OSPF and ISDN Backup

Discussion in 'Cisco' started by NetWorkMan, Oct 13, 2007.

  1. NetWorkMan

    NetWorkMan Guest

    Hi Guys,

    I have a question. I'm currently running OSPF on all of the routers
    and I'm having problems with ISDN backup links. I use floating static
    routes to activate the BRI interfaces. Just to mention; My network is
    set up in a hub & spoke topology where remote routers dial into the
    central site. When a branch router fails over to ISDN it forms a
    neighbor relationship with the central site, but I believe all of the
    other remote routers instantly perform STP calulations because of the
    topology changes. It creates a few second times-outs on all of the
    routers as the topology changes. So my question is:

    Should ISDN backup links be placed in separate OSPF areas ? I
    currently have the ISDN links in area 0. ? Any suggestions on what I
    should pay attention to or a web link that would help me with setting
    this up ?

    Thank you for all your help.
     
    NetWorkMan, Oct 13, 2007
    #1
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  2. NetWorkMan

    AFRO Guest

    Sounds kida like a timer issues to me.

    You may just need to cut down on some timers so the network converges
    faster.

    "NetWorkMan" <> wrote in message
    news:...
    > Hi Guys,
    >
    > I have a question. I'm currently running OSPF on all of the routers
    > and I'm having problems with ISDN backup links. I use floating static
    > routes to activate the BRI interfaces. Just to mention; My network is
    > set up in a hub & spoke topology where remote routers dial into the
    > central site. When a branch router fails over to ISDN it forms a
    > neighbor relationship with the central site, but I believe all of the
    > other remote routers instantly perform STP calulations because of the
    > topology changes. It creates a few second times-outs on all of the
    > routers as the topology changes. So my question is:
    >
    > Should ISDN backup links be placed in separate OSPF areas ? I
    > currently have the ISDN links in area 0. ? Any suggestions on what I
    > should pay attention to or a web link that would help me with setting
    > this up ?
    >
    > Thank you for all your help.
    >
     
    AFRO, Oct 14, 2007
    #2
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  3. NetWorkMan

    Guest

    On Oct 13, 4:51 pm, "AFRO" <> wrote:
    > Sounds kida like a timer issues to me.
    >
    > You may just need to cut down on some timers so the network converges
    > faster.
    >
    > "NetWorkMan" <> wrote in message
    >
    > news:...
    >
    > > Hi Guys,

    >
    > > I have a question. I'm currently running OSPF on all of the routers
    > > and I'm having problems with ISDN backup links. I use floating static
    > > routes to activate the BRI interfaces. Just to mention; My network is
    > > set up in a hub & spoke topology where remote routers dial into the
    > > central site. When a branch router fails over to ISDN it forms a
    > > neighbor relationship with the central site, but I believe all of the
    > > other remote routers instantly perform STP calulations because of the
    > > topology changes. It creates a few second times-outs on all of the
    > > routers as the topology changes. So my question is:

    >
    > > Should ISDN backup links be placed in separate OSPF areas ? I
    > > currently have the ISDN links in area 0. ? Any suggestions on what I
    > > should pay attention to or a web link that would help me with setting
    > > this up ?

    >
    > > Thank you for all your help.



    The previous poster is correct decreassing timers will spead up
    convergence when running OSPF. You will need to do this on all routers
    in the area. If you are truly doing a hub and spoke design, and the
    all traffic to other sites and the internet flows thru the hub you may
    want to consider an option of static routes point to the hub. and with
    floating statics in place for the ISDN connections. Under this design
    your remote areas do not need to know what is happening at each one if
    the have to cross the hub anyhow.

    Option 2. Use hierarchay in your design with a seperate area for the
    remote sites, and then perform address aggregation into the hub. This
    way the remote sites will not reconverge for every flap.
     
    , Oct 14, 2007
    #3
  4. NetWorkMan

    Merv Guest

    How many routers are in your network ?

    Do you have a single OSPF area or multiple OSPF areas ?

    What is the reason for using OSPF as the IGP (i.e. is this a multi-
    vendor router network) ?

    How many routers at the hub site have ISDN interfaces ( assuming they
    are PRI's) ?
     
    Merv, Oct 14, 2007
    #4
  5. NetWorkMan

    stephen Guest

    <> wrote in message
    news:...
    > On Oct 13, 4:51 pm, "AFRO" <> wrote:
    > > Sounds kida like a timer issues to me.
    > >
    > > You may just need to cut down on some timers so the network converges
    > > faster.
    > >
    > > "NetWorkMan" <> wrote in message
    > >
    > > news:...
    > >
    > > > Hi Guys,

    > >
    > > > I have a question. I'm currently running OSPF on all of the routers
    > > > and I'm having problems with ISDN backup links. I use floating static
    > > > routes to activate the BRI interfaces. Just to mention; My network is
    > > > set up in a hub & spoke topology where remote routers dial into the
    > > > central site. When a branch router fails over to ISDN it forms a
    > > > neighbor relationship with the central site, but I believe all of the
    > > > other remote routers instantly perform STP calulations because of the
    > > > topology changes.


    you might want to not allow OSPF over the ISDN route, which should simplify
    the topology change.

