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Networking Routing Believability and Administrative Distance by Anthony Sequeira

If a network is running more than one routing protocol (maybe as a result of a corporate merger), and a router receives two route advertisements from different routing protocols for the same network, which route advertisement does the router believe? Interestingly, some routing protocols are considered to be more believable than others. For example, a Cisco router would consider EIGRP to be more believable than RIP.

The index of believability is called administrative distance (AD). Table 10-1 shows the AD values for various sources of routing information. Note that lower AD values are more believable than higher AD values.

Routing Information Source AD value
Directly connected network 0
Statically configured network 1
EIGRP 90
OSFP 110
RIP 120
External EIGRP 170
Unknown or unbelievable 255

You can configure a router to prefer RIP routes over IGRP using the distance command as shown below.

R1(config)#router rip R1(config-router)#distance 80

This will configure router R1 with an administrative distance of 80 for RIP protocol.

About The Author

Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, StormWind.com. Anthony is also a VMware Certified Professional.

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