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:
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:
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.
When a router is first brought up, it:
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.
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.
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.