
Configuring IP, ARP, RARP, RIP, and OSPF Services
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• OSPF considers the cost of a path, not the hop count, when choosing the best
path. Each interface, however, is assigned the default cost 1 for the path to
which it interfaces. If you have a preferred path, you must change the metric
cost for your interfaces. Assign a higher metric cost to those paths that are the
least preferred paths.
• If BGP is also configured on the router, the OSPF and BGP router IDs must be
set to the same IP address.
• If you change the topology (for example, if you add an area, combine two
areas, or move routers), you must reconfigure the appropriate OSPF elements
(OSPF area ranges/interfaces/neighbors/virtual links, and so on).
Router Discovery Protocol
Before a host can send IP datagrams beyond its directly attached subnet, the host
must discover the address of at least one operational router on that subnet. Router
Discovery is an extension of the Internet Control Message Protocol (ICMP) that
enables hosts attached to multicast or broadcast networks to discover the IP
addresses of their neighboring routers.
Routers configured with Router Discovery periodically multicast or broadcast a
route advertisement from each of their interfaces, announcing the IP address or
addresses of that interface. Hosts discover the addresses of their neighboring
routers by listening for these advertisements. Hosts will use the router with the
highest preference level as a gateway.
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