Subject: [Ospf-wireless-design] Another example for self-selected MPRs

From: Richard Ogier <[email-masked]>
Date: Tue, 29 Mar 2005 18:14:16 -0800

To help understand how my proposed solution uses Hellos to learn
2-hop neighbor information and to update self-selected MPRs (or MDRs),
I would like to present another example.

Let's assume that (differential) Hellos must be used to learn
2-hop neighbor information, e.g., so that partial-topology
LSAs can be generated.  Also, assume that Hellos are sent
every 2 seconds, and that no early Hellos are sent.

First, I want to point out that, for self-selected MPRs/MDRs, only
DRs and BDRs need to advertise their full neighborhood (periodically)
in Hellos. This is an additional benefit of using self-selected MPRs,
because they are in the "middle of the action", i.e., they can directly
see both the upstream neighbors and the downstream neighbors.
However, for this example, I will assume that all nodes periodically
advertise their full neighborhood in Hellos.

       1
      / \
     /   \
    2 --- 3
     \   /
      \ /
       4

Assume that initially node 3 is an MPR, either self-selected, or
neighbor-selected by node 1.

Now assume that node 4 detects the failure of its link to node 3 at
time 0, either from a link-layer failure notification, or from
not hearing Hellos from node 3 for RouterDeadInterval.

Then node 4 will send a Hello at time 2 (worst case) that indicates
node 3 is no longer a neighbor. Assume that node 3 can still hear
Hellos from node 4, so both nodes 2 and 3 will receive the Hello.

If MPRs are self-selected, then node 2 will select itself as an MPR
at time 2, when it receives the Hello from node 4.

If MPRs are neighbor-selected, then node 3 will send a Hello at
time 4 (worst case) indicating that node 4 is no longer a neighbor.
When node 1 receives this Hello, it will select node 2 as an MPR,
and will send a Hello at time 6 including the updated MPR.
Node 2 will then know it is an MPR at time 6, compared to time 2
for self-selected MPRs.

(I know that non-active overlapping relays will forward MPRs
during this recovery period, but with an added delay of
PushbackInterval, and possibly additional overhead if
mutiple non-active relays flood the LSA at nearly the same time,
a real possibility if the number of non-active relays is large.)

Richard

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