CCIE RS – Written – L2 Multicast – Explain PIM Snooping

PIM Snooping

Switch restricts multicast packets for each IP multicast group to only those multicast router ports that have downstream receivers joined to that group

  • Learn within specific VLAN listening to PIM hello messages, PIM join and prune and Bidrectional PIM designated forwarder election
  • Must enable IGMP snooping on switch to use PIM snooping
  • IGMP snooping restricts multicast traffic that exits through the LAN port to which hosts are connected
  • IGMP snooping does not restrict traffic that exists through the LAN ports to which one or more multicast routers are connected


Disabled by default

  • Must use PIM-SM
  • Dense mode is seen as unknown traffic and is dropped
  • Auto-RP groups ( and is always flooded
  • Can enable and disable on per-VLAN basis



  • Ip pim snooping – Global or under vlan interface
  • Show ip pim snooping


Router(config)# ip pim snooping 
Router# show ip pim snooping
Global runtime mode: Enabled
Global admin mode  : Enabled
Number of user enabled VLANs: 1
User enabled VLANs: 10

CCIE RS – Written – L2 Multicast – Explain MLD

Multicast Listener Discovery Protocol


  • ICMP is used to carry messages
  • Messages are link local with hop limit of 1
  • Used by IPv6 to discover multicast listeners on directly attached links
  • PIM-SM is used between deices to track multicast packets to forward to each other
  • PIM-SSM – Ability to report interest in receiving packets from SSM to an IP multicast address
  • Message Types
    • Query – General, group specific and multicast address specific
      • Multicast address field is set to 0 when MLD sends a general query
    • Report – Multicast address field is that of the specific IPv6 multicast address to which the sender is listening
      • Must be sent with a valid IPv6 link local source address or the unspecified (::)
      • Unspecified address (::) is the allow support of Neighbor Discovery Protocol
    • Done – Multicast address field is that of the specific IPv6 multicast address to which the source of the MLD message is no longer listening
  • MLDv1
    • Based on IGMPv2
  • MLDv2
    • Based on IGMPv3
    • Backwards compatible with MLDv1

IPv6 Multicast

  • Ipv6 mutlicast-routing


  • ipv6 mld join-group [group-address] [[include | exclude] {source-address | source-list [acl]} 
  • ipv6 mld access-group access-list-name 
  • ipv6 mld static-group [group-address] [[include| exclude] {source-address | source-list [acl]} 
  • ipv6 mld query-max-response-time seconds 
  • ipv6 mld query-timeout seconds 
  • ipv6 mld query-interval seconds

CCIE RS – Written – L2 Multicast – Implement and Troubleshoot IGMP

Implement and Troubleshoot IGMP

Specifies how a host can register with a router in order to receive specific multicast traffic

  •  To inform a local multicast router that a host wants to receive multicast traffic for a specific group
  •  To inform local multicast routers that a host wants to leave a multicast group (in other words, the host is no longer interested in receiving the multicast group traffic)




RFC 1112

  • Membership reports are issued by hosts that want to receive a specific group (GDA)
  • Membership queries are issued by routers at regular intervals to check whether there is a host interested in the GDA
  • Host membership reports are issued either unsolicited or in response to a membership query
  • Membership queries are sent by routers to all multicast address
      • All multicast capable hosts
    • Host for each group must respond to the query or the router will stop forwarding all traffic for that GDA after 3 attempts
  • IGMPv1 has no leave mechanism

Default Query Interval – 60 Seconds




RFC 2236

  • Host should send a leave message to destination
    • All multicast capable routers




RFC 3376

  • Backwards compatible with v1 and v2
    • V1 membership report
    • V2 membership report and leave group
  • Source Specific Multicast (SSM)



IGMP Snooping

  • Allows switch to listen to IGMP conversations between host and router
  • When switch hears an report for host to multicast group it adds a host port number to the GDA list
  • When it hears a IGMP leave it removed the hosts port from the CAM table entry
  • Learning Router Port
    • IGMP Membership query send to 01-00-5e-00-00-01
    • PIMv1 hello send to 01-00-5e-00-00-02
    • PIMv2 hello send to 01-00-5e-00-00-0d
    • DVMRP probes send to 01-00-5e-00-04
    • MOSPF message send to 01-00-5e-00-05 or 06

Enabled on per-VLAN basis



IGMP Querier

Sends periodic IGMP queries that trigger IGMP report messages from hosts that want to receive IP multicast traffic



IGMP Proxy

Enables hosts in undirectional link routing (UDLP) environment tha are not directly connected to a downstream router to join a multicast group sources from an upstream network

Untitled picture

CCIE RS – Written – L2 – Describe Spanning-Tree Concepts

CCIE RS – Written – L2 – Describe Spanning-Tree Concepts

Compatibility between MST and RSTP

Talked about in the earlier blog post on STP

Common Spanning Tree

  • Interconnecting different MST regions and non-MST
  • Common Instance Spanning-Tree (CIST)
  • Single spanning tree that spans entire switch topology
    • Can have multiple root switches
    • CIST Root – One for entire CIST
    • CIST Regional Root – one per IST inside of each region

STP Dispute

Checks consistency of port role and state in the received BPDU to detect unidirectional links that could cause L2 loops

  • Moves port to discarding state

STP Bridge Assurance

6500 switch optional feature – Enabled by default

  • Supported only by RPVST+ and MST
  • Both ends must have Bridge Assurance enabled
  • Runs only on P2P STP ports
  • BPDU’s are sent out all operational network ports each hello period
    • Including alternate and backup ports