Pop Quiz - EIGRP and DUAL experts

Question #1 EIGRP

Exactly when is the RTP protocol used? Is it used only when traffic
crosses WAN links or is it also used on a local LAN? Is it used only
for certain packet types or all packet types? Is it always used on
routing updates?

THE BOOKS DISAGREE ON THE ANSWER TO THIS QUESTION.


Question #2 DUAL
DUAL will reject a route update if the REPORTED DISTANCE is less then
the FEASIBLE DISTANCE of the successor - to prevent a possible routing
loop. But, if the successor is removed and DUAL performs a diffused
computation, and gets a reply advertising THE SAME EXACT NETWORK IT
REJECTED BEFORE, DUAL will accept the route. Right?

So how does DUAL know for sure the route is not looped when it accepts
it. How is the guaranteed 100% loop-free promise kept in this
circumstance?

Thanx!
Jimbo

 

Jimbo,

From Cisco's site...

Reliable Transport Protocol (RTP) is responsible for guaranteed, ordered
delivery of Enhanced IGRP packets to all neighbors. It supports intermixed
transmission of multicast or unicast packets. For efficiency, only certain
Enhanced IGRP packets are transmitted reliably. On a multiaccess network
that has multicast capabilities, such as Ethernet, it is not necessary to
send hello packets reliably to all neighbors individually. For that reason,
Enhanced IGRP sends a single multicast hello packet containing an indicator
that informs the receivers that the packet need not be acknowledged. Other
types of packets, such as updates, indicate in the packet that
acknowledgment is required. RTP contains a provision for sending multicast
packets quickly when unacknowledged packets are pending, which helps ensure
that convergence time remains low in the presence of varying speed links.
The DUAL finite-state machine embodies the decision process for all route
computations by tracking all routes advertised by all neighbors. DUAL uses
distance information to select efficient, loop-free paths and selects routes
for insertion in a routing table based on feasible successors. A feasible
successor is a neighboring router used for packet forwarding that is a
least-cost path to a destination that is guaranteed not to be part of a
routing loop. When a neighbor changes a metric, or when a topology change
occurs, DUAL tests for feasible successors. If one is found, DUAL uses it to
avoid recomputing the route unnecessarily. When no feasible successors exist
but neighbors still advertise the destination, a recomputation (also known
as a diffusing computation) must occur to determine a new successor.
Although recomputation is not processor-intensive, it does affect
convergence time, so it is advantageous to avoid unnecessary recomputations.

Hope that helps,

Jim

 

Hey, thanx for the reply. The stuff on RTP is helpful. I pulled down a
couple of white papers from the site, but it did not include this
detail.

As far as EIGRP goes, I did not see anything in what you posted that
goes to my question. Did it make sense to you? I understand the
mechanics of what EIGRP does for route selection, but I have yet to
hear clear reasoning on why EIGRP will accept a route it previously
rejected for failing to meet the ferasibility condition. How does DUAL
know the route is loop-free the second time around?

Thank you
Jimbo

 

DUAL accepts previously rejected route

Hi ,


DUAL will accept the route it rejected previously( call it routeA).
The route (route A) was rejected earlier
since there was a possibility of forming a loop . Note that another successor
route was present at that time and there was a chance for loop if routeA gets
installed in RT.

Now the successor route gets removed.

DUAL happens and it finds that the best route is routeA

Now DUAL can safely install routeA , since the earlier successor route is
not present and there is no chance of forming a loop including routeA and the
earlier successor route.


thanks
anish