EIGRP Dual Algorithm

Here Router C has 3 path

Via	FD	AD	Topology
B	3	1	Successor
D	4	2	FS
E	4	3	---------

Here Router D has 3 path

Via	FD	AD	Topology
B	2	1	Successor
C	5	3	---------

Here Router E has 3 path

Via	FD	AD	Topology
C	4	3	---------
D	3	2	Successor

Now suppose Link between the Router B and Router D got down means lets see how EIGRP Dual algorithm works.

Router D checks the topology and mark the network 10.1.1.0/24 as unusable path via B and check if its find any F Successor. Since in EIGRP Topology there is no F Successor its mark the route as Active.

Here Router D has 3 path

Via	FD	AD	Topology
B	2	1	Successor
C	5	3	---------

Router D send the query to its neighbor that is Router C and Router E. Router D Marks Routers C and E as having a query pending (q).

At Router E, DUAL marks the path to network 10.1.1.0/24 through Router D as unusable.

At Router C, DUAL marks the path to network 10.1.1.0/24 through Router D as unusable.

Here Router C has 3 path

Via	FD	AD	Topology
B	3	1	Successor
D	4	2	FS
E	4	3	---------

Here Router E has 3 path

Via	FD	AD	Topology
C	4	3	---------
D	3	2	Successor

The following steps then occur

At Router D:

— DUAL receives a reply from Router C that indicates no change to the path to

network 10.1.1.0/24.

— DUAL removes the query flag from Router C.

— DUAL stays active on network 10.1.1.0/24, awaiting a reply from Router E to its

query (q).

At Router E, there is no FS to network 10.1.1.0/24, because the AD from Router C (3) is not

less than the original FD (also 3).

— DUAL generates a query to Router C.

— DUAL marks Router C as query pending (q).

At Router C, DUAL marks the path to network 10.1.1.0/24 through Router E as unusable.

The following steps then occur

At Router D, DUAL stays active on network 10.1.1.0/24, awaiting a reply from Router E (q).

At Router E:

— DUAL receives a reply from Router C indicating no change.

— It removes the query flag from Router C.

— It calculates a new FD and installs a new successor route in the topology table.

— It changes the route to network 10.1.1.0/24 from active to passive (converged).

At Router D

— DUAL receives a reply from Router E.

— It removes the query flag from Router E.

— It calculates a new FD.

— It installs new successor routes in the topology table. Two routes (through Routers C  and E) have the       same FD, and both are marked as successors.

— It changes the route to network 10.1.1.0/24 from active to passive (converged).

— The network is stable and converged.