Subject: Re: Neighbor state change

From: Paresh Khatri <[email-masked]>
Date: Thu, 24 Apr 2003 09:44:17 +1000

Thanks guys, I finally understand !!

Regards,
Paresh.

>>> [email-masked] 04/24/03 09:40AM >>>
Paresh,

>Say you have this scenario.  You have two routers, RT1 and RT2 on a
>broadcast network.  RT1 comes online first and sends out a Hello packet
>that is received by RT2.  Having seen the hello from RT1, RT2 transitions
>to Init.  Since it is now in Init, it sends out a Hello packet which
>includes RT1 in its list of neighbors seen.  Rt1 (which is still in a Down
>state), receives the hello and interprets its as '2-Way Received', since
>it can see itself in the list of neighbors in the hello
>packet.  Therefore, a router in a 'Down' state has received a '2-Way
>Received' event.  Is this then possible ?  This example is a subset of the
>example given in Section 10.10 of rfc2328...
>
>             +---+                                         +---+
>             |RT1|                                         |RT2|
>             +---+                                         +---+
>
>             Down                                          Down
>                             Hello(DR=0,seen=0)
>                        ------------------------------>
>                          Hello (DR=RT2,seen=RT1,...)      Init
>                        <------------------------------

First two bullet points on page 98 (section 10.5 - Receiving Hello Packets):

 >        o   Each Hello Packet causes the neighbor state machine to be
 >           executed with the event HelloReceived.
 >
 >        o   Then the list of neighbors contained in the Hello Packet is
 >            examined.  If the router itself appears in this list, the
 >            neighbor state machine should be executed with the event 2-
 >            WayReceived.  Otherwise, the neighbor state machine should
 >            be executed with the event 1-WayReceived, and the processing
 >            of the packet stops.

The first point will cause the HelloReceived event, causing the neighbor
FSM to transition from Down to Init.

The second point will cause the 2-WayReceived event but the neighbor FSM
has already transitioned to Init. The 2-WayReceived is a valid event for
the Init state. Further actions will be as described in section 10.3.

Hope this helps.

-Ashish.