Hi there ,can anyone clarify how this is worked out please-here is my take on things The priority takes 1st priority! higher number wins Then its the IP addy o fthe router(locally)so even if the higher IP is on a different network(say S1 is 200.10.10.1 and e0 is 192.168.10.10) S1 IP is the router ID-even if it is just applied to the 192 network? This has confused me today-I did a practice exam for Sem3 and had an OSPF question-I got it correct by using the Serial link IP address to work out who was the DR-however on the quiz at the end of the chapter for OSPF routing ,my tutor explained that the serial link is not included...... In the question there were 3 routers -1,2,3 router 2 had the highest priority so it was DR R1 had ip 192.168.1.3 and a Loopback of 7.1.1.1 R3 had ip eo 192.168.1.1 and a S1 of 200.20.20.1
I know R1 uses the loopback so its ID is 7.1.1.1 however if that wasnt there would R3 still be the BDR because it has the S1 ip addy? Thanks alot ,
router ID is a box wide ID, so it doesnt care about which interface it is.
if you want better control:
1.add a loopback address to the router - the router ID will use that by default,
manually config router ID - which should override either.
Although it looks like an IP address it is just a unique 32 bit number....
DR is a different issue to router ID - it is a per subnet / interface election. all the parameters configured are per interface so, you can get different choices on different interfaces of the same box.
Although DR priority breaks a "tie" for DR election, the DR choice is sticky - so often it gets chosen before the highest priority router comes up
once chosen the DR stays until the current DR interface goes away for some reason.
Thankyou for your reply, To clear things up-if the election is taking place ,for xample on the
192.168.10.0 network-which ever has the higher IP is DR(assuming priorty setting is the same) unless a loopback has been set(which would have to have a higher IP unique 32 bit number to be DR) . If a serial connection was invloved this would have no bearing on the election process?even if the IP was 200.10.10..... Many thanks. My tutor has not explained this to me ,even though I aks every lesson-he says that you work out all BIDs locally and then apply it the entire network.........what confuses me is why you do not use the Serial port IP as the highest router ID. Hope this makes a little sense ;)
If I recall correctly there is no concept of OSPF DR and BDR on a serial link.
The point of having a DR is that the DR is responsible for broadcasting the topology database (strictly Link State Advertisments) on to a multi-access network (such as Ethernet) where many routers may be listening. This is done to avoid having multiple routers all broadcasting (well strictly multicasting) the same information.
On a serial link there is always only exactly one other OSPF router. For some reason the detail of which escapes me at present there is no need for designated routers.
formatting link
"On point-to-point and point-to-multipoint networks, there are no designated routers (DRs) or backup designated routers (BDRs)."
Your question does disturb me slightly. You are focussing on things (router priority for DR in many and great detail) which are pretty much irrelevent since a professional design will simply create loopbacks and specify the router ID in the router ospf secion of the config. If desired router priority will be specified for DR election.
There is simply no need to worry about it. On the other hand fundamentals, like whether we have a DR at all may be passing you by. I this of course may be a consequence of the syllabus that you are following.
Anyway good luck, when you work it out let us know why a DR is not required on point-to-point links.
Irritatingly enough I seem to have missed the boat for CCNP before 31 Dec.
If I recall correctly there is no concept of OSPF DR and BDR on a serial link.
The point of having a DR is that the DR is responsible for broadcasting the topology database (strictly Link State Advertisments) on to a multi-access network (such as Ethernet) where many routers may be listening. This is done to avoid having multiple routers all broadcasting (well strictly multicasting) the same information.
On a serial link there is always only exactly one other OSPF router. For some reason the detail of which escapes me at present there is no need for designated routers.
formatting link
"On point-to-point and point-to-multipoint networks, there are no designated routers (DRs) or backup designated routers (BDRs)."
Your question does disturb me slightly. You are focussing on things (router priority for DR in many and great detail) which are pretty much irrelevent since a professional design will simply create loopbacks and specify the router ID in the router ospf secion of the config. If desired router priority will be specified for DR election.
There is simply no need to worry about it. On the other hand fundamentals, like whether we have a DR at all may be passing you by. I this of course may be a consequence of the syllabus that you are following.
Anyway good luck, when you work it out let us know why a DR is not required on point-to-point links.
Irritatingly enough I seem to have missed the boat for CCNP before 31 Dec.
"On point-to-point and point-to-multipoint networks, there are no
Of course! As it is a Point 2 point link-what I am unsure of is whether the serial IP is used as the BID?or whether it is ignored ,and only the ethernet address is used(or the loopback). Thanks alot
this was nagging so i looked it up (OSPF - anatomy of a routing protocol - John Moy)
This only matters if there are 2 or more routers connected at the time a segment comes up, or the old DR goes away. At any other time the 1st router elects itself, or the existing DR stays as DR.
if you do get an election - priority is the important setting.
The OSPF ID only matters if there are at least 2 routers with the highest priority configured when it is used as a tie breaker.
However - elections for DR are fairly rare in practice. you need to have several routers on a segment, and the DR "goes away". BDR gets promoted to DR, new BDR gets elected....... so priority has little effect.
and it has no practical effect if you have only 2 routers on a segment, since 1 is Dr and the other BDR, so carry the same workload. DR may not be the one with highest priority as the 1st to come up will elect itself DR - it is difficult to actually bring up 2 simultaneously and get a "real" election......
formatting link
> "On point-to-point and point-to-multipoint networks, there are no
the only times it makes much sense to worry about DR are:
when you have a topology that isnt fully meshed (ie using DR on Frame Relay) - not common any more.
when you want to avoid some load on 1 box, eg a L3 LAN switch with lots of interfaces when you would setDR to 0 to avoid that box ever being elected. Since IOs started supporting it - "passive" is a much simpler way to cut OSPF processing load in most cases.
when there are so many devices on a link segment you want to make sure you put the DR on specific boxes - for load, stability etc.
it depends.
if the serial link is set as OSPF point to point - then each end identifies the other and they exchange databases - only 2 devices, so no need to elect a DR.
however - the OSPf "logical" link type doesnt have to match the physical link type - so you can have a "broadcast" OSPF type interface on a serial link - and then the routers will elect a DR etc.
these odd looking configs are sometimes used to get around compatibility issues.
> > "On point-to-point and point-to-multipoint networks, there are no
That makes sense-point to pont or point to multipoint- the confusing thing for me is the intorduction of the loopback that acts as the BID-because that is being used,as well as the e0 addressess, I mistakenly thought the serial addresses could be used as well. Thanks for taking time out-much appreciated
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