Multicast Introduction

Traditional IP communications allow a host to send packets to another host (unicast transmissions) or to all hosts (broadcast transmissions). IP Multicast provides a third communication alternative: allowing a host to send packets to a group that is made up of a subset of the hosts on the network. IP Multicast is a bandwidth-conserving technology specifically designed to reduce traffic by simultaneously delivering a single stream of information to potentially thousands of corporate recipients or homes. By replacing copies for all recipients with the delivery of a single stream of information, IP Multicast is able to minimize the burden on both sending and receiving hosts and reduce overall network traffic. Within a multicast network, routers are responsible for replicating and distributing multicast content to all hosts that are listening to a particular multicast group. Cisco? routers employ Protocol Independent Multicast (PIM) to build distribution trees for transmitting multicast content, resulting in the most efficient delivery of data to multiple receivers.

Alternatives to IP Multicast require the source to send more than one copy of the data. Traditional application-level unicast, for example, requires the source to transmit one copy for each individual receiver in the group.

Mutlicast Applications and Environments

IP Multicast solutions offer benefits relating to the conservation of network bandwidth. In the case of a high-bandwidth application, such as MPEG video, IP Multicast can benefit situations with only a few receivers because a few video streams would otherwise consume a large portion of the available network bandwidth. Even for low-bandwidth applications, IP Multicast conserves resources when transmissions involve thousands of receivers. Additionally, IP Multicast is the only nonbroadcasting alternative for situations that require simultaneously sending information to more than one receiver.

For low-bandwidth applications, an alternative to IP Multicast could involve replicating data at the source. This solution, however, can deteriorate application performance, introduce latencies and variable delays that impact users and applications, and require expensive servers to manage the replications and data distribution. Such solutions also result in multiple transmissions of the same content, consuming an enormous amount of network bandwidth. For most high-bandwidth applications, these same issues make IP Multicast the only viable option.

Course Objectives

• Determine the need for IP multicasting services
• Explain the IP multicast model and its applications
• Describe multicast distribution trees and configure Protocol Independent Multicast (PIM) protocol
• List the solutions for IP multicast reliability
• Identify and solve the problems of IP multicast in switched LAN environment and in WAN networks
• Deploy IP multicast in network scenarios

IP Multicasting Concepts

• Determine IP multicasting services and implementation needs
• List various types of  multicast applications and determine their requirements
• Identify the IP multicast conceptual model and its implementation details
• Explain the issues of multicast technologies on data-link layer

IP Multicast Addresses

Multicast addresses specify an arbitrary group of IP hosts that have joined the group and wish to receive traffic sent to this group.

Internet Group Management Protocol (IGMP)

IGMP is used to dynamically register individual hosts in a multicast group on a particular LAN. Hosts identify group memberships by sending IGMP messages to their local multicast router. Under IGMP, routers listen to IGMP messages and periodically send out queries to discover which groups are active or inactive on a particular subnet.

Multicast in the Layer 2 Switching Environment

The default behavior for a Layer 2 switch would be to forward all multicast traffic to every port that belongs to the destination LAN on the switch. This would defeat the purpose of the switch, which is to limit traffic to the ports that need to receive the data.

There are two methods to deal with multicast in a Layer 2 switching environment efficiently¡ªCisco Group Management Protocol (CGMP) and IGMP Snooping.

• Configure Cisco routers and LAN switches for CGMP, IGMP snooping.
• Troubleshoot Cisco routers and LAN switches for IP multicast in switched LAN networks
• Identify the problems of IP multicast on a data link layer of NBMA networks
• Configure and troubleshoot PIM NBMA mode on Cisco routers

Multicast Distribution Trees

Multicast capable routers create distribution trees that control the path which IP Multicast traffic takes through the network in order to deliver traffic to all receivers. The two basic types of multicast distribution trees are source trees and shared trees.

• Source Trees
• Shared Trees

Multicast Forwarding

In multicast routing, the source is sending traffic to an arbitrary group of hosts that are represented by a multicast group address. The multicast router must determine which direction is upstream (towards the source) and which direction (or directions) is downstream. If there are multiple downstream paths the router will replicate the packet and forward it down the appropriate downstream paths¡ªwhich is not necessarily all paths. The concept of forwarding multicast traffic away from the source, rather than to the receiver, is called Reverse Path Forwarding.

