Multicast traffic is typically sent by one source and received by a group of recipients that  migh be spread throughout a network and that might change over time. A typical example of multicast traffic is video stream.

Types of packets:

  • Unicast: Packets that are sent from one source address to a single destination host address.

Unicasts are forwarded by a router or Layer 3 switch by finding the destination IP address in its routing table. A Layer 2 switch relies on the destinationś MAC address. Unicast forwarding is turned on by default, and is the type  of routing familiar to you already.

  • Broadcast: Packets sent to a broadcast destination address.

Broadcasts are one way to comunicate the exact same information to a group. Broadcasts are single transmissions that are received and acted upon by all devices.Is useful  when a destination address is unknown.

Ehernet broadcast are sent to the reserved MAC address FFFF.FFFF.FFFF. Layer 2 switches flood broadcasts out all ports in the same VLAN. IP broadcast use the reserved destination IP Routers do not forward broadcasts by default.

  • Multicast:  Packets sent to a special group-based destination address.

Multicast addresses are “group” addresses. An IP device joins a group by recognizing group IP addresses and reprogramming its network interface card(NIC) to cop traffic destined for the group MAC. Because a multicast goes to a different MAC, some hosts will pay attention to it and others will ignore it. For example, EIGRP uses and corresponding MAC 0100.5E00.000A Routers pay attention to this traffic, but your PC can safaly ignore it.

Multicast traffic is generally unidirectional and sent in a best-effort connectionless format. UDP(connectionless) is commonly used, whereas TCP(connection-oriented) is not. By default, Layer 2 switches flood Ethernet multicast to all ports on the destination VLAN.

Multicast Addressing.

End systems and intermediate devices must have a way to distinghish multicast traffic from unicasts or broadcasts.

At Layer 3, this is done by reserving class D IP addresses( through for multicasting. Network devices can quickly distinguish multicast IP  addresses by looking at the first four bits, which are always 1110. Ethernet devices similarly have a range of addresses set aside for multicast. The low-oder bit in the first byte of a MAC address is a unicast/multicast bit, and setting it indicates the frame is a multicast. Beyond this single bit, IP multicast are mapped to a specific range of MAC addresses to aid hosts in discriminating between multicasts.

Multicast IP Addressing

In addition to the Class D multicast address space, some IP multicast addresses have been reserved for particular uses, such as the following:

  • Link-local addresses( – Used on a local segment (TTL=1) only. Routers do not forward these packets because of TTL. These are known as fixed-group addresses because they are well-known and predefined. Examples:

Address —————————————- Destination ————————————– All hosts ————————————– All OSPF routers ————————————– All OSPF DRs ————————————– All RIPv2 routers ———————————— All EIGRP routers

  • Source-specific multicast( – An extension of multicasting wherein hosts only receive traffic from a particular server instead of from any server using a multicast channel.
  • GLOP ( – Allocates 256 multicast IP addresses to each registered autonomous system (AS). The 16-bit AS number  is used for the middle-two octets, so that AS 1000 hast
  • Administratively scoped addresses ( – This space can be used in private multicast domains, much like the private IP address ranges from RFC 1918. These addresses are not supposed to be routed between domains; this way, they can be reused.

Within the administratively scoped addresses, is reserved for site-local multicast and the rest of is reserved for organization-local scope.

* Globally scoped addresses ( and – These addresses can be used by aany entity and can be routed across an organization or the Internet. They must be unique and globally significant, and therefore are usually temporaly assigned( think of this range as neither local nor private; it is the rest  of the multicast range).
Multicast Issues
There are downsides to multicasting that must be recognized. Because multicast traffic is generally UDP, it lacks windowing functions, packet ordering, and capability for retransmission. It also lacks the abaility to allow some users to receive multicast while preventing others  on the segment (although there are ways to block multicasts at the router).
These challenges can be met creatively  at the protocol level. For multimedia traffic, ordering is handled by the Real Time Protocol(RTP). Security could be addressed by encrypting the traffic using IPSec.
Source: CCNP BSCI  – Official Exam Certification Guide by Stewart Gough