From the blog.

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Complexity – My Friend, My Enemy
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OECG – Chapter 10

767 Words. Plan about 5 minute(s) to read this.

<rant>I have to admit that tonight I’m crabby and irritable. My new PC didn’t get here because no one was here to sign for it. UPS won’t try again until Monday afternoon, where there might be (but might not be) anyone here to sign for it. Meaning I’ll have to go get it. That’s not the end of the world, but just annoying. They’d normally leave a body in my atrium with no one to sign for it…but if it’s something I really want and am excited to have…then they have to make me jump through 20 hoops to get it. Grr. Next time I’ll have them ship stuff to work. Never minding all that…this week was sort of blah, and I’m glad it’s Friday. But guess what…snowstorm of the season is ramping up to make sure my Friday commute sucked (it did, more than twice as long to get home as normal), and my Saturday will suck too, with a driveway full of snow to do something about. It’s the middle of March! Time to move on, Old Man Winter. Let it go. You lost. It was 70 degrees earlier this week. Die, winter…die die die.</rant>

To the strains of Mozart, on to more important topics.

An OSPF designated router is a router in an area that’s responsible to flood LSA’s to every other router in the area. This saves every OSPF router from exchanging their LSDB with every other router in an OSPF area, and keeps redundant LSA exchanges to a minimum. OSPF areas on LAN’s elect designated routers (DRs) and backup designated routers (BDRs). There may or may not be a DR or BDR on a segment, depending on what type of network OSPF is running across.

  • Here’s an walkthrough of how the DR process reduces the number of LSA exchanges that would normally occur:
    • A non-DR router (DROther) sends his DD (an LSA header, remember) to a multicast address of 224.0.0.6. 224.0.0.6 is the “all DR” multicast address – remember that there’s generally a DR and a BDR, so there’ll be more than one.
    • The DR & BDR see the LSA header. The DR will send a unicast acknowledgement of the DD by sending back the same DD again.
    • The DR will then multicast the DD to 224.0.0.5, which is all OSPF routers in the area.
    • The non-DR routers will acknowledge the DD with a unicast identical DD in response.
  • This implies something important about OSPF neighbor relationships in a LAN. Not all OSPF routers that find each other will need to continue the Hello process until the “FULL” state. In on OSPF segment with a DR, routers only need to form “FULL” relationships with the DR and BDR. Routers that are “DROther” (ergo, not DRs), will only advance to the “2WAY” state. All the DD exchanges from there will only happen with the DRs, so there’s no point in DROther routers forming FULL relationships with one another.
  • The last point in mind, OSPF thinks of “neighbors” and “adjacent” differently. Neighbors are simply OSPF routers that have found one another. They’ve swapped Hellos, and the Hellos match up for the needed number of parameters. Adjacent or “fully adjacent” routers are OSPF routers in a “FULL” state.
  • If you’re thinking there’s a risk that not all routers in the OSPF area will get the same LSDB, think again. Remember, all OSPF routers will exchange a full database with the DR. It’s the DR’s job to make sure that all the OSPF routers are performing link-state computations from the same sheet of music, so to speak.

Routers do go through an OSPF DR election process.

  • A router with an OSPF priority of anywhere from 1-255 can attempt to become the DR by popping his own RID in the DR field of his Hellos.
  • Routers look at the DR field in the Hellos to determine who wants to be the DR.
  • If an inbound Hello lists a better DR, the router then decides that he DOESN’T want to be the DR, but that the better router does.
  • Routers with the highest priority will be DR’s.
  • In case of a tied priority, the router with the highest RID is better.
  • The router not claiming to be the DR, with the higher priority (or RID) of the remaining routers will become the BDR.
  • If a new router shows up or another router has a higher priority, they can NOT preempt the existing DR.
  • If the DR falls off the network, the BDR will take over as DR. A BDR election will follow.