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Home » Certification » Cisco CCNP 642-901 BSCI Exam Tutorial: EIGRP Adjacencies And Secondary Addresses
Cisco CCNP 642-901 BSCI Exam Tutorial: EIGRP Adjacencies And Secondary AddressesI've read some non-Cisco documentation that EIGRP will not allow adjacencies to form when secondary addresses are used. This is incorrect, but there is one common error that can result if both addresses are not secondary. To fully prepare for the 642-R2(config)#interface ethernet0 R2(config-if)#ip address 172.12.23.2 255.255.255.0 R2(config-if)#ip address 23.23.23.2 255.255.255.0 secondary R2(config)#router eigrp 100 R2(config-router)#no auto-summary R2(config-router)#network 23.23.23.0 0.0.0.255 R3(config)#interface ethernet0 R3(config-if)#ip address 172.12.23.3 255.255.255.0 R3(config-if)#ip address 23.23.23.3 255.255.255.0 secondary R3(config)#router eigrp 100 R3(config-router)#no auto-summary R3(config-router)#network 23.23.23.0 0.0.0.255 Here's the partial output of show ip eigrp neighbor on R3: R3#show ip eigrp neighbor IP-EIGRP neighbors for process 100 H Address Interface 0 172.12.23.2 Et0 The adjacency has formed! Note the address is actually the primary IP address on the interface, even though we used the secondary network number in the EIGRP network command. Personally, I stay away from secondary network numbers if at all possible, but you should know that secondary IP addresses can be used to create EIGRP adjacencies. What's the common error with using secondary addresses, you asked? It's when an address from the same subnet is the primary interface address on one neighbor and the secondary interface address on another. Let's say we had configured R2 and R3 as follows: R2(config)#int e0 R2(config-if)#ip address 23.23.23.2 255.255.255.0 R3(config)#interface ethernet0 R3(config-if)#ip address 172.12.23.3 255.255.255.0 R3(config-if)#ip address 23.23.23.3 255.255.255.0 secondary On R2, we get this message: 01:54:05: IP-EIGRP: Neighbor 172.12.23.3 not on common subnet for Ethernet0 (23.23.23.2 255.255.255.0) Since we configured 23.23.23.2 as a primary interface address on R2, the EIGRP process is looking at the primary interface address on potential neighbors. R3's primary ethernet0 address is 172.12.23.2, so you get the "not on common subnet" error message - and what you don't get is an adjacency! That's something to be aware of on your 642-901 BSCI exam as well as when working with EIGRP in production networks. Article Directory: http://www.articledashboard.com Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of over 200 free certification exam tutorials, including CCNA certification training articles. His exclusive CCNA study guide is also available! Visit his blog and sign up for Cisco Certification Central, a daily newsletter packed with CCNA, Network+, Security+, A+, and CCNP certification exam practice questions! A free 7-part course, ?How To Pass The CCNA?, is also available, and you can attend an in-person or online Cisco CCNA training boot camp with The Bryant Advantage! Permalink: http://expert-talk.com/tips/733/cisco-ccnp-642-901-bsci-exam-tutorial-eigrp-adjacencies-and-secondary-addresses-21733.htm Related Tips and Advices
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