for same group
➢ Problem R2 & R6 Missed RP Mappings (PIM-Sparse Mode Issue)
✓ Auto-RP packets 224.0.1.40 is only forwarded after PIM Join is sent to the RP.
✓ R2/R6 couldn’t join because they didn’t know the RP address yet

➢ Why R3 Worked:
✓ Directly connected to RPs (R1/R5), so received announcements without joins.

➢ Issue:
✓ R2 and R6 need the RP address to send PIM Join,
✓ But they can’t learn the RP without receiving the RP mapping packets first.

➢ Solutions:
1.PIM Sparse-Dense Mode:
✓ If the RP is unknown, routers use Dense Mode to flood multicast traffic.
✓ Once the RP is known, routers switch back to Sparse Mode.

2.Auto-RP Listener (Cisco Proprietary):

✓ Forces dense-mode behavior only for Auto-RP groups (224.0.1.39 and 224.0.1.40).
✓ Advantage: More targeted than sparse-dense mode (avoids flooding all unknown groups).

3.Static RP assignment for (224.0.1.39 and 224.0.1.40)

 ❑ PIM Bootstrap (BSR) is a standard (Part of PIMv2) :
➢ Two main roles:
✓ Candidate RP: Routers that want to become an RP.
✓ Candidate BSR: Collects RP info and floods it to the network (like Auto-RP’s Mapping Agent).

➢ Candidate BSR:
✓ Sends BSR messages to 224.0.0.13 (TTL = 1, hop-by-hop flooding).
✓ Routers re-flood the BSR messages after an RPF check.
✓ Only one active BSR (highest priority wins, if tie → highest IP wins).
➢ Candidate RP:
✓ Learns BSR IP from BSR messages.
✓ Registers to BSR using unicast packets (not multicast like Auto-RP).

➢ BSR Behavior:
✓ Builds a group-to-RP mapping list.
✓ Does NOT select the RP itself — just advertises all RPs.
✓ Multicast routers choose their RP based on the list.

➢ RP Selection:
❑ BSR advertises all RPs (doesn’t choose one).
❑ Each router picks the best RP based on:
✓ Highest RP priority
✓ Highest hash value (if priority matches).
✓ Highest IP address (if all else fails).
➢ Hash Function in PIM BSR
✓ To decide which RP will handle a multicast group.
✓ Helps load-balance multicast traffic among multiple RPs.
➢ Hash Function Formula:
✓ G = Multicast Group address , M = Hash Mask , C(i) = Candidate RP IP address
✓ Value(G,M,C(i)) = (1103515245 * ((1103515245 * (G&M) + 12345) XOR C(i)) + 12345) mod 2^31
✓ (No need to calculate manually, routers handle it automatically)

➢ Hash Mask:
✓ Default = 0 (only RP IP is used in hash calculation).
✓ Hash mask advertised inside BSR messages.
✓ The hash mask controls how multicast groups are divided among RPs.
➢ Effect of Hash Mask Size:
•31-bit mask → 1 bit left → 2 values → Multicast groups split across 2 RPs.
•30-bit mask → 2 bits left → 4 values → Multicast groups split across 4 RPs.

➢ Example:
✓ 8 multicast groups: 239.0.0.0 to 239.0.0.7
✓ No Hash Mask: All groups assigned to one RP.
❑ 31-bit Hash Mask:
✓ RP1: 239.0.0.0, 239.0.0.1
✓ RP2: 239.0.0.2, 239.0.0.3
✓ RP1: 239.0.0.4, 239.0.0.5 Configuration
✓ RP2: 239.0.0.6, 239.0.0.7
❑ 30-bit Hash Mask: Router(config)# ip pim rp-hash <mask>
•RP1: 239.0.0.0 to 239.0.0.3
✓ RP2: 239.0.0.4 to 239.0.0.7
➢ Main Benefit:
✓ Load Balancing multicast groups across multiple RPs automatically.
Feature Auto-RP (Cisco Proprietary) BSR (PIMv2 Standard)
Roles Candidate RP, Mapping Agent Candidate RP, Candidate BSR
RP Advertisement Multicast to 224.0.1.39 Unicast to BSR address

RP Mapping Mapping Agent floods to 224.0.1.40 BSR floods hop-by-hop to 224.0.0.13

Flooding Method Dense mode flooding required Hop-by-hop (TTL = 1)

RP Election Mapping Agent Selects RP Multicast routers Select RP themselves

BSR Election Not applicable Highest priority wins (then highest IP)

Special Mode Needed? Needs dense mode or Auto-RP listener No special mode needed
Multicast Bidirectional PIM
 Multicast Bidirectional PIM
✓ PIM Bidirectional (PIM-Bidir) is a different mode of multicast routing.
✓ Ideal for many-to-many communication scenarios (many sources and receivers)
(video conferencing , collaborative Tools - financial trading).

➢ Differences from PIM Sparse Mode

✓ Because Sparse Mode is Unidirectional
✓ PIM Sparse Mode builds two forwarding states: (*,G) and (S,G).
✓ PIM-Bidir only builds shared tree (*,G) entries – no (S,G) created.
✓ No SPT(Shortest Path Tree) is built. in PIM-Bidir traffic always via shared tree
✓ Traffic flows: The Bidir (*,G) trees facilitate traffic flow
❖ Senders forward traffic toward the RP (upstream).
❖ RP distributes traffic to Receivers (downstream ).
✓ No Register/Register-Stop messages to notify the RP of sources.
✓ Sources send directly without prior registration the RP first.

