OSPF single area network DR and BDR elections

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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 ;)


Re: OSPF single area network DR and BDR elections

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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.....



Re: OSPF single area network DR and BDR elections
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play

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.

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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
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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....

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

stephen_hope@xyzworld.com - replace xyz with ntl



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