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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Figure 5-16 illustrates the different types of link state advertisements.<br />

Router Links Network Links<br />

Router<br />

- Advertised by router<br />

- Describes state/cost of<br />

router's links<br />

Summary Links<br />

ABR<br />

Area X Area 0<br />

- Advertised by router<br />

- Describes state/cost of<br />

router's links<br />

Figure 5-16 OSPF link state advertisements<br />

Area X<br />

- Advertised by designated router<br />

- Describes all routers attached to<br />

network<br />

External Links<br />

- Advertised by router<br />

- Describes state/cost of<br />

router's links<br />

OSPF packet types<br />

OSPF packets are transmitted in <strong>IP</strong> datagrams. They are not encapsulated<br />

within <strong>TCP</strong> or UDP packets. The <strong>IP</strong> header uses protocol identifier 89. OSPF<br />

packets are sent with an <strong>IP</strong> ToS of 0 <strong>and</strong> an <strong>IP</strong> precedence of internetwork<br />

control. This is used to obtain preferential processing for the packets. We discuss<br />

ToS <strong>and</strong> <strong>IP</strong> precedence further in “Integrated Services” on page 288.<br />

Wherever possible, OSPF uses multicast facilities to communicate with<br />

neighboring devices. In broadcast <strong>and</strong> point-to-point environments, packets are<br />

sent to the reserved multicast address 224.0.0.5. RFC 2328 refers to this as the<br />

AllSPFRouters address. In non-broadcast environments, packets are addressed<br />

to the neighbor’s specific <strong>IP</strong> address.<br />

DR<br />

ASBR<br />

Area 0<br />

Chapter 5. Routing protocols 203

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