Well, being away from a while presents me with the dilemma of trying to decide where to go from here for the next few weeks and months. I could ramble on whatever's on my mind that day, changing topics each time, or try to get something more cohesive going. Should I talk about the value of certs? Lab gear (again)? Pick a techie topic? Alas, so many choices, so little time.
I'll probably jump around a bit in the coming month or two, but I've decided to start out with a post or two on OSPF. Not new, not snazzy, but in my most recent survey in this blog, asking about CCNA topics people might like to hear about, OSPF was near the top. (IPv6 is there already, and I did a few posts on it this past June, here and here.)
I also surveyed on CCNP topics, if you want to look there - so maybe I'll do something on MPLS or BGP coming up.
To do this right, I need your help. I'm wondering what is it about OSPF that makes it a challenge for CCNA candidates. Is it the link state theory? Is it figuring out how much Link State theory you need to really know? Is it the trickier-than-average config? Is it the sometimes-confusing output from the various "show ip ospf..." commands? If it's hands on, maybe I could post a lab exercise that could be done on gear, or maybe not, something like this:
Three routers connect to one LAN switch, with the switch placing all three routers in the same VLAN. Assume all hardware is cabled and has no physical problems. Each router uses their respective Fa0/0 interfaces. Configure each router as shown here, and save the configs. Then reload the switch. Finally, using show commands, determine:
That's not a lot of detail on a lab, but it's enough to try if you've got three routers handy. If you don't, you could look at the config, and think about what should happen just based on the configuration.
Just for grins, while I give you some time to weigh in on what in particular is troublesome about OSPF, you try the above "lab" with the following configs. Note that this lab, in my opinion, requires much more thought and time than any particular CCNA exam question - it's just a tool for learning and discussing. Some of this may well even be more like CCNP level topics, but it makes for a more interesting discussion.
R1:
- Hostname R1
- Interface loopback 0
- Shutdown
- Ip address 5.5.5.5 255.255.255.255
- Interface fa0/0
- Ip addr 10.1.1.1 255.255.255.192
- No shutdown
- Router ospf 1
- Network 10.1.1.0 0.0.0.255 area 0
- Network 5.5.5.5 0.0.0.0 area 1
R2:
- Hostname R2
- Interface Fa0/0
- Ip address 10.1.1.222 255.255.255.192
- No shutdown
- Router ospf 1
- Network 10.1.1.0 0.0.0.255 area 0
R3:
- Hostname R3
- Interface loopback 2
- Ip address 3.3.3.3 255.255.255.0
- Interface loopback 4
- Ip address 2.2.2.2 255.255.255.255
- Interface fa0/0
- Ip address 10.1.1.33 255.255.255.192
- No shutdown
- Router ospf 2
- Network 10.0.0.0 0.255.255.255 area 0
- Network 2.2.2.2 0.0.0.0 area 0
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.
The opinions expressed in this Weblog are those of the writer and may not represent the opinions of Network World.
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OSPF
I think the two areas of OSPF that I would love to gather more information about is LSA's and multi-area setups.
Yours is first in line
Ok that1guy15,
Yours is the first request in line ;-) I'll do it next!
Wendell
Welcome back,
Welcome back, Wendell!
Tricky lab, i had to start PacketTracer to check some things. I already have CCNA so i'll let the others answer the questions. Just an advice for them : think twice :D .
Enough to get confused over
Adrian,
Thanks, it's good to be back. Yep, I wanted enough tricky stuff to hve a robust discussion - defintely more to it than 1 little problem. Good advice - don't stop after finding the first issue!
Wendell
This is what I think: # The
This is what I think:
# The Router ID of each router
R1 - 5.5.5.5
R2 - 10.1.1.222
R3 - 3.3.3.3
# The DR (if it exists)
R2 is DR
# The BDR (if it exists)
R1 is BDR
# OSPF intra-area routes learned at each router
2.2.2.2/32
10.0.0.0/8
10.1.1.0/24
# OSPF inter-area routes learned at each router
5.5.5.5/32
# Number of Type 2 LSAs known on each router
one
# Number of Type 3 LSAs known on each router
one
# Number of neighbors in the full state on each router
three
comments?
Couple of issues...
Hi Almandau,
Well, I've decided to drag it out a little, so for any of you still working on this problem...
SPOILER ALERT!
That said, I posted more background here. If you follow that through, you'll see that there's an issue with one of the routers (R2) becoming neighbors with the other two. That'll impact who becomes DR/BDR. Also, I'd disagree with one of your choices for the RIDs (check that the interface must be up/up for its IP address to be chosen.)
I'll be getting to the OSPF LSA stuff by next week - so I'm going resist the urge to say more until I at least give it one longer shot in the blog. Feel free to post your revised thoughts here or in today's post. Thanks!
Troubleshooting
"I could ramble on whatever's on my mind that day, changing topics each time..."
Wendell, I've already got a corner on that in my blog. Stay away.
I suggest doing an OSPF troubleshooting series. While OSPF configuration is pretty straightforward for most designs, sometimes OSPF can break in interesting ways. Looking at a few curious scenarios and, as you say, learning to decipher the many OSPF "show" commands, could be valuable.
--Jeff
Happy Thanksgiving!
1. The Router ID of each router
10.1.1.1
10.1.1.222
3.3.3.3
2. The DR (if it exists)
10.1.1.1
10.1.1.222
3. The BDR (if it exists)
3.3.3.3
4. OSPF intra-area routes learned at each router
R1 - 2.2.2.2/32
R2 - none
R3 - none
5. OSPF inter-area routes learned at each router
R1 - none
R2 - none
R3 - none
6. Number of Type 2 LSAs known on each router
R1 - 1
R2 - 0
R3 - 1
7. Number of Type 3 LSAs known on each router
R1 - 0
R2 - 0
R3 - 0
8. Number of neighbors in the full state on each router
R1 - 1(R3)
R2 - 0
R3 - 1(R1)
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