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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Area 1<br />

Area 0<br />

Figure 5-18 OSPF virtual link <strong>and</strong> transit areas<br />

This diagram shows that area 1 does not have a direct connection to the<br />

backbone. Area 2 can be used as a transit area to provide this connection. A<br />

virtual link is established between the two ABRs located in area 2. Establishing<br />

this virtual link logically extends the backbone area to connect to area 1.<br />

A virtual link is used only to transmit routing information. It does not carry regular<br />

traffic between the remote area <strong>and</strong> the backbone. This traffic, in addition to the<br />

virtual link traffic, is routed using the st<strong>and</strong>ard intra-area routing within the transit<br />

area.<br />

5.6.4 OSPF route redistribution<br />

Route redistribution is the process of introducing external routes into an OSPF<br />

network. These routes can be either static routes or routes learned through<br />

another routing protocol. They are advertised into the OSPF network by an<br />

ASBR. These routes become OSPF external routes. The ASBR advertises these<br />

routes by flooding OSPF AS external LSAs throughout the entire OSPF network.<br />

208 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

ABR<br />

ABR<br />

Virtual link<br />

Area 2<br />

Transit Area

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