Cisco Systems RJ-45-to-AUX manual Multicast at Layer, Time to Live, Igmp Snooping, 147

Time to Live

Using the IP time to live (TTL) field is an important IGMP topic. The TTL field is a value in an IP packet that tells a network router whether the packet has been in the network too long and should be discarded. A multicast datagram with a TTL of 0 is allowed only on the same host. A TTL of 1 (the default value) is allowed only on the local subnet. If a higher TTL is set, the router is allowed to forward the packet.

By increasing the TTL value, a multicast application can look further from its origin for a server. This process works in the following manner: The host first sends a multicast message with a TTL of 1. If it doesn’t receive a response, it sends a multicast message with a TTL of 2, then 3, and so on, until it reaches its limit. In this manner, the application can locate the nearest server that can be reached.

You can also change the multicast TTL value on a router’s interface to prevent multicast traffic from leaving your network. If you set the TTL value higher than the multicast application’s TTL value, the traffic will never leave the network.

Multicast at Layer 2

You must be wondering by now why we keep talking about routers when this book’s title is Cisco Switching Black Book. Let’s get down to how multicast behaves in a switch environment. As you are well aware, the difference between a hub and a switch is how the device forwards the frames out the ports: A hub will forward a frame out every port except the receiving port, whereas a switch will forward the frame to the ports based on the destination MAC address.

A switch accomplishes this task by comparing the destination MAC address with a forwarding table. This forwarding table is populated by looking at the source MAC addresses of frames sent between every port and keeping track of the port from which the frame arrived. This information creates a forwarding table that is usually kept in Content Addressable Memory (the table is commonly referred to as a CAM table). When the switch receives a frame with a destination MAC address that is not in the forwarding table, it has no choice but to send it out every port in hopes that it will reach its destination. This usually happens in two situations: when the destination MAC address has not been seen before, or when the destination MAC address is a broadcast/multicast address.

If the MAC address has never been seen before, the switch will eventually receive a frame with that MAC address as the source; the address will then be put into the forwarding table. This process takes a little time. If the MAC address is a broadcast/multicast address, the switch will never see a frame with that MAC address as the source address. If you have multicast traffic, this situation will turn your Catalyst 5000 into a very expensive hub. To overcome this problem, Cisco has the following solutions:

∙IGMP snooping

∙Cisco Group Management Protocol (CGMP)

∙Router Group Management Protocol (RGMP)

∙GARP Multicast Registration Protocol (GMRP)

IGMP Snooping

IGMP snooping works as its name implies: The switch snoops on the traffic between the host and the multicast router. When the switch sees an IGMP report from a host for a multicast group, it associates that port with the multicast MAC address in the CAM table. When the switch sees an IGMP leave group message, it removes the port from the association.

Note The source port must also be added to the association for that multicast group in the CAM table.

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