eigrp, query, reply scenarios

[sarahr202]

hi every body

how is every one doing?

I am reading about e query and reply  messages in eigrp.

i have questions  if you can answer them.

secario 1:

Let say  routerc has following topology table( for the sake of simplicity , only one route is considered)

10.0.0.0   metric= 12 via router d   ( it is successor route)

Let say routerC receives a query  about 12.0.0.0/8, how would router respond?  will it send queries to its neighbor  or it will simply send the reply to the originator of query that it does not know? simply put how does router react to  query about a route it never knew?

  I was reading  online,  according to one web side  ,  if router receives a query about a route that it never knew, router simply sends the reply indicating it does not know the route.  According to webside, that is how summary route is used to control  scope of query in hub and spoke topology.  But then i read somewhere, if  router receives a query about a route it never knew, it will query its own neighbor.

Scenario:

Let say routerC has following in its toplogy table>

11.0.0.0 metric= 12 via router A( successor )

11.0.0.0 metric=24 via router B ( not a feasiblr route)

Let say routerC receives a query from router A , about 11.0.0.0.  I understand router C will remove the entry  : 11.0.0.0  m=12 via router A,   but    will it send the reply to Router A with  11.0.0.0.0 metric=24 ? or  will it query its neighbor trying to find the route 11.0.0.0 as it does not have a feasible route?

thanks a lot.   

[Mohamed Sobair]

Hi Sarah,

If a router running Eigrp recieved query of a route thaqt is not in the Eigrp topology table of the router, it will simply start query all directly connected routers, The Intermediate routers DOESNT send query reply indicating the route is not in thier table, This process continous until the Query message recieved by the last egress Eigrp router which doesnt have a neighbor but only the Querior.

The LAst Egress Eigrp router then replies to the querior router indicating that it doesnt have a loop free path to the mentioned Network, this query reply traverses until it reach the original querior which finally remove the Route from its routing table and send UPdate message to its neighbor indicating that Network x.x.x.x/x is Down and no possible path to it.

The neigbors then start to reemove the Network x.x.x.x from thier Eigrp Topolgy Table.

So, the Website is incorrect in thier explanation, however , they are partially correct saying that Summarization limit the Scope of queries, and this is true, because Summarization creates a Boundary for Eigrp Query message.

HTH

Mohamed

[sarahr202]

blog.ine.com/2009/08/02/eigrp-query-scoping/

Please look ath the above link. thanks for your answer.

[sarahr202]

Please look at the following link. thanks for your response.

blog.ine.com/2009/08/02/eigrp-query-scoping/

[Mohamed Sobair · Accepted solution]

Hi Sarah,

As I said, the Website is partially correct , the reason I am saying they are partially correct because they didnt mention all the circumistances where How the Eigrp router reacts or responds when it recieves a query.

I have had read this before from different Sources , and I would like to cnfirm it to you now according to the documentation.

Please see below Section:-

Routes that have a valid successor are said to be in a “passive” state. If, for some reason, a router loses a route through its successor and does not have a feasible successor for that route, then the route transitions to an “active” state. In the active state, a router sends queries out to its neighbors requesting a path to the lost route.

When an EIGRP neighbor receives a query for a route, it behaves as follows:

• If the EIGRP topology table does not currently contain an entry for the route, then the router immediately replies to the query with an unreachable message, stating that there is no path for this route through this neighbor.

• If the EIGRP topology table lists the querying router as the successor for this route and a feasible successor exists, then the feasible successor is installed and the router immediately replies to the query.

• If the EIGRP topology table lists the querying router as the successor for this route and a feasible successor does not exist, then the router queries all of its EIGRP neighbors except those sent out the same interface as its former successor. The router will not reply to the querying router until it has received a reply to all queries that it originated for this route.

• If the query was received from a neighbor that is not the successor for this destination, then the router replies with its successor information.

According to the above, what I mentioned and originally described that if a router sends a query to an Eigrp neigbor, and the Eigrp neighbor knows this router as a successor for its path before, then the router queries all its directly connected neighbors and it wont reply untill it recieves a reply.

HOWEVER, if the router recieves a query for a Network that wasnt in its topolgy table (NO Successor or Feasible Successor), it will immediately reply with unreachable message to the querior.

Here is a reference:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008010f016.shtml (See background information)

HTH,

Mohamed

[Kishore Chennupati]

Good explanation Mohammed 5 points from me to you as well.