Re: Virtual links Failover and design in OSPF

From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Tue Aug 26 2008 - 16:09:38 ART

Hi Hammad,

As I understand it, the reason DR/BDR election will not normally affect
the virtual link operation is that the ABR which has a virtual link sends:
a) External LSAs into the transit area subnet as multicasts. These
reach all routers on that subnet (inc. BDR), and after flooding reach
all in the area (including the endpoint virtual router).
b) Other LSAs using unicast direct to the endpoint virtual router.

With virtual links I find their operation easier to understand when
thinking about areas as multiple subnets, rather than single subnets. A
single virtual link can span multiple routers/subnets but only one area.

If you build a virtual link over an area which already has multiple
ABRs, you could have a case where a failure occurs which splits this
non-backbone transit area into two halves. These two halves will then
operate as independently areas, separated by area 0.
    Say your virtual link was configured over the now failed link; you
may still be able to ping between the configured endpoints, but your
virtual link will no longer work as it requires intra-area connectivity
between them. Then you would need a new virtual link.

One other point, which is not specific to virtual links, is that on
links with two ethernet routers the DR/BDR election wait time can delay
exchange of routes longer than would occur with point-to-point ospf
nework type (which doesn't elect so doesn't need to wait). After link
recovery, any such additional delay will obviously have a knock on
effect to virtual links as well as any other routes.

Paul.

Rick Mur wrote:
> If your router with router-id 4.4.4.4 goes down, the virtual-link goes
> down. So if you are required to keep the ospf working if that router
> goes down, you should have another router that's an ABR from area 1 to
> area 0.
>
> The DR/BDR election has totally no effect on virtual-links, you can
> configure a virtual-link on any ABR that's touching area 0.
>
> Rick
>
>
> On 26 aug 2008, at 09:45, Zealist Hamamd wrote:
>
>> Hie Experts,
>>
>> I came across a question that If I have a multi area ospf network,
>>
>> AREA 0 --- AREA 1 --- AREA 3
>>
>> If I configure a virtual link from area 3 to area 0 using AREA 1 as
>> transit. area 1 virtual-link 4.4.4.4
>>
>> 1- now, what happens if the router 4.4.4.4 in bw area 0 and 100
>> goes down
>> ?? do I have to reconfigure the virtual link ?
>> 2- What happens if the DR router in area 0 goes down, and an election
>> takes place, so the BDR on area 0 becomes the DR.do I have to reconfigure
>> the
>> virtual link??
>>
>>
>> My understanding in this regard is
>>
>> Ans 1- If router 4.4.4.4 down the link will down and untill u have some
>> backup link configure , area 3 wil be dosconnected(Kindly shed some
>> views
>> on how to configure backup Virtual links).
>>
>> Ans 2- DR / BDR have no effect on Virtual links, ABR wil calc the
>> current DR
>> and fwd LSAs to that.
>>
>> Kindly correct me if i m wrong somewhere.
>> Regards
>> Hammad
>> zealist@gmail.com
>>
>>
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