OSPF Virtual-Links, Cabling

Really quick post tonight just to re-hash some of the material I already covered but haven’t posted about, and finally got my virtual link in Area 34 which is an Ethernet segment off the Fa0/0 of both R3 and R4 off the usual Hub-and-Spoke NBMA network. This is achieved by going into the Router Configuration mode, and enter “area 34 virtual-link x.x.x.x” where x.x.x.x is the remote routers RID.

Also, the router that you are creating the virtual link towards to bring it’s network into the ‘backbone area / grouping’ must not have an interface in Area 0, again this cannot be a Backbone router (that I have been made aware of yet) – This may be an important point to keep in mind.

The virtual link is a way of including a network on a non-backbone router (non Area-0) into the backbone router grouping (not sure if that is the correct word), and allow it to propagate it’s network to the rest of the OSPF routers.

Kind of odd that the router does not treat virtual-links as for example a Loopback interface, as I keep getting muddled typing ‘show ip int …’ which does not show virtual links, the actual command is “show ip ospf virtual-links” from User Exec mode.

Another odd but logical behavior that it appears as a second Adjacency / Neighbor when running ‘show ip ospf nei’:

R4#show ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface           0   FULL/  –           –     OSPF_VL2           1   FULL/BDR        00:00:31     FastEthernet0/0

One final thought, I tripped on this so hard and its eaten up so much time, but I learned the hard way that an interface that shows the following output can be a bad / incorrect cable: FastEthernet0/0 is up, line protocol is down

I pulled the cable from another working link I originally was using, but of course in my wisdom I didn’t check the cable before plugging it in (or did and just honestly didn’t see it was incorrect), and I thought if there was a complete mismatch of a cable type it would have shown down / down. This really put my study on the back burner for a couple days there as I only try to study on work days and take weekends off, so I found this today, and moving forward I will keep in mind this is in fact a physical lab with physical issues.

I was going to go over some material I’ve already covered such as cost, adding ethernet (broadcast) segments, etc, but I will wait until I cover the bandwidth video and hopefully actually tank to the end of OSPF Fundamentals and prepare to crack into the Advanced OSPF videos – Cannot wait 🙂

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