Magoo's Wise Words
CCNP Study guides

Posted:
23 Aug 07

 

 

 


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Multicast

 

  1. Types

    1. One to many

    2. many to one

    3. many to many

    4. few to many, many to few

  2. Advantages & disadvantages

    1. Advantages

      1. Efficiency

      2. enables distributed apps

    2. Disadvantages - UDP

      1. best effort delivery

      2. no congestion control

      3. Duplicate packets

      4. out of sequence delivery

  3. IP Ranges

    1. Aggregate Range - binary 1110 = 224:: - 239.255..

    2. Reserved local - 224::/24 - Reserved by IANA

      1. 224::1 - All hosts

      2. 224::2 - All multicast routers

      3. 224::4 - All Distance Vector multicast routers

      4. 224::5 - All OSPF routers

      5. 224::6 - All OSPF DR's

      6. 224::9 - All RIPv2 routers

      7. 224::10 - All EIGRP routers

    3. Global dynamic scope - 224.0.1.0 - 238.255..

      1. 224.2/16 - Mbone

    4. Limited scope - 239::/8

      1. Site local - 239.255/16

      2. Organization local - 239.192:: - 239.251.255..

  4. Multicast on Layer 2

    1. Bit 0 of 1st octet in MAC set to 1 - broadcast/multicast bit

    2. 0100.5e:: - 100.5e7f.ffff available from IANA for multicast MAC addresses

    3. map lower 23 bits of multicast IP address to MAC address

    4. creates an overlap of 32 IP's per MAC, but unneeded packets are simply discarded on Layer 3

  5. Joining a multicast session

    1. Finding sessions

      1. Well-know multicast groups can advertise other available groups

      2. directory services can be used to list groups

      3. Web pages could list groups as URL's

      4. Email announcements could announce a group

      5. SDP - session directory protocol

    2. Session Directory

      1. Client application runs on user's pc

      2. listens to either Session Description Protocol or Session Announcement Protocol (also call sdr)

      3. Handles session description and its announcement

      4. transport of session announcement via well-know multicast groups (224.2.127.254)

      5. Handles creation of new sessions

      6. Avoids address conflicts by consulting cache of active groups and selecting unused address

      7. SDR packets can be carried many ways

        1. SAP

        2. SIP (yes, the voice one)

        3. RTSP - real time streaming protocol - allows VCR like controls

        4. Email

        5. Web pages

  6. IGMP and CGMP

    1. IGMP is used between a host and a router so routers know which network segments have interested hosts for a session

    2. IGMPv1

      1. routers send periodic membership queries to all-hosts address

      2. interested hosts send membership reports to the multicast address of the groups they want to join

      3. can also send an unsolicited membership report in order to join a group

      4. only one host per segment sends report for each group

      5. no mechanism for hosts to leave a group - results in extra traffic between the time when hosts leave the group and when the membership expires

    3. IGMPv2

      1. Improvements

        1. Group-specific queries

        2. leave group message

        3. designated querier election mechanism

        4. query-interval response time - controls burstiness

      2. Group specific queries sent to group multicast address instead of all hosts

      3. router with the lowest IP is designated querier for IGMP

      4. sh ip igmp group - shows active groups and last reporting host

      5. Leaving a group

        1. Host sends leave group message to router

        2. router sends group membership query to see if there are other hosts interested

    4. IGMPv3

      1. Proposed standard

      2. adds the ability to filter multicasts based on source

      3. Host sends report to 224::22 after joining, which could include a source list

    5. show ip igmp interface interface - shows which version is active

  7. Multicast with Layer 2 switches - frames would be flooded unless controls are used

    1. Manually associate multicast MAC addresses with various switch ports

    2. Use CGMP or IGMP snooping

    3. CGMP

      1. Cisco proprietary

      2. Router informs switch of multicast memberships and switch forwards only to those ports

      3. Maps multicast MAC to actual MAC

      4. uses well-know CGMP MAC of 0100.0cdd.dddd

    4. IGMP snooping

      1. Switch listens to IGMP transactions to learn about multicast hosts and what port they are on

      2. listens to join messages and statically maps the multicast group to the host port and all router ports

  8. PIM routing protocol (Protocol Independent Multicast)

    1. Used by multicast routers to route multicast packets

    2. Uses normal routing table, so it is independent of the unicast routing protocol running

    3. Two distribution tree types - Source and Shared

      1. Source Tree -

        1. Root at the multicast source

        2. separate tree for each multicast source

      2. Shared Tree -

        1. single tree shared by all multicast sources for each group.

        2. Has common root at the rendezvous point (RP).

        3. Traffic flows from the source to the RP, then out to the network

    4. Reverse Path forwarding - the routing table is used to forward multicast packets away from the source rather than toward the destination (as in unicast). This avoids loops by ensuring that multicast traffic is not sent back toward the source.

    5. PIM modes

      1. Sparse mode

        1. uses a pull model to send traffic.

        2. uses a shared tree

        3. sources register with the RP

        4. Default forwarding address is notated (*,G)

        5. consume less router CPU and memory because there is only one route, but may result in suboptimal routing and extra delay

        6. Preferred over dense mode for all production networks.

      2. Dense Mode

        1. uses a push model that floods traffic to the entire network

        2. Uses source trees

        3. Forwarding state notated by (S,G)

        4. data is initially flooded to entire network through all non-RPF interfaces. Routers that have no need for multicast data request that the tree is pruned. (S,G) state remains in all routers. Flood and prune repeats every 3 mins

        5. Takes more memory because there is a separate entry for each source, but minimizing delay in packet delivery

      3. Sparse-Dense mode

        1. Allows a router to operate in sparse mode for sparse groups and dense mode for dense groups at the same time

        2. multiple RP's can be implemented, each in an optimum location

        3. supports automatic selection of RP's for each source

        4. Cisco recommended best mode

    6. Optimizations to PIM

      1. Bidirectional PIM - allows for many-to-many

      2. Source Specific Multicast - variant of Sparse Mode that builds only source specific SPF trees and does not need an active RP for source-specific groups (232::/8 range)

    7. Configuring PIM-SM and PIM S-D

      1. Enable ip multicast-routing global command

      2. Use ip pim sparse-mode interface command, or ip pim sparse-dense-mode

      3. Configure the ip pim send-rp-announce interface scope ttl group-list access-list global command to make a router RP

        1. Sends auto-RP message to 224.0.1.39 address announcing the router as an RP candidate

        2. candidate for groups in the range described by the access list

        3. candidate within a hop radius dictated by ttl

        4. address of the specified interface is used as the RP address

      4. ip pim send-rp-discovery interface type scope ttl command sets the router as an RP mapping agent.

        1. listens to 224.0.1.39 and sends a RP-to-group mapping to 224.0.1.40 within a hop radius of ttl.

        2. Other PIM routers listen to 224.0.1.40 to automatically discover the RP.

      5. PIM v. 2 supports an open standard auto-rp

        1. ip pim bsr-candidate int mod/num hash-mask-length [priority] - makes this router a bootstrap router candidate

        2. ip pim rp-candidate int ttl - configures this router as an RP candidate

      6. ip pim spt-threshold {rate | infinity }command controls win a PIM leaf router will switch from shared tree to source tree.

  9. References

    1. Cisco Press BSCI Official Exam Certification Guide

    2. Cisco Press BSCI Authorized Self-Study Guide

    3. cisco.com

    4. RFC 2236

Back to IPv6 - Up to index - Next to EIGRP
Updated:
23 Aug 07

 

 

 


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