TCP IP Illustrated

These advertisements are not exactly periodic but are

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Unformatted text preview: starts up it transmits periodic advertisements on all interfaces capable of broadcasting or multicasting. These advertisements are not exactly periodic, but are randomized, to reduce the probability of synchronization with other routers on the same subnet. The normal time interval between advertisements is between 450 and 600 seconds. The default lifetime for a given advertisement is 30 minutes. Another use of the lifetime field occurs when an interface on a router is disabled. In that case the router can transmit a final advertisement on the interface with the lifetime set to 0. In addition to the periodic, unsolicited advertisements, a router also listens for solicitations from hosts. It responds to these solicitations with a router advertisement. If there are multiple routers on a given subnet, it is up to the system administrator to configure the preference level for each router as appropriate. For example, the primary default router would have a higher preference than a backup. Host Operation Upon bootstrap a host normally transmits three router solicitations, 3 seconds apart. As soon as a valid advertisement is received, the solicitations stop. A host also listens for advertisements from adjacent routers. These advertisements can cause the host's default router to change. Also, if an advertisement is not received for the current default, that default can time out. As long as the normal default router stays up, that router will send advertisements every 10 minutes, with a lifetime of 30 minutes. This means the host's default entry won't time out, even if one or two advertisements are lost. Implementation The router discovery messages are normally generated by and processed by a user process (a daemon). This adds yet another program updating the routing table in Figure 9.1, although it would only add or delete a default entry. The daemon would have to be configured to act as a router or a host. These two ICMP messages are new and not supported by all systems. Solaris 2.x is the only system in our network that supports these messages (the in.rdisc.daemon). Although the RFC rec...
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