➢ RP (Rendezvous Point) Behavior

✓ The RP forwards traffic only if receivers exist.
✓ No way for RP to stop unwanted multicast traffic from sources.
✓ RP placement is critical ,should be centrally located between sources and receivers.
 Multicast Bidirectional PIM
➢ Loop Prevention
❑ No RPF (Reverse Path Forwarding) check used in PIM-Bidir for Loop prevention .
❑ Loop prevention is achieved via a Designated Forwarder (DF):
✓ Only the DF on a segment is allowed to forward multicast traffic to the RP.

➢ DF election criteria:
✓ Router with the lowest metric to RP wins.
✓ If equal, router with the highest IP address becomes DF.

➢ Note-1:-
✓ In some articles on PIM BiDir, it is mentioned that there is no RPF check.
✓ This is not entirely true since RPF is used to find the right interface toward RPA
✓ PIM BiDir does not use RPF to ensure loop free forwarding
✓ Use DF mechanism for this.

Note-2:-
✓ RP Behavior & Phantom RP
✓ RP can be virtual (Phantom RP), not tied to a physical device.
 ➢What Is the Concept of Anycast?
✓ Traffic is routed to the “Nearest destination’’ based on the Unicast Routing Table (Best path)
✓ Anycast is a Network addressing and Routing method
✓ Multiple devices share the same IP address

❑One IP, many locations. Route to the closest one.”

❑Used for load balancing, redundancy, and failover.

Use Case Example

Anycast DNS Public DNS servers like Google (8.8.8.8) route you to the nearest DNS server.
CDNs Content Delivery Networks use Anycast to deliver content from the nearest data center.
Anycast RP In multicast, multiple routers act as RP using the same IP, and traffic goes to the closest RP.
➢ RP Redundancy Methods
✓ AutoRP / BSR (Bootstrap Router):
✓ Multiple RPs advertise themselves, but with different IPs.
✓ Enables failover: If one RP fails, another can take over.

➢ Anycast RP Overview
✓ A single IP address is configured on multiple RP routers.
✓ This RP IP is advertised in the IGP (OSPF, EIGRP).
✓ Devices (Sources and Receivers) are routed to Nearest RP based on unicast routing table.

➢ Challenges & Solution: Traffic Handling with Anycast RP

❖ Split Control Plane:

✓ PIM Register messages from sources may go to a different RP.
✓ PIM Join messages from receivers may go to one RP.
❖ MSDP (Multicast Source Discovery Protocol):
✓ This protocol is run between RPs to share information about Source-Active.
✓ When a source registers with one RP, it sends (SA) message to other RPs so it know about the source too.
✓ If a receiver joins different RP, that RP can still establish the path to the source because it learned about it via MSDP.
1- Source Registration:
✓ The first-hop router (closest to the source) sends a PIM Register to the nearest RP (RP1).
✓ This register contains the first multicast packet from source S for group G.

2- MSDP SA Message:
✓ RP1 sends an MSDP Source-Active (SA) message to RP2.
✓ This SA message advertises that source S is active for group G.

3- Receiver RP Joins Source Tree:

✓ If RP2 has a receiver interested in group G, it sends a (S,G) PIM Join toward the source S.
✓ This creates a Shortest Path Tree (SPT) from RP2 to the source.

4- Multicast Packet Forwarding:

✓ RP2 starts forwarding multicast packets to the receiver using its shared tree.
✓ The last-hop router, after receiving the packet, also joins the SPT optimizing the path.

5- Ongoing MSDP Messages:

✓ MSDP SA messages are sent every 60 seconds while the source is active to keep RPs updated.
✓ If RP1 fails, traffic shifts to the next closest RP automatically.
➢ Benefits:
✓ Optimal Path Selection: Receivers/sources always use the nearest RP
✓ Automatic Failover: If RP1 fails, traffic shifts to the next closest RP automatically.
✓ No Impact on Active Sessions:
❑If data flow is already established between a source and receiver, RP failure doesn’t break it.
❑The RP is only needed to start the session.
 Source Specific Multicast (SSM):
1.ASM vs SSM:
✓ ASM (Any Source Multicast): Used in PIM sparse/dense mode with IGMPv2. Receivers accept multicast traffic from any source.
✓ SSM (Source Specific Multicast): Receivers specify which sources they want to receive multicast traffic from. Requires IGMPv3.
2. SSM Characteristics& Benefits :
1. Source Specification: Receivers can request multicast traffic only from specified sources.
2. No Rendezvous Point (RP): No Auto-RP , or Bootstrap protocols.
3. No Shared Trees: Only Shortest Path Trees (SPTs) are built directly to the sources.
4. Networks can run both ASM and SSM simultaneously, with SSM for critical streams. Financial data feeds (only trusted sources).
5. More secure and efficient: receivers only connect to trusted sources
3. SSM Range (IPv4):
✓ By default, the SSM address range is: 232.0.0.0/8 for Global

4. Router and Host Requirements:

✓ Routers must support PIM-SM & IGMPv3
✓ Receivers must send IGMPv3 reports specifying the (S,G) pair.
 Source Specific Multicast (SSM):

Feature ASM (Any Source Multicast) SSM (Source Specific Multicast)

IGMP Version IGMPv2 IGMPv3
Source Selection Any source Specific source
Uses RP (Rendezvous Point) Yes (Auto-RP / Bootstrap / Manual) No
Tree Type Shared Tree + SPT Only Shortest Path Tree (SPT)
Receiver Control Less control over sources Full control over source selection
Complexity Higher (due to RP and shared trees) Lower (simpler design)