Enhanced Interior Gateway Routing Protocol is a hybrid routing protocol developed by Cisco systems for routing many protocols across an enterprise Cisco network. It has characteristics of both distance vector routing protocols and link state routing protocols. It is proprietary which requires that you use Cisco routers. EIGRP will route the same protocols that IGRP routes (IP, IPX, Decnet and Appletalk) and use the same composite metrics as IGRP to select a best path destination. As well there is the option to load balance traffic across equal or unequal metric cost paths. Summarization is automatic at a network class address however it can be configured to summarize at subnet boundaries as well. Redistribution between IGRP and EIGRP is automatic as well. There is support for a hop count of 255 and variable length subnet masks.
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IGRP, which stands for Internet Gateway Routing Protocol, is a relatively old routing protocol that was invented by Cisco. It has been largely replaced by the newer and more superior Enhanced-IGRP, more commonly known as EIGRP, since 1993. Even in Cisco the Cisco curriculum, IGRP is only discussed as an obsolete protocol as an introduction to EIGRP.
The main reason behind the advent of EIGRP is to move away from classful routing protocols like IGRP because of the rapidly depleting IPv4 addresses. IGRP simply assumes that all elements in a given class belong to the same subnet. EIGRP utilizes variable length subnet masks (VLSM) to make more efficient use of the short supply of IPv4 addresses, prior to the advent of IPv6 .
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The characteristics of EIGRP follow:
- Hybrid routing protocol (distance vector that has link-state protocol characteristics).
- Use DUAL, first proposed by E. W. Dijkstra and C. S. Scholten, to perform distributed shortest-path routing while maintaining freedom from loops at every instant. Although many researchers have contributed to the development of DUAL, the most prominent work is that of J. J. Garcia-Luna-Aceves.
- Cisco Proprietary created in 1994.
- First released in IOS 9.21
- Uses IP protocol 88.
- Makes Automatic summarization on network Class boundary.
- Classless protocol (supports VLSM).
- Have the power to shut the Auto-summarization And make a configured manual Summarization.
- Default composite metric of bandwidth and delay.
- You can factor load, MTU and reliability into the metric.
- Eigrp metric is the same as IGRP*256, It uses the smallest B.W,Reliablity,Load & MTU with the Comulative delay upon the path…..The MTU doesn’t actually used in the Metric calculations, But is included in the EIGRP Routing updates.
- Sends route updates to multicast address 126.96.36.199, and nei. Reply’s back with Unicast Address.
- Sends non-periodic, partial, and bounded updates.
- Send Hello packets every 5 sec. and Hold down timer is 15 sec.
- For Low speed Hello is every 60 sec. with hold down time 180 sec.
- By default, EIGRP uses no more than 50 percent of the bandwidth of a link.
- Support for authentication via MD5 Only.
- Uses DUAL for loop prevention, and generating Succ./Fesible Succ.
- Maximum paths for Load-balancing are 6 & default is 4 , maximum are 16 in IOS 12.3(2)T and later IOS releases
- By default, Equal-Metric load balancing. If Unequal-Metric load sharing is used the router will load share inversely proportional to the metrics of the paths.
- Administrative distance is 90 for EIGRP internal routes, 170 for EIGRP external routes, and 5 for EIGRP summary routes.
- Potential routing protocol for the core of a network; used in large networks.
- For neighbor relation to be established, both routers must send and receive Hello or Ack packets from each other, they must have the same AS #, and the same Metric K values.
- Eigrp doesn’t restrict that neighbors must have the same Hello & dead interval timers, Unlike OSPF.
- Has a Maximum hop count of 255, the default is 100 in the last IOS releases.
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