Learning Multicast Part 7 Bringing Clarity

David Hedley’s
Tuesday Tech Talks
Multicast Part 7 Bringing Clarity
Turning networking on it’s head
© 2018-2020 David M. Hedley All Rights Reserved.

David Hedley's Tuesday Tech Talks – Multicast Pt. 7
• Cisco Certified Network Professional: Enterprise, Routing & Switching
• Cisco Certified Design Professional
• Cisco Certified Specialist: Enterprise Core, Enterprise Design, Wireless
Design, Wireless Implementation, Advanced Infrastructure
Implementation.
• Cisco Certified Network Associate: Routing & Switching, Wireless, Data
Center, Security
• Cisco Certified Design Associate
• ITIL v4 Foundations
• CompTIA A+
• https://www.youracclaim.com/users/david-hedley/badges

• 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: Tying to bring all the knowledge together.
• Question: What don’t I yet understand?

• 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) Layer 3
• Source (Class A, B, C) 1.0.0.0 – 223.255.255.255
• Destination (Class D) 224.0.0.0 – 239.255.255.255
• 232.0.0.0 – 232.255.255.255 Source Specific Range
• 239.0.0.0 – 239.255.255.255 Administratively Scoped Addresses
• SOURCE CAN NEVER BE CLASS D GROUP ADDRESS!!!!

• Host-Router – Internet Group Management Protocol (IGMP) Layer 2/3
• 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.

• IP Multicast MAC Address Mapping Layer 2
• In IPv4, all multicast address start with bits 1110, so the last 28 bits
differentiate the different multicast addresses
• Each Multicast MAC begins with 0x01005e with a zero in the 25th bit
• The last 23 bits are taken from the group address
• Example
• 239.200.128.1 = 11101111 11001000 10000000 00000001
• Mac 0000001 00000000 01011111 01001000 10000000 00000001 (01-00-5e-48-80-01)
• 224.72.128.1 = 11100000 01001000 10000000 00000001
• Mac 00000001 00000000 0101111 01001000 10000000 00000001 (01-00-5e-48-80-01)
• 32:1 Address Overlap!

• Multicast Global Commands
• Ip multicast-routing [distributed]
• Ip multicast multipath

• Multicast Boundaries
• ip multicast boundary access-list [filter-autorp]
• Placed at the boundary interfaces of a multicast region.
• Do NOT use the filter-autorp option on an RP.
• Filter-autorp will filter Auto-RP messages which for ranges denied by the
boundary ACL.
• Example
• Ip access-list standard MCAST_BOUNDARY
• Permit 239.200.0.0 0.0.255.255
• Permit 224.0.0.0 7.255.255.255
• Deny 224.0.0.0 15.255.255.255

• Hot Standby Router Protocol (HSRP) and PIM
• When there are 2 or more routers running PIM on a single segment, one will
be elected the Designated Router (DR)
• DR is responsible for PIM Joins upstream
• Chosen by highest IP Address.
• Highest IP device might not be optimal path.
• DR can be manually set with ip pim dr-priority command.
• Default priority is 1.
• Highest priority is the DR..

• Orphaned VLAN
• Scenario – Small Branch Office
• Router and a Switch. On switch, uplink is built as an access port.
• Now I need to add a Voice VLAN, so I create the SVI on the switch.
• I enable routing between the User VLAN and the Voice VLAN, using a routing protocol
such as EIGRP
• I have no demand for multicast on the User VLAN
• Where do I enable PIM?
• Voice VLAN SVI?
• Routers Uplink to the switch?
• ?????
• What other configuration do I need?

• Key Points
• Layer 1
• IGMP snooping is normally enabled by default, you can turn off for troubleshooting, but
good to verify that it’s on, when touching a device for the first time in troubleshooting.
• Layer 2
• Remember the 32:1 oversubscription.
• IGMP from host to Last Hop Router
• Reports, queries, and leaves.
• IGMP snooping helps with keeping load of Last Hop Router
• IGMP snooping keeps multicast packets from being flooded to all ports

• Key Points
• Layer 3
• Each host may subscribe to multiple groups
• PIM between each Layer 3 Interface
• Sparse mode as Dense is not supported across all platforms
• Be careful about orphaned VLANs
• Multicast routing enabled on each device from first hop to last.
• Any break will cause Reverse Path Forwarding to fail. Use mtrace.
• Multipath enabled on all devices, just in case.
• Not used unless needed, but essential with equal cost paths.
• AutoRP Listener turned on, on all devices for Any Source Multicast (AutoRP)
• Setup Source Specific Multicast on Last Hop Router, but best to do all.

