Routers and Broadcast Traffic

Internetwork-level broadcasts are Media Access Control (MAC)-level broadcast frames with a special destination internetwork address that informs the router that the packet is to be forwarded to all other networks except the network on which it was received. Routers must be configured to pass internetwork-level broadcast traffic. A MAC-level broadcast frame is used to reach all the hosts on a network. Routers, unlike bridges, do not forward MAC-level broadcast traffic. However, to reach all the hosts on an internetwork, some routable protocols support the use of internetwork-level broadcasts.

The inherent danger of forwarding internetwork-level broadcasts is the possibility of an internetwork-level broadcast storm in which a host malfunctions and continuously sends out the same internetwork-level broadcast packet. If the routers forward this traffic, the result is that all the hosts on the internetwork process each broadcast frame, possibly crippling the entire internetwork.

The NetBIOS over IPX broadcast is an internetwork-level broadcast. NetBIOS applications on an IPX internetwork use a NetBIOS over IPX broadcast to perform name registration, resolution, and release. When the NetBIOS over IPX broadcast packet is received by an IPX router, the router records the network on which the packet was received in the NetBIOS over IPX header. Thus, the internetwork path is recorded in the NetBIOS over IPX header as it traverses the IPX internetwork.

Before being forwarded, the IPX router checks the internetwork path information to prevent the forwarding of the NetBIOS over IPX broadcast onto a network on which it has already traveled. This prevents the broadcast from looping and causing more broadcast traffic. As an additional safeguard, NetBIOS over IPX broadcast packets can only propagate across eight networks using seven routers. At the eighth router, the packet is discarded without notifying the sending host. This is known as a silent discard. For more information about NetBIOS over IPX broadcasts, see "IPX Routing" in this book.



An IPX internetwork path is recorded in a similar fashion to the MAC-sublayer routing information in a Token Ring source routing Explorer frame. However, unlike Token Ring source routing, the IPX internetwork path is not used in the subsequent communication. The IPX internetwork path is only used to prevent the broadcast packet from being forwarded on the same IPX network more than once.