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xarqi, Generalist
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Routers using RIP

Customer Question

Assume a router has 30 routes to advertise using RIP, requiring one datagram with 25 routes and another with the remaining 5. What happens if once an hour the first datagram with 25 routes is lost?
Submitted: 11 years ago.
Category: Computer
Customer: replied 11 years ago.
My question is still the same. Any help will be greatly appreciated.
Expert:  xarqi replied 11 years ago.

OK - I'll take a swing at it.

My analysis is based on the RFC:

It says, in part:

As an example, in RIP every gateway that participates in routing
sends an update message to all its neighbors once every 30
seconds. Suppose the current route for network N uses gateway G. If we don't hear from G for 180 seconds, we can assume that either the gateway has crashed or the network connecting us to it has become unusable. Thus, we mark the route as invalid.
When we hear from another neighbor that has a valid route to N, the valid route will replace the invalid one.

It seems that upon loss of the 25 route datagram,

neighbours of the node sending it may, within a short timespan,
invalidate any route information they contain derived from
receipt of that information. Data from other nodes reporting
alternate paths to the distant node that could not now be
reached via the presumably faulty link will be used to update the
routing table.

Note, however, that the RFC goes on to say that it may not actually be a
good idea to drop table entries too quickly, as spontaneous loss
of single datagrams is not uncommon. Whether or not this
occurs may be up to the administrator of each RIP node.

In the quote from the RFC, it is suggested that although messages
should arrive every 30 seconds, no action might be taken until a
silence of 180
seconds is seen.  For the scenario you proposed, the loss of a
single message once per hour, the next datagram superceding the one
was lost should have arrived before the receiving nodes take drastic
action.  It seems that the loss of the single
datagram will actually be without significant consequence!

The phrasing of your question implies that  there is a
consequence, so doubtless I have missed the point.  Anyway, it may
give you some thoughts to ponder.


Customer: replied 11 years ago.
Reply to xarqi's Post: Could you please break your answer down into simplier terms. I understand what you are saying but I don't understand why the differences in time and how it affects the datagram routes within RIP.
Expert:  xarqi replied 11 years ago.
Each gateway maintains a routing table based on information it
receives from adjacent gateways.

It monitors the time between messages from each neighbour,
expecting to see a fresh message each 30 seconds.

If it does not see a message from a neighbour whose data has
been used to build its own routing table, then it knows
something may be wrong. This is the situation that would
apply in the scenario you propose.

If it decides to act (based on the time since the last
message was seen and the parameters set by the network
administrator), it will invalidate the entries in its routing table
that are based on information from the gateway which has not
been responding.

In reality, and in line with the example given in the RFC, a
gateway probably would not act immediately upon
detecting the loss of a single RIP datagram, since dropped
packets are not that rare.

In the case you describe, 30 seconds after the packet should
have arrived, but didn't, a new one would arrive, and all would
be as it was. If the gateway had in fact invalidated and replaced
routing table data as a result of loss of one packet, it would then
have to reverse that process when the fresh packet arrived 30
seconds later.

The RFC says that the gateway may invalidate after a
missing message, but suggests that it should wait a bit longer
and see. In the example in the RFC, 180 seconds is used as a
period of time after which action might be taken to alter the
routing table. That would mean that six successive RIP
datagrams from the gateway in question had been lost.

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