Learning Multicast Part 4 Rendezvous Points

David Hedley’s
Tuesday Tech Talks
Multicast Part 4
Turning networking on it’s head
© 2018-2020 David M. Hedley All Rights Reserved.

David Hedley's Tuesday Tech Talks – Multicast Pt. 4
• Cisco Certified Network Professional: Enterprise, Routing & Switching
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Design, Wireless Implementation, Advanced Infrastructure
Implementation.
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Center, Security
• Cisco Certified Design Associate
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• Specific brands and models are for illustration purposes only.
• They do not imply any endorsement by the vendor, in any way.
• This talk does not represent the business process of any employer or
client, past or present, this is based on my own work and study.
• I only include equipment that I have experience with.
• At the time of writing, I have not received any compensation, or
inducement from any vendor.

• Definition
• In computer networking, multicast is group communication[1] where data
transmission is addressed to a group of destination computers
simultaneously. Wikipedia https://en.wikipedia.org/wiki/Multicast retrieved July 5,2018
• Originally defined in RFC 966 (1985)

• Purpose: Understanding the fundamentals of Multicast.
• Question: How do I connect my senders (sources) and my receivers
(group members)?

• Triple Constraints
• Time
• Quality or Scope
• Money or Budget

• Unicast vs. Multicast

• Multicast Advantages
• Eliminates Traffic Redundancy = Less Bandwidth
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Unicast vs Multicast
Mulicast Unicast

• Multicast Addressing (IPv4)
• Source (Class A, B, C) 1.0.0.0 – 223.255.255.255
• Destination (Class D) 224.0.0.0 – 239.255.255.255
• SOURCE CAN NEVER BE CLASS D GROUP ADDRESS!!!!

• Host-Router – Internet Group Management Protocol (IGMP)
• Hosts tells routers about group membership.
• Routers solicit group membership from hosts.
• IGMP v. 1 RFC 1112 (1989) https://www.rfc-editor.org/rfc/rfc1112.txt
• For IPv6 Multicast Listener Discovery (MLD) v.1 is equivalent to IGMP v. 2
• MLD v. 2 is the IPv6 equivalent for IGMP v. 3.

• Unicast v. Multicast L3 Forwarding
• Unicast is all about where the packet is going.
• Destination IP indicates where the packet is going.
• Hop by hop forwarding.
• Multicast is all about where the packet is coming from.
• Multicast group tells you nothing about where the packet is going.
• Sender starts broadcasting with no idea if there are any receivers or where they may be.
• Receivers need to be connected to the sender.
• Protocol Independent Mulitcast (PIM) joins follow unicast routing
• Build Distribution Trees to forward packets
• Distribution trees rebuild dynamically.

• Reverse Path Forwarding
• RPF Calculation
• Multicast source is a unicast address of a sender to the Multicast Group.
• Using that source, the unicast route table is used to determine the interface and next
hop, where the PIM joins are sent.
• That interface becomes the “incoming” or RPF interface
• Multicast is only forwarded if received on an incoming interface.
• The Source isn’t trying to get to the receiver, the receiver is trying to get to the source.

• RPF Calculation
• Shortest Path to Source from unicast
routing table
• Sends joins until it completes the
tree.
• Multicast traffic flows down the tree.

• RPF Calculation
• What happens with equal cost paths.
• CAN’T USE BOTH!*
• Uses highest next hop IP address.
• What is the normal router behavior
with equal cost paths?
• Load is balanced either by
destination or by packet.
• * Cisco uses the command ip
multicast multipath to allow for
equal cost paths.

• Distribution Tree Creation (Shared Tree (*.G))

• PIM SM Sender Registration
• (S,G) State created on Source Tree Only

• (S,G) traffic arrives along Source Tree
• RP sends a Register-Stop back to 1st Hop

• Source flow on SPT to RP
• Traffic flows from RP to Receivers

• PIM SM Sender Switchover
• Last Hop Router joins Source Tree
• New (S,G) along new Source Tree

• Traffic flows down new Source Tree
• LHR sends (S,G) prune towards RP

• (S,G) Traffic pruned from shared tree.

• Why Any Source Multicast (ASM)?
• Traffic is only on joined branches.
• Can switch to optimal source-trees
• Unicast routing protocol independent.

• Components

• Tech Talk #2
• To do list!
• Where are my current rendezvous points ?
• Are my RP’s redundant?
• Given the application boundaries I have/need, where should the RP’s be placed, and
what groups to they need to advertise?

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hedley-541985/
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Learning Multicast Part 4 Rendezvous Points

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