Routing information broadcasts

On a network, routers are constantly exchanging information with each other to make sure that their routing information tables reflect up-to-the-minute changes in the layout of the network. To accomplish this, routers transmit a series of broadcasts from the time they come up until they are brought down. Routing information broadcasts can be classified by the time at which they occur:

• initial broadcast of directly connected network segments

• initial request to receive routing information from other routers

• periodic broadcasts (every 60 seconds) of current list of active network numbers

• broadcast of change in network configuration

• final broadcast when brought down

• Each broadcast must be a local broadcast, addressed so that it will not be passed on by the routers that receive it. This reduces the network traffic created by these information exchanges.
  
  

• Routers must follow a ``best information'' algorithm when providing information to other routers through a broadcast (because the second broadcast listed above is a request for information, this rule does not apply to it).

Best information algorithm

A router sends routing information broadcasts to every network segment to which it is directly connected, as illustrated in Figure 4-11.

Figure 4-11 The best information algorithm

The purpose of routing information broadcasts is to:

• allow a router to share its current impression of the layout of network with other routers.

• inform the routers of a network change so the routers can update their tables.

The first rule of the best information algorithm dictates that a router about to broadcast to a particular network segment should not include any information about other segments that it has received from the particular network segment to which the information is being sent.

For example, if the router within server FS2 in Figure 4-11 is going to broadcast routing information to network segment BB, it will not include information that it received from FS1 about network segment AA. If it did, someone on segment BB might erroneously conclude that there are two paths to segment AA, one through FSl and another through FS2.

The best information algorithm also states that routers should not include information about the network segment to which they are sending routing information broadcasts. For example, FS2 would not include information about BB in its broadcast to BB.

Taking these rules into account, the information that FS2 would broadcast to segment BB would be information about segments CC and DD.

Initial and periodic broadcasts

When a router is first brought up, it:

1. Places the network numbers of its directly connected segments into its routing information table.

2. Sends a routing information broadcast, following the best information algorithm, to inform the routers on its directly connected segments of the segments that the router will be making available.

3. Broadcasts a request to each of its directly connected segments for information about all other network segments that exist on the network. This request is responded to by all the routers (each using the best information algorithm) on these directly connected segments.

4. Places the information from these responses in its routing information table.

Figure 4-12 Sequence used to build and maintain the routing information table

Once the router has performed these initial broadcasts and updated its routing information table, it is ready to accept routing requests and route packets. In addition to routing packets, the routers will broadcast all the information in their routing information table (except that excluded by the best information algorithm) to each of their connected network segments every 60 seconds. Routers perform these periodic broadcasts to make sure that all routers on the network remain synchronized.

Because of the lower bandwidth of X.25 and asynchronous links, routers do not perform 60-second broadcasts on these links; only initial broadcasts, changed information broadcasts, and final broadcasts are sent over these links.

Changed information broadcasts

When a router receives information that causes it to change its routing information table, the router immediately passes that information on to its other directly connected network segments, except the segment from which the router received the information. If a new network segment comes up or an existing segment goes down, all the routers on the network will learn about the change in a short period of time.

The primary causes of a change in the network's configuration are file servers or external routers coming up or going down. If a router needs to be brought down, the router will inform its directly connected segments of the fact before discontinuing service. The router issues broadcasts, using the best information algorithm, that indicate that the network segments that the router had made available will no longer be accessible through this router.

The aging process

If a router goes down due to a hardware failure, power glitch, or power outage, other routers will not be notified that a change has occurred. To safeguard against this eventuality, an ``aging'' mechanism has been built into NetWare routers.

Routers maintain a timer for each entry in their routing information table. Every time that information is received concerning the entry, the timer is reset to zero. If the timer reaches three minutes, the router assumes that the route to the network number is down and broadcasts that fact to its other segments. Because this information is new or changed, the routers that receive the information will pass it on immediately and the change will quickly permeate the network.

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