• Key Points
• Quiz Layer 2
• PIM or IGMP?
• What does IGMP Snooping Do?
• A vendor is using a group 239.200.127.54 and has set the TTL to 4, you see the (*,G) and
(S,G) on the last hop router, but the shortest path is NOT set, nor do you seem to be
getting the traffic, how can you troubleshoot?
• When do you need IGMP v3?
• Quiz Layer 3
• On which interfaces should you enable PIM?
• VLAN interface where hosts might be interested in the multicast traffic
• Layer 3 links between devices (WAN, Layer 3 Uplinks to Cores, etc.)
• Loopback, if and only if, you are an RP

• Key Points
• Quiz Layer 3
• Where do I place boundary statements?
• Where do I NOT place boundary statements?
• Filter-autorp, what is this used for?
• I have a campus range that I used at multiple campuses, assume we block that range at
the campus edge. What would happen if you did not block the autorp announcement for
the range, and a layer 3 device at another campus receives that annoucement?
• Would MSDP help?
• I announce the range 239.200.0.0/16, and my boundary has permit 239.200.128.0/22,
what would filter-autorp do to this annoucement?
• How do I figure out which is the RP for a particular group?
• Does Source Specific Multicast use a Rendezvous Point?
• How does SSM connect the receiver and source?

• Key Points
• Quiz Layer 3
• What command(s) show me if my Layer 3 uplink is properly connected via PIM?
• What command(s) show me that an interface is interested in particular groups?
• What command will show me that my Last Hop Router knows about a particular source?
• What is MSDP, and what command(s) can help me see the it is working?
• What is Anycast for IPv4, and what application does it have to multicast?
• I just installed a new RP on the WAN, all of a sudden it is now acting as the BSR, what did
I do wrong?
• I’m using BSR, why do I see multiple RP’s for the same groups?

• Troubleshooting
• PIM
• Show ip pim neighbor interface (any PIM enabled device)
show ip pim neighbor
PIM Neighbor Table
Mode: B - Bidir Capable, DR - Designated Router, N - Default DR Priority,
P - Proxy Capable, S - State Refresh Capable, G - GenID Capable
Neighbor Interface Uptime/Expires Ver DR
Address Prio/Mode
192.168.254.1 Vlan108 5w0d/00:01:31 v2 1 / G
• What if no neighbor?
• Missing PIM statements
• Unicast addresses not on same subnet
• Link down

• Troubleshooting
• PIM
• Show ip pim mroute [ group-address ] [ count ]
show ip mroute verbose
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
L - Local, P - Pruned, R - RP-bit set, F - Register flag,
T - SPT-bit set, J - Join SPT, M - MSDP created entry, E - Extranet,
X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
U - URD, I - Received Source Specific Host Report,
Z - Multicast Tunnel, z - MDT-data group sender,
Y - Joined MDT-data group, y - Sending to MDT-data group,
V - RD & Vector, v - Vector
Outgoing interface flags: H - Hardware switched, A - Assert winner
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 224.0.1.39), 5w1d/stopped, RP 0.0.0.0, flags: DCL
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Vlan108, Forward/Sparse, 5w1d/stopped
(192.168.254.4, 224.0.1.39), 5w1d/00:02:27, flags: PLT
Incoming interface: Vlan108, RPF nbr 0.0.0.0
Outgoing interface list: Null

• Troubleshooting
• PIM
• Show ip pim mroute [ group-address ] [ count ]
show ip mroute count
IP Multicast Statistics
6 routes using 2736 bytes of memory
4 groups, 0.50 average sources per group
Forwarding Counts: Pkt Count/Pkts per second/Avg Pkt Size/Kilobits per second
Other counts: Total/RPF failed/Other drops(OIF-null, rate-limit etc)
Group: 239.255.255.250, Source count: 0, Packets forwarded: 0, Packets received: 13350
RP-tree: Forwarding: 0/0/0/0, Other: 13350/0/13350
Source: 192.168.254.4/32, Forwarding: 0/0/0/0, Other: 0/0/0
Source: 192.168.254.4/32, Forwarding: 0/0/0/0, Other: 0/0/0

• Components

• Tech Talk #2
• To do list!
• What standards do you need to document before the deployment can begin?
• Administrative Scopes (chopping up 239.0.0.0/8 to suite your needs)
• Boundary statements (how you’ll divide up your network, where and what)
• Are you using Source Specific or Any Source Multicast, why, and write out the standard
configuration.
• Where do you need Rendezvous Points, how you create redundancy? (MSDP peers, etc.)
• You are going to use PIM Sparse Mode, that’s one of your standards, not Sparse-Dense.
• What question have you not asked yet?

• Thanks for watching!
• You can subscribe to my YouTube Channel
https://www.youtube.com/channel/UCZ3pcIh5Zmbp3rdjhfR7BOg
• Or connect with me on Linkedin https://www.linkedin.com/in/david-
hedley-541985/
• You can suggest topics in the comments!

Learning Multicast Part 7 Bringing Clarity

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