EIGRP SIA - Basics

[jas2061461]

So I am studying EIGRP, and I am missing a fundamental concept of EIGRP.  I have read several documentations, Routing TCP/IP Volume I, CCNP Route, etc and have still not seen a simple answer.

Scenario 1:  LAN1 - R1 - R2 - LAN2

So, a cable between LAN1 and R1 is disconnected in scenario 1.  This is recognized by EIGRP and the path to that network is no longer available. EG That successor route is no longer available.  R1 does not have a feasible successor in its topology table.  So, it begins the SIA timer and sends Query messages to all its neighbors.  Obviously R2 does not have a route to LAN1.  Does it Reply with a "no" and remove the current route via R1 out of the topology table or does it just make the route via R1 active because there are no other neighbors to query to and wait?

Scenario 2: 

LAN2

  |      |

R2   R3

  \    /

   R1

    |

LAN1

So, a cable between LAN1 and R1 is disconnected in scenario 2. This is recognized by EIGRP and the path to that network is no longer available. EG That successor route is no longer available. R1 does not have a feasible successor in its topology table. So, it begins the SIA timer and sends Query messages to all its neighbors.  For simplicity, R2 and R3 receiving the Query packet at the same time.  Obviously they don't have a feasible successor route to LAN1 and cannot reply immediately, so they mark the current route as Active.  They then Query their neighbors (each other).  What happens at this point?  Do they recognize they have received multiple queries from all of there neighbors and simply reply with no, or???

All the help is appreciated...for study purposes and work related issues.

-Jason C.

[Mohamed Sobair]

Hello Jason,

Scenario 1:

When R1 LAN is disconnected, R1 doesnt have valid Successor or Feasible Successor route to the LAN, R1 starts querying all directly connected neighbors (R2 in this case) for alternative path. R2 obviously doesnt have a valid path to R1 LAN so it replies with NO valid path to R1.  R1 remove the network from its Eigrp Topology Table and sends update message to R2 that netwok X is longer reachable and R2 removes it from its Topology table as Well.

Scenario2:

Like Wise, R1 has no valid successor or feasible successor to its LAN, it queries R2, R3 respectively, R2,R3 queries each other and sends Query reply to each other and then to R1. R1 remove Network X from the Topology table and sends update Message to R2, R3 to remove the Network from their EIGRP Topology table.

Regards,

Mohamed