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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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In this figure, the ASBR is advertising a single summary route for the 64<br />

subnetworks located in the R<strong>IP</strong> environment. This single summary route is<br />

flooded throughout the entire OSPF network. In addition, the ABR is generating a<br />

single summary route for the 64 subnetworks located in area 1. This summary<br />

route is flooded through area 0 <strong>and</strong> area 2. Depending of the configuration of the<br />

ASBR, the inter-area summary route can also be redistributed into the R<strong>IP</strong><br />

network.<br />

5.7 Enhanced Interior Gateway Routing Protocol<br />

(EIGRP)<br />

The Enhanced Interior Gateway Routing Protocol (EIGRP) is categorized as a<br />

hybrid routing protocol. Similar to a distance vector algorithm, EIGRP uses<br />

metrics to determine network paths. However, like a link state protocol, topology<br />

updates in an EIGRP environment are event driven.<br />

EIGRP, as the name implies, is an interior gateway protocol designed for use<br />

within an AS. In properly designed networks, EIGRP has the potential for<br />

improved scalability <strong>and</strong> faster convergence over st<strong>and</strong>ard distance vector<br />

algorithms. EIGRP is also better positioned to support complex, highly redundant<br />

networks.<br />

EIGRP is a proprietary protocol developed by Cisco Systems, Inc. At the time of<br />

this writing, it is not an IETF st<strong>and</strong>ard protocol.<br />

5.7.1 Features of EIGRP<br />

EIGRP has several capabilities. Some of these capabilities are also available in<br />

distance vector or link state algorithms.<br />

► EIGRP maintains a list of alternate routes that can be used if a preferred path<br />

fails. When the path fails, the new route is immediately installed in the <strong>IP</strong><br />

routing table. No route recomputation is performed.<br />

► EIGRP allows partial routing updates. When EIGRP discovers a neighboring<br />

router, each device exchanges their entire routing table. After the initial<br />

information exchange, only routing table changes are propagated. There is no<br />

periodic rebroadcasting of the entire routing table.<br />

► EIGRP uses a low amount of b<strong>and</strong>width. During normal network operations,<br />

only hello packets are transmitted through a stable network.<br />

► EIGRP supports supernetting (CIDR) <strong>and</strong> variable length subnet masks<br />

(VLSM). This enables the network administrator to efficiently allocate <strong>IP</strong><br />

address resources.<br />

212 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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