Even Jeff Doyle tells me that I should do a little blogging with OSPF. What's a networker to do with that kind of backing? ;-) And a few others expressed some interest as well - in understanding LSAs better, in multi-area designs, in deciphering some of the show commands, and in some of the oddball cases.
When prepping for a Cisco exam that covers OSPF - which is several of them - you need be ready to notice whether a pair of routers that share the same layer 2 network (same VLAN, same FR PVC, same pt-pt link...) can become neighbors. Once the two routers are neighbors, they can exchange LSAs (or not), depending on other factors. But as the learning theorists would say, becoming neighbors is necessary but not sufficient for routers to use OSPF to learn routes.
The problem is that if you know how to learn how to configure OSPF correctly, you avoid the issues that prevent OSPF neighborships. But for Cisco exams, you need to be ready to analyze existing config problems in some questions. There are items that must match between two routers, and just as important, items that appear to impact neighborship but do not.
So, if you haven't looked at my last post, and want to give it a shot before looking at the list of things that prevent OSPF neighborship, then go here now. When you're ready, go to page 2 of today's post for the list and the explanation of that part of my last post.
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Odom, CCIE No, 1624, splits time between writing books for Cisco Press and teaching classes for Skyline ATS. In his 25-ish years in the networking industry, he has worked as as a pre-sale and post-sale SE for a few networking vendors, as well as a network engineer implementing network technology. Wendell has spent the majority of the last 15 years teaching, consulting, and writing about networking technologies, most of which in some way relate to Cisco products. His books include titles on QoS, CCIE R/S, as well as several titles related to CCNA certification, including the September 2007 book CCNA Official Exam Certification Library (CCNA Exam 640-802) (Read a sneak peek of chapter 7). Click for the list of current titles by Wendell.
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HI Wendell, Great what you
HI Wendell,
Great what you explained!
But i was wondering why you didn't make mention of the fact that instead of using the network command we could also use the ip ospf area command at the interface level. I have been preparing for BSCI since April this year and i find this command to be very useful in the sense that it also to be more specific on what you actually want to do (What interface you want to have in the ospf process)
Keep up the good work !
The ip ospf area command
The ip ospf area command under interface config mode is for OSPFv3. You likely won't be dealing with that unlesss you're working with IPv6. The network command under the router process does the same thing, if you want to specify an interface exactly just include the 0.0.0.0 wild-card mask while specifying the network (i.e. network 192.168.10.1 0.0.0.0 area 0); this will start advertising OSPF hello's and advertisements out of that interface only.
Looks like it's V2 as well?!
Stacy,
I just plain forgot about ip ospf area on the interface - oversight by me. Yep, that'll do the trick, and a good one to add to the list.
Two small related points - one from Stacy's post, one from Anon, in this thread:
I don't see how ip ospf area are gives you any more granulatiry, since the network command with a 0.0.0.0 wildcard lets you select each interface individually. But the ip ospf area interface command is more intuitive, IMHO.
I believe the ip ospf area command is also OSPF v2 - I've done it in labs, and the IOS doc says V2. (See http://www.cisco.com/en/US/partner/docs/ios/iproute/command/reference/irp_osp1.html#wp1013708
But hey, that's why we all want a couple of routers in the home office, to try it out, right? Thanks to you both!
Wendell
Wendell, good topic my man.
Wendell, good topic my man. I looooove OSPF. It's definitely the most complicated IGP to config and troubleshoot (a vote could probably be given for IS-IS mostly due to how different it is than any other IGP) but for some reason I just like it more. Keep up the good work, you are performing a great service to certification candidates!!!
MTU?
Hi Wendell.
Shouldn't the interface MTU be in the neighborship criteria list as well?
Great blog!
Yep, I should've listed it!
Jeff, Yep, I oughta have listed mtu. For real life, mismatched MTUs prevent adjacency (a higher form of neighborship), and that's very important. Pickily, the neighbors are discovered, and listed in "show ip ospf neighbbors", which was what I was driving at. But I was being too picky. Thanks for the heads up...
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