On Apr 28, 11:12*am, hyperboogie wrote:
> Hi
>
> I'm taking a course in computer networks and right now we're studying
> about Link State protocols (i.e. - OSPF)
> My question is regarding loops.
>
> It's my understanding that when a broken connection goes back up, the
> two routers on both sides have to exchange their information and
> rebuild their internal maps in order to avoid loops. But how can a
> loop occur? let's assume that a broken line goes back up and the two
> routers don't exchange information, how can a loop occur then. Can
> someone please give an example or an illustration?

Take a simple case:

In this mesh, routers A and B each have a link to C. There is also a
slow, i.e. costly, standby link between A and B.

A-------C
|
B---+

Now the A-C link breaks, so the mesh reconfigures with the spare link,
to:

A------B------C

Okay, so now all packets from A to C must go through B. B knows it has
to forward all packets from A to C, and from C to A.

Now, the A-C link is restored, B figures this out, but C does not.
What happens when C wants to send a packet back to A?

Well, C sends the packet to B, thinking that's still the only way to
get to A. But B has placed that slow link in standby again, so its
route to A is through C.

So when B receives a packet addressed to A, and sends it back to C,
along its computed best route. And so on.

This packet from C to A will bounce back and forth between C and B,
until C finally updates its routing tables, and finds that direct link
to A.

Bert

On Apr 28, 9:56 pm, Albert Manfredi wrote:
> On Apr 28, 11:12 am, hyperboogie wrote:
>
> > Hi
>
> > I'm taking a course in computer networks and right now we're studying
> > about Link State protocols (i.e. - OSPF)
> > My question is regarding loops.
>
> > It's my understanding that when a broken connection goes back up, the
> > two routers on both sides have to exchange their information and
> > rebuild their internal maps in order to avoid loops. But how can a
> > loop occur? let's assume that a broken line goes back up and the two
> > routers don't exchange information, how can a loop occur then. Can
> > someone please give an example or an illustration?
>
> Take a simple case:
>
> In this mesh, routers A and B each have a link to C. There is also a
> slow, i.e. costly, standby link between A and B.
>
> A-------C
> |
> B---+
>
> Now the A-C link breaks, so the mesh reconfigures with the spare link,
> to:
>
> A------B------C
>
> Okay, so now all packets from A to C must go through B. B knows it has
> to forward all packets from A to C, and from C to A.
>
> Now, the A-C link is restored, B figures this out, but C does not.
> What happens when C wants to send a packet back to A?
>
> Well, C sends the packet to B, thinking that's still the only way to
> get to A. But B has placed that slow link in standby again, so its
> route to A is through C.
>
> So when B receives a packet addressed to A, and sends it back to C,
> along its computed best route. And so on.
>
> This packet from C to A will bounce back and forth between C and B,
> until C finally updates its routing tables, and finds that direct link
> to A.
>
> Bert

Thank you so very much :-)
You've been very helpful
This is precisely what I needed to understand