This chapter covers the following key topics about Enhanced IGRP (EIGRP):
• EIGRP neighbor relationships
• The Diffusing Update Algorithm (DUAL)
• DUAL finite-state machine
• EIGRP reliable transport protocol
• EIGRP packet format
• EIGRP behavior
EIGRP query process Default routes and EIGRP
• Unequal-cost load balancing in EIGRP
As the size of network grows larger, you can see that the classical distance vector routing protocols such as IGRP and RIP won't scale to the needs of the network. Some of the biggest scalability problems of IGRP and RIP are as follows:
• Full periodic routing updates that consume bandwidth? RIP sends out its entire routing table every 30 seconds; IGRP sends out its entire routing table every 90 seconds. This consumes significant bandwidth.
• RIP hop-count limitation of 15 hops? This limitation makes RIP protocol a non-scalable routing protocol in today's networks because most medium-sized networks have more than 15 routers.
• No support of VLSM and discontiguous networks? This also hinders the capability to scale large networks for RIP and IGRP. Because of this factor, router summarization is not supported.
• Slow convergence time? Because RIP and IGRP send periodic routing updates, a network that is not available in one part of the network could take minutes for the other part of the network to discover that it's no longer available.
• Not 100 percent loop-free? RIP and IGRP do not keep topology tables, so there is no mechanism for them to ensure a 100 percent loop-free routing table.
Because of these shortcomings of IGRP and RIP, Cisco developed an enhanced version of IGRP that not only fixed all the problems of IGRP and RIP but also developed a routing protocol robust enough to scale to today's network growth. This enhanced version is called Enhanced Interior Gateway Routing Protocol (EIGRP).
EIGRP is neither a classic distance vector routing protocol nor a link-state protocol? it is a hybrid of these two classes of routing protocol. Like a distance vector protocol, EIGRP gets its update from its neighbors. Like a link-state protocol, it keeps a topology table of the advertised routes and uses the Diffusing Update Algorithm (DUAL) to select a loop-free path. The convergence time in a network is the time that it takes for all the routers in the network to agree on a network change. The shorter the convergence time is, the quicker a router can adapt to a network topology change. Unlike a traditional distance vector protocol, EIGRP has fact- rnnx/ornonrQ f i mo anH H r\oc nr\f conH fiill na r\r\r]\r~ rr\ i ifinn i i nH |-qc I I n I i Uq a I i n
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