Reverse Path Forwarding (RPF)

RPF is a fundamental concept in multicast routing that enables routers to correctly forward multicast traffic down the distribution tree. RPF makes use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will only forward a multicast packet if it is received on the upstream interface. This RPF check helps to guarantee that the distribution tree will be loop free.

RPF Check

When a multicast packet arrives at a router, the router will perform an RPF check on the packet. If the RPF check is successful, the packet will be forwarded. Otherwise it will be dropped.

Protocol Independent Multicast (PIM)

PIM gets its name from the fact that it is IP routing protocol independent. PIM can leverage whichever unicast routing protocols are used to populate the unicast routing table including EIGRP, OSPF, BGP or static routes. PIM uses this unicast routing information to perform the multicast forwarding function, therefore it is IP protocol independent. Although PIM is called a multicast routing protocol it actually uses the unicast routing table to perform the Reverse Path Forwarding (RPF) check function instead of building up a completely unrelated multicast routing table. PIM does not send and receive multicast routing updates between routers like other routing protocols.

1. PIM Dense Mode (PIM-DM)

• Explain the principles and detailed operation of PIM DM
• Identify the roles of various PIM DM control packets
• Identify the deficiencies of PIM DM
• Configure and troubleshoot Cisco routers for PIM Dense mode

2. PIM Sparse Mode (PIM-SM)

• Explain the principles and detailed operation of PIM SM
• Identify the roles of various PIM SM control packets
• Configure and troubleshoot PIM SM in nonredundant deployment
• Describe the variants of PIM SM (bidirectional PIM and Source Specific Multicast)

3. Sparse-dense Mode

4. Bidirectional PIM (Bidir-PIM)

Multiprotocol Border Gateway Protocol (MBGP)

MBGP provides a method for providers to distinguish which route prefixes they will use for performing multicast RPF checks. The RPF check is the fundamental mechanism that routers use to determine the paths that multicast forwarding trees will follow and successfully deliver multicast content from sources to receivers.

MBGP is based on RFC 2283, Multiprotocol Extensions for BGP-4. Since MBGP is an extension of BGP it brings along all the administrative machinery that providers and customers like in their inter-domain routing environment. Including all the inter-AS tools to filter and control routing (e.g. route maps). Therefore, by using MBGP, any network utilizing internal or external BGP can apply the multiple policy control knobs familiar in BGP to specify routing (and thereby forwarding) policy for multicast.

Multicast Source Discovery Protocol (MSDP)

In the PIM Sparse mode model, multicast sources and receivers must register with their local Rendezvous Point (RP). Actually, the closest router to the sources or receivers registers with the RP but the point is that the RP knows about all the sources and receivers for any particular group. RPs in other domains have no way of knowing about sources located in other domains. MSDP is an elegant way to solve this problem. MSDP is a mechanism that connects PIM-SM domains and allows RPs to share information about active sources. When RPs in remote domains know about the active sources they can pass on that information to their local receivers and multicast data can be forwarded between the domains. A nice feature of MSDP is that it allows each domain to maintain an independent RP which does not rely on other domains but it does enable RPs to forward traffic between domains.

The RP in each domain establishes an MSDP peering session using a TCP connection with the RPs in other domains or with border routers leading to the other domains. When the RP learns about a new multicast source within its own domain (through the normal PIM register mechanism), the RP encapsulates the first data packet in a Source Active (SA) message and sends the SA to all MSDP peers. The SA is forwarded by each receiving peer using a modified RPF check, until it reaches every MSDP router in the interconnected networks¡ªtheoretically the entire multicast internet. If the MSDP peer is an RP, and the RP has a (*,G) entry for the group in the SA (there is an interested receiver), the RP will create (S,G) state for the source and join to the shortest path tree for the source. The encapsulated data is decapsulated and forwarded down that RP's shared tree. When the packet is received by a receiver's last hop router, the last-hop may also join the shortest path tree to the source. The source's RP periodically sends SAs which include all sources within that RP's own domain. Figure 12 shows how data would flow between a source in domain A to a receiver in domain E.

MSDP was developed for peering between Internet Service Providers (ISPs). ISPs did not want to reply on an RP maintained by a competing ISP to service to their customers. MSDP allows each ISP to have their own local RP and still forward and receive multicast traffic to the Internet.

Source Specific Multicast SSM

SSM is extension of the PIM protocol than allows for an efficient data delivery mechanism in one-to-many communications. SSM enables a receiving client, once it has learned about a particular multicast source through a directory service, to then receive content directly from the source, rather than receiving it using a shared RP.