OSPF router ID question

Discussion in 'Cisco' started by daytime, Nov 26, 2006.

  1. daytime

    daytime Guest

    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 ,
     
    daytime, Nov 26, 2006
    #1
    1. Advertising

  2. daytime

    stephen Guest

    "daytime" <> wrote in message
    news:...
    > 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?


    yes

    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,
    2. manually config router ID - which should override either.

    Although it looks like an IP address it is just a unique 32 bit number....

    > 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


    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.
    >
    > 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 ,
    >

    --
    Regards

    - replace xyz with ntl
     
    stephen, Nov 26, 2006
    #2
    1. Advertising

  3. daytime

    daytime Guest

    stephen wrote:
    > "daytime" <> wrote in message
    > news:...
    > > 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?

    >
    > yes
    >
    > 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,
    > 2. manually config router ID - which should override either.
    >
    > Although it looks like an IP address it is just a unique 32 bit number....
    >
    > > 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

    >
    > 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.
    > >
    > > 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 ,
    > >

    > --
    > Regards
    >
    > - replace xyz with ntl


    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 ;)
     
    daytime, Nov 26, 2006
    #3
  4. daytime

    Guest

    daytime wrote:
    > stephen wrote:
    > > "daytime" <> wrote in message
    > > news:...
    > > > 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?

    > >
    > > yes
    > >
    > > 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,
    > > 2. manually config router ID - which should override either.
    > >
    > > Although it looks like an IP address it is just a unique 32 bit number....
    > >
    > > > 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

    > >
    > > 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.
    > > >
    > > > 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 ,
    > > >

    > > --
    > > Regards
    > >
    > > - replace xyz with ntl

    >
    > 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.

    http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml
    "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.
     
    , Nov 26, 2006
    #4
  5. daytime

    Guest

    daytime wrote:
    > stephen wrote:
    > > "daytime" <> wrote in message
    > > news:...
    > > > 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?

    > >
    > > yes
    > >
    > > 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,
    > > 2. manually config router ID - which should override either.
    > >
    > > Although it looks like an IP address it is just a unique 32 bit number....
    > >
    > > > 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

    > >
    > > 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.
    > > >
    > > > 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 ,
    > > >

    > > --
    > > Regards
    > >
    > > - replace xyz with ntl

    >
    > 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.

    http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml
    "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.
     
    , Nov 26, 2006
    #5
  6. daytime

    daytime Guest

    wrote:
    > daytime wrote:
    > > stephen wrote:
    > > > "daytime" <> wrote in message
    > > > news:...
    > > > > 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?
    > > >
    > > > yes
    > > >
    > > > 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,
    > > > 2. manually config router ID - which should override either.
    > > >
    > > > Although it looks like an IP address it is just a unique 32 bit number....
    > > >
    > > > > 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
    > > >
    > > > 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.
    > > > >
    > > > > 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 ,
    > > > >
    > > > --
    > > > Regards
    > > >
    > > > - replace xyz with ntl

    > >
    > > 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.
    >
    > http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml
    > "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.


    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
     
    daytime, Nov 26, 2006
    #6
  7. daytime

    stephen Guest

    "daytime" <> wrote in message
    news:...
    >
    > wrote:
    > > daytime wrote:
    > > > stephen wrote:
    > > > > "daytime" <> wrote in message
    > > > > news:...
    > > > > > 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?
    > > > >
    > > > > yes
    > > > >
    > > > > 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,
    > > > > 2. manually config router ID - which should override either.
    > > > >
    > > > > Although it looks like an IP address it is just a unique 32 bit

    number....
    > > > >
    > > > > > 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
    > > > >
    > > > > 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.
    > > > > >
    > > > > > 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 ,
    > > > > >
    > > > > --
    > > > > Regards
    > > > >
    > > > > - replace xyz with ntl
    > > >
    > > > 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) .


    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......

    > > > 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.
    > >
    > >

    http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml
    > > "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.


    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.
    > >
    > > 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.

    >
    > 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).


    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.

    > Thanks alot

    --
    Regards

    - replace xyz with ntl
     
    stephen, Nov 26, 2006
    #7
  8. daytime

    daytime Guest

    stephen wrote:
    > "daytime" <> wrote in message
    > news:...
    > >
    > > wrote:
    > > > daytime wrote:
    > > > > stephen wrote:
    > > > > > "daytime" <> wrote in message
    > > > > > news:...
    > > > > > > 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?
    > > > > >
    > > > > > yes
    > > > > >
    > > > > > 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,
    > > > > > 2. manually config router ID - which should override either.
    > > > > >
    > > > > > Although it looks like an IP address it is just a unique 32 bit

    > number....
    > > > > >
    > > > > > > 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
    > > > > >
    > > > > > 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.
    > > > > > >
    > > > > > > 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 ,
    > > > > > >
    > > > > > --
    > > > > > Regards
    > > > > >
    > > > > > - replace xyz with ntl
    > > > >
    > > > > 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) .

    >
    > 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......
    >
    > > > > 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.
    > > >
    > > >

    > http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml
    > > > "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.

    >
    > 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.
    > > >
    > > > 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.

    > >
    > > 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).

    >
    > 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.
    >
    > > Thanks alot

    > --
    > Regards
    >
    > - replace xyz with ntl


    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
     
    daytime, Nov 27, 2006
    #8
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