Ethernet LAN addressing identifies either individual devices or groups of devices on a LAN. Unicast Ethernet addresses identify a single LAN card or a NIC (network interface card). Each address is 6 bytes long, is usually written in hexadecimal, and, in Cisco devices, typically is written with periods separating each set of four hex digits. For example, 0000.0C12.3456 is a valid Ethernet address. The term unicast addresses, or individual addresses, is used because it identifies an individual LAN interface card. Unicast traffic is used in contrast with the terms broadcast, multicast traffic, and group addresses. See also more detail about IP address design.
Unlike Unicast addresses, Multicast addressesare used to allow a subset of devices on a LAN to communicate. Some applications need to communicate with multiple other devices. By sending one frame, all the devices that care about receiving the data sent by that application can process the data, and the rest can ignore it. The IP protocol supports multicasting.
IP multicast services can provide new feature rich applications which include video conferencing and interactive message collaboration. IP multicast traffic can generate very high bandwidth data-streams which are normally destined for a number of concurrent registered receivers.
Without IP multicast switching enabled, all multicast traffic is unnecessarily flooded to all network devices in the LAN. Each device is then required to process every multicast packet which results in performance degradation in the network and slows processing on every host.
If IP multicast support is not provided, then the use of multicast applications should be avoided. If you should deploy the multicast application, you should follow the minimum requirement with regards to LAN IP Multicast:
- If IP Multicast support cannot be provided then IP Multicast applications and services are to be avoided.
- IP multicast should be enabled on all LAN switches.
- Multicast support is to be provided via either CGMP, IGMP snooping or equivalent protocol.
- IP multicast support must not be enabled on the WAN without analysis.
NOTE: If using CGMP on the switch a CGMP aware multicast router must also be configured to support it on the same LAN. See also LAN topologies.
CGMP (Cisco Group Management Protocol) is a proprietary protocol developed by Cisco Systems to control IP multicast traffic distribution in layer-2 switched networks. A CGMP aware router identifies multicast group receivers and then dynamically programs CGMP capable switches. The MAC-address forwarding table of the switch is programmed such that IP multicast streams are only forwarded to the correct multicast-group receivers.
If using CGMP for IP multicast support then a CGMP aware router is also required. CGMP also requires a multicast routing protocol such as PIM to be configured.
Rich IP multicast based applications (video conferencing) can generate massive traffic streams that have the potential to cripple any low bandwidth network.
Multicast support should be enabled on the LAN, but should not be enabled on any WAN links without proper planning and analysis. Enabling IP multicast support on a LAN involves using CGMP (or an equivalent protocol) on the LAN switch and on the connecting router’s LAN port. If VLANs have been configured, IP multicast support will need to be configured on a per VLAN basis. The CGMP aware router also requires an IP multicast routing protocol such as PIM (Protocol Independent Multicast) to be configured. See also IP routing guide.
IGMP snooping is an all-in-one hardware based method for performing intelligent IP multicast forwarding. It uses a superior method as it very quick to identify, maintain and when required terminate IP multicast streams. It works by listening to multicast IGMP messages generated by multicast clients and then dynamically configures the switches so that traffic streams are directed to only those devices that are a part of the correct multicast-group.
A multicast routing protocol is required to build a multicast distribution tree. Many such protocols exist, each with its own advantages and disadvantages. However PIM (Protocol Independent Multicast) is the most accepted protocol due to it being independent of the underlying IP routing protocol and its ability to build economically scaled distribution pathways. There are two operating modes for PIM, dense and sparse mode, with sparse mode being the preferred choice.
To support CGMP on a router, PIM must be configured on a per interface basis. The recommended version of PIM to use is PIM Sparse mode. Do not enable multicast support on WAN interfaces due to the risk of multicast meltdown and WAN link saturation. A single high-quality multicast video stream can be very bandwidth intensive and can easily oversubscribe any low speed WAN links. Enabling IP multicast applications on a LAN that doesn’t support intelligent multicast switching will result in all traffic streams being flooded throughout the LAN. The result of which will cause network saturation, slow network response times and client utilization problems.
Enabling IP multicast in the WAN must be carefully planned. It should never be enabled over slow WAN links due to link saturation
HP ProCurve Switch 4000M 10/100 40-Port w/5 Module Slots ( J4121A#ABA )
This is a feature-rich, modular 10/100/Gigabit switch that provides scalable, low-cost switching and all the benefits of HP Proactive Networking.It is ideal for medium-to-large businesses looking for scalable, expandable, low-cost migration to 10/100/Gigabit switching to the desktop. It features easy installation with 40 autosensing 10/100 ports that support any combination of 10Mbps and 100Mbps connections with no configuration required and with open universal slots that accept any combination of modules, advanced monitoring with RMON (4 groups) and RMON extensions, IEEE 802.1Q VLAN tagging which puts a secure firewall between workgroups, port mirroring which lets you monitor any combination of ports with a single RMON probe, and IP multicast (IGMP) software that prevents the flooding of multicast video traffic and supports QoS for IP multicast traffic.
See also the following:
- Layer 3 switching devices
- LAN Switch 24 ports
- Understanding STP – spanning tree protocols
- ISDN E1 and T1
- Understanding ATM WAN
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