Re: OSPF Router ID & Network Types

You missed the part

"All other packet types are sent/received only on adjacencies. This means
that the packet must have been sent by one of the router's active
neighbors."

Here is the question: when an OSPF packet is received on the interface,
should it be processed or dropped? What are the rules for distinguishing
legitimate packets?

Obviously, the packets need to have proper header structure first. Next, if
the interface is configured for authentication, the authentication
information in the packet should be verifiable using the interface key
settings. What next? If the packet is a HELLO packet, it could signal a new
adjacency establishment or belong to an existing adjacency. If packet is
anything else but HELLO, there MUST be an adjacency matching this packet -
otherwise the packet should be dropped.

Now we approach your question: how would OSPF router match a packet to an
adjacency? OSPF maintains list of neighbor IP addresses, so it can simply
use the source IP address to find matching neighbor from the list. However,
there is a problem area here: point-to-point links and virtual-links.

P2P links could be unnumbered. That is, the source IP address used in OSPF
packets sent over P2P link may actually correspond to MULTIPLE adjacencies.
Simply imagine two routers connected by two unnumbered point-to-point links.
For this reason, if a packet is received on a P2P interface, it is matched
against the neighbor table using the Router-ID found in the OSPF header.
This resolves the unnumbered problem.

The next problematic link type is virtual link. Virtual link neighbors could
be separated by multiple hops. The source and destination IP addresses in VL
packets are calculated dynamically based on the existing topology state.
When OSPF receives an unicast HELLO packet, it finds out it belongs to area
0 and inspects the Router-ID in OSPF header to find a matching virtual-link.
Obviously, relating on source IP will not help in this case, even though it
is theoretically possible to scan the LSDB for Type 3 links and see what
router has the IP address.

HTH,

-- 
Petr Lapukhov, petr_at_INE.com
CCIE #16379 (R&S/Security/SP/Voice)
CCDE #20100007
Internetwork Expert, Inc.
http://www.INE.com <http://www.ine.com/>
Toll Free: 877-224-8987
Outside US: 775-826-4344
2011/1/21 Nish Vamadevan <ipnish_at_gmail.com>
> Hi,
>
> As I am reading through RFC 2328, Came up with this statement and I can't
> seem to come up with a logical explanation, why... Maybe someone can help
> me
> out?
>
> When it comes to the Hello Packet,
>
> //
> If the receiving interface connects to a point-to-point network or a
> virtual
> link, the sender is identified by the Router ID (source router) found in
> the
> packet's OSPF header.
> //
>
> I can understand the part about the virtual-link because it is created
> using
> the Router ID.
>
> What I can't understand is why a point-to-point network is identified by
> the
> Router ID. And having said that, Point-To-MultiPoint isin't. (It also
> states
> that, as we know Point-To-MultiPoint is a collection of point-to-point)
>
> I'd appreciate it if anyone can shed some light in this...
>
> Thanks,
>
> --
> Regards,
> Nish
>
> www.twitter.com/nish
>
>
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