ZyXEL Communications Corporation P873HNUP51B ユーザーズマニュアル

Chapter 13 IGMP

P-873HNUP-51B User’s Guide

To allow better network performance, you can use IGMP proxy instead of a multicast routing 
protocol in a simple tree network topology. 

Note: Your ZyXEL Device is an IGMP proxy.

In IGMP proxy, an upstream interface is the port that is closer to the source (or the root of the 
multicast tree) and is able to receive multicast traffic. There should only be one upstream interface 
(also known as the query port) for one query VLAN on the ZyXEL Device. A downstream interface is 
a port that connects to a host (such as a computer).

The following figure shows a network example where A is the multicast source while computers 1, 2 
and 3 are the receivers. In the figure A is connected to the upstream interface and 1, 2 and 3 are 
connected to the downstream interface.

Figure 77   IGMP Proxy Network Example

The ZyXEL Device will not respond to IGMP join and leave messages on the upstream interface. The 
ZyXEL Device only responds to IGMP query messages on the upstream interface. The ZyXEL Device 
sends IGMP query messages to the hosts that are members of the query VLAN.

The ZyXEL Device only sends an IGMP leave message via the upstream interface when the last host 
leaves a multicast group.

The router alert option provides a way to let routers intercept packets not addressed to them 
directly, without incurring any significant performance penalty. The router alert option in the IP 
header of an IGMP control packet tells the router to examine the packet more closely for routing 
information. Regular data packets do not receive the extra checking and are forwarded with little or 
no performance penalty. IGMP v2 and IGMP v3 both require the router alert option while IGMP v1 
does not use it at all. See RFC 2113 for more information.