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OSPF question about adjacencies

 
 
Sameer
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      11-12-2003
I'm a bit confused about adjacencies. Is the router adjacent only to other
routers that are directly connected to an interface, or could it be adjacent
to any router that it gets a hello packet from that contains it's own
RouterID?

Also, are hello packets sent out only to directly connected routers, or do
the directly connected routers also pass this hello packet to all other
routers?

Finally, what is the mechanism that allows a DR and BDR to send out link
state packets on the entire network that makes sure all routers, within the
OSPF area, receive all the link states?

-Sameer


 
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shope
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      11-12-2003
"Sameer" <(E-Mail Removed)> wrote in message
news:%3lsb.25$(E-Mail Removed)...
> I'm a bit confused about adjacencies. Is the router adjacent only to

other
> routers that are directly connected to an interface, or could it be

adjacent
> to any router that it gets a hello packet from that contains it's own
> RouterID?


Thats the mechanism, although the spec says the router should only form
adjacencies when other things match, such as subnet mask, MTU and
authentication.

In practice the adjacency is usually over a layer 2 connection - i.e. the 2
OSPF devices are connected to the same segment - main exception you could
argue is a virtual link.
>
> Also, are hello packets sent out only to directly connected routers, or do
> the directly connected routers also pass this hello packet to all other
> routers?


Hello packets are sent on each attached interface set up to run OSPF -
unless there are configured neighbours.

Protocol details are a bit different for a multicast capable network or an
OSPF point to point style interface.

Note that on cisco and several other boxes you can set a device to "passive"
where the hellos dont get generated - this means the interface is part of
the OSPF database, but you wont form an adjacency - mainly useful for star
points such as big layer 3 switches where you dont want 50 or more
adjacencies to the same device over all the alternate paths.
>
> Finally, what is the mechanism that allows a DR and BDR to send out link
> state packets on the entire network that makes sure all routers, within

the
> OSPF area, receive all the link states?


Flooding. Just like all the other OSPF topology info.

DR description packets are part of the link state database that gets
replicated within an area.
>
> -Sameer

--
Regards

Stephen Hope - remove xx from email to reply


 
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phil~~
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      11-13-2003
"Sameer" <(E-Mail Removed)> wrote in message news:<%3lsb.25$(E-Mail Removed)>. ..
> I'm a bit confused about adjacencies. Is the router adjacent only to other
> routers that are directly connected to an interface, or could it be adjacent
> to any router that it gets a hello packet from that contains it's own
> RouterID?


Hello are sent with a destination 224.0.0.x therefore the packet is not
forwarded, that's why the adjacency remains on the layer2 connection.
As stated by the other member the virtual-link is an exception.

> Also, are hello packets sent out only to directly connected routers, or do
> the directly connected routers also pass this hello packet to all other
> routers?


No, they are not passed

> Finally, what is the mechanism that allows a DR and BDR to send out link
> state packets on the entire network that makes sure all routers, within the
> OSPF area, receive all the link states?
>
> -Sameer

 
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