[HECnet] "Dropped by adjacent node" ....

[Paul_Koning at Dell.com]

On May 27, 2014, at 10:48 AM, <Paul_Koning at Dell.com> <Paul_Koning at Dell.com> wrote:


On May 27, 2014, at 10:35 AM, Bob Armstrong <bob at jfcl.com> wrote:

On 2014-05-27 07:07, Paul_Koning at Dell.com wrote:
In other words, the listen timeout for
Ethernet is 3 * hello time, for point to point it is 2 * hello time.

But can you clarify who is dropping who (i.e. which end is timing out)?     The message on LEGATO says "dropped by adjacent node" not "dropping adjacent node", which makes it sound as if MIM is timing out, not LEGATO (and then somehow telling LEGATO that, which is another mystery).

Or is this just a poorly worded message?

No, the message is very precise.

If you see a message on LEGATO which reports adjacency MIM down for that reason, it means LEGATO received an Ethernet router hello message from MIM that no longer lists LEGATO as one of the routers that MIM can see.

MIM will have a reason for not reporting LEGATO any longer.   It should have logged a message stating that reason.

Let me spell out the sequence of protocol exchanges a bit, that may help make this clear.

Suppose I have an Ethernet with two routers on it (and nothing else, to keep it simple).   

1. Start A.   It will periodically send router hello messages with R/S List (see Phase IV routing spec, page 92) that is empty.

2. Start B.   It will send out a router hello message with R/S List empty.

3. A receives that hello.   It adds B to the list of routers it has heard from.   It sends out (immediately, usually) a router hello with one R/S List entry: B, NOT known to be 2-way.

4. B receives that hello from A.   It adds A to the list of routers it has heard from.   It adds A to the R/S List, known 2-way (because A says it has heard B).   It sends that updated hello.   It also generates an Adjacency Up for A.

5. A receives that hello from B.   It sees itself mentioned in the R/S List, so it changes its own R/S list entry for B to say that it now has 2-way connectivity.   It sends out that updated router hello, and generates an Adjacency Up event for B.

So the key thing here is that an adjacency B is not up at A unless A hears from B that B can hear A.   When that stops being true, you get the    dropped    event.

	paul