    Also unless you inject a default route into OSPF to act as a black hole, the
    ISDN lines will tend to come up when a packet to an unknown address finds
    the static.....

    It creates a few second times-outs on all of the
    > > > routers as the topology changes. So my question is:

    > >
    > > > Should ISDN backup links be placed in separate OSPF areas ? I
    > > > currently have the ISDN links in area 0. ? Any suggestions on what I
    > > > should pay attention to or a web link that would help me with setting
    > > > this up ?


    No - adding areas isnt going to help much durectly, although summarising
    routes is at the area boundary would cut down how far the update has to go,
    and where it has an effect.

    The Dykstra should only affect routes across the "downed" serial link.

    If that isnt happening, you might want to try incremental updates.

    dont forget that the ISDN is going to take a second or 2 to come up - so you
    may get a 2nd set of OSPF updates when the link is actually up.

    > >
    > > > Thank you for all your help.

    >
    >
    > The previous poster is correct decreassing timers will spead up
    > convergence when running OSPF. You will need to do this on all routers
    > in the area.


    if should only matter for sites where the routes change (and where there is
    a 2nd path to the site) - anywhere else the old and new routing tables will
    be the same, so calculating them a couple of sec later should not have any
    effect.

    If you are truly doing a hub and spoke design, and the
    > all traffic to other sites and the internet flows thru the hub you may
    > want to consider an option of static routes point to the hub. and with
    > floating statics in place for the ISDN connections. Under this design
    > your remote areas do not need to know what is happening at each one if
    > the have to cross the hub anyhow.
    >
    > Option 2. Use hierarchay in your design with a seperate area for the
    > remote sites, and then perform address aggregation into the hub. This
    > way the remote sites will not reconverge for every flap.
    >

    there are interface timers that affect how quickly a link is actually
    declared down - whether they alter the convergence time depends on the
    interface type and exactly which type of "down" you get in practice.

    try altering carrier-delay (default is either 500 or 5000 mSec). if you put
    it to zero, then add "dampening" to the interface in case the link flaps.

    if the link doesnt send a physical "down" event (eg an Ethernet WAN without
    link loss forwarding), then alter the hello and dea timers to detect a
    "down" faster.

    Note this can seriously load a router with a lot of adjacencies, so be
    cautious on the hub box.
    Note2 - tinkering with the timers will kill an adjacency on the 1st change -
    so do the far end 1st unless you have a back door........

    there are also a couple of global timers for OSPF itself - the defaults
    defer a recalc for to allow updates to arrive from multiple neighbours.
    flip side is, the routing table doesnt get undated until all this lots kicks
    off.

    look for the ospf "throttle / lsa" commands - some IOS versions claim this
    is off by default, but i had to tweak this on a Cat 6509 with 12.2 to get
    down to 100 mSec convergence (on GigE, so likely not the same as your
    problem).


    --
    Regards

    - replace xyz with ntl
     
    stephen, Oct 14, 2007
    #5
  6. NetWorkMan

    stephen Guest

    "Merv" <> wrote in message
    news:...
    >
    > How many routers are in your network ?
    >
    > Do you have a single OSPF area or multiple OSPF areas ?
    >
    > What is the reason for using OSPF as the IGP (i.e. is this a multi-
    > vendor router network) ?


    how about - even on a Cisco router, OSPF is more predictable / reliable than
    EIGRP? :)

    here is an SRND (actually for campus) that gives some good tips on using
    OSPF (and EIGRP) to provide high availability and fast convergence.
    http://www.cisco.com/application/pdf/en/us/guest/netsol/ns432/c649/ccmigration_09186a0080811468.pdf

    in reality, fast convergence is only really about how quickly you find an
    alternate to a lost path.

    bringing links back up is much less of an issue (as long as you dont get a
    forwarding hit during the topology change)

    if you had a backup path then getting the main link working quickly and with
    minimal side effects is much more important than doing that instantly.

    and if you didnt - well everything stopped working when the main link died,
    so it will take plenty of time and effort to get everything back anyway, and
    the users probably went home / for coffee when it happened.....
    >
    > How many routers at the hub site have ISDN interfaces ( assuming they
    > are PRI's) ?
    >

    --
    Regards

    - replace xyz with ntl
     
    stephen, Oct 14, 2007
    #6
  7. NetWorkMan

    Merv Guest

    Also would be useful for OP to post the output of "show ip ospf
    statistics"

    for the hub and several of the spoke routers
     
    Merv, Oct 14, 2007
    #7
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