OSPF single area network DR and BDR elections

Hello- I had a question that had 4 routers all connected in a circle-there were 3 subnets between them (10.2.0.0 -10.4.0.0 and 10.5.0.0) my limited knowledge understood that the router with the highest local IP address became the DR and the next highest became the BDR-this is if no loopback addy is conifged or prioritys set

What I am unsure about is that do you count the IP for both i/f on the router and apply that to both subnets? eg-if S0 is 10.5.0.2 and e0 is 10.5.0.3 -and they are the highest in the whole area(not just the individual subnet) does that router become DR and BDR? Also if Router A has an S0 IP of 10.5.0.2 and its E0 is 10.4.0.1 and the E0 is connected to Router B which has an IP of 10.4.0.2 -does Router A still take priority ?even though its S0 is connected on another subnet? Or do you take the whole area ? Many thanks ,I hope this makes sense ;)

Reply to
gregg johnstone
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DRs will not come into play on your serial segments. DRs will come into play on the Ethernet segments. On that Ethernet segment, the first_router_that_comes_up with OSPF will become the DR, regardless of priority/IPs, etc. The IP address of that DR will be the highest IP address on that router, even if it is on a different subnet. (IE - you may see the DR on your ethernet is the IP address of the serial port.) It's best to use the "router-id" command to fix the DR identifier to something that may make more sense.....

Reply to
John Agosta

unless you make the serial link into a "broadcast" type OSPF interface - you sometimes need this to connect to other non cisco kit that doesnt understand pt-pt.

To put it another way that may make more sense if you have notes based on the OSPF standard: the OSPF DR election is "sticky" - you only get an election when you lose the current DR - (and the BDR should just get promoted to DR rather than a complete election). Priority and OSPF ID only matter in an election, so usually dont make much difference (unless you take an interface out of the election - pri 0, or OSPF "passive"?). So elections dont happen much in practice, and the tie break settings make very little difference to what happens.

The IP address of that DR will be the highest IP address

i usually call it OSPF ID - since on some kit it isnt an address, just an arbitary number. In IOS, if you have a loopback address defined then that is the address used rather than an interface - or do as John suggests below.

The main reason to avoid using an interface address for the OSPF ID is that if that interface goes down, OSPF seems to do a restart, and the router chooses a new ID.

So - you get a topology change, and the old router info may not vanish until LSA timeout, since the replacement data now has uses the new ID. Seriously confusing if you are debugging at the time....

Yes! - but make sure all routers have unique IDs (thru out the AS) or you will get other problems.

Reply to
stephen

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