| Junos Router | Enhanced Interior Gateway Routing Protocol | | Print | |
EIGRP is sometimes said to be a "DV protocol that thinks it's an LS." EIGRP does in fact share some of the characteristics normally associated with LS routing, including rapid convergence and loop avoidance, but the lack of LSA flooding and the absence of the resulting LSDB expose EIGRP's true DV nature. This section highlights the major operational characteristics and capabilities of EIGRP. The goal is not an exhaustive treatment of EIGRP's operation or configuration—this subject has been covered in numerous other writings. Instead, the purpose here is to understand EIGRP to the degree necessary to effectively replace this proprietary legacy protocol with another IGP, while maintaining maximum network availability throughout the process.
The operational characteristics of EIGRP are as follows:
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It uses nonperiodic updates that are partial and bounded. This means that unlike typical DV protocol operation, EIGRP generates only triggered updates, that these updates report only affected prefixes, and that the updates are sent to a bounded set of neighbors.
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It uses a Diffusing Update Algorithm (DUAL) to guarantee a loop-free topology while providing rapid convergence. The specifics of DUAL operation are outside the scope of this book; suffice it to say that DUAL is the muscle behind EIGRP's rapid converge and loop guarantees.
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It uses a composite metric that, by default, factors delay and throughput. Also, it supports the factoring of dynamically varying reliability and loading, but users are cautioned not to use this capability. EIGRP uses the same metric formula as IGRP, but it multiplies the result by 256 for greater granularity.
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It supports VLSM/CIDR and automatic summarization at classful boundaries by default.
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It supports unequal cost load balancing using a variance knob.
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It supports neighbor discovery and maintenance using multicast.
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It automatically redistributes to IGRP when process numbers are the same.
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It features protocol-independent modules for common functionality (reliable transport of protocol messages).
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It features protocol-dependant modules for IP, IPX, and AppleTalk that provide multiprotocol routing via the construction of separate route tables using protocol-specific routing updates.
At first glance, the multiprotocol capabilities of EIGRP may seem enticing. After all, this functionality cannot be matched by today's standardized routing protocols. There was a time when many enterprise backbones were in fact running multiple network protocols, and the lure of a single, high-performance IGP instance that could handle the three most common network suites was hard to resist. However, there has been an unmistakable trend toward IP transport for virtually all Internetworking suites, including IBM's SNA/SAA. (We have a hard time recalling the last time we knew of an enterprise still deploying the native Netware or AppleTalk transport protocols.) In contrast, these proprietary-routed protocols are being phased out in favor of native IP transport, which serves to render EIGRP's multiprotocol features moot in this modern age of IP internetworking.
EIGRP can load-balance across paths that are not equal in cost, based on a variance setting, which determines how much larger a path metric can be as compared to the minimum path metric, while still being used for load balancing. This characteristic remains unique to IGRP/EIGRP given that neither RIP nor OSPF supports unequal cost load balancing.
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