EIGRP INE 1

Question 1

Why use EIGRP?

Answer 1

Guarantees loop-free topology
-Diffusing Update Algorithm (DUAL)
Fast convergence
-Fastest of all IGP in certain designs
Reliable & Efficient Updating
-Forms active neighbor adjacencies
-Guratesss packet delivery with (RTP)
-Supports partial updates

Question 2

Why use EIGRP, cont?

Answer 2

Unequal cost load balancing
-Only IGP that supports true load distribution
Control plane security
-Support MD5 based authentication

Question 3

How EIGRP works?

Answer 3

Step 1 - Discover EIGRP Neighbors
Step 2 - Exchange Topology Information
Step 3 - Choose Best Path via DUAL
Step 4 - Neighbor and Topology Table Maintenance

Question 4

How does Step 1 - Discover EIGRP Neighbors works?

Answer 4

EIGRP uses multicast “HELLO” packets to discover neighbors on EIGRP-enabled links
-Transport via IP protocol 88 (EIGRP)
-Multicast Destination Address 224.0.0.10
Hello, packets contain:
-Autonomous System Number
-Hold Time
-Authentication
-Metric Weightings (K values)
EIGRP neighbors found are inserted into the EIGRP neighbor table
Neighbors that agree on attributes and exchange updates form active adjacency

Question 5

How does Step 2 - Exchange Topology Information works?

Answer 5

Once neighbors are found, EIGRP “UPDATE” messages are used to exchange routes.
-Sent as multicast to 224.0.0.10 or as unicast
RTP uses sequence numbers and acknowledgments (ACKs) to ensure delivery.
Update messages describe attributes of a route
-prefix + Length
-Next-Hop
-Bandwidth
-Delay
-Load
-Reliability
-MTU
-Hop Count
-External Attributes
All routes learned from all neighbors make up the EIGRP topology table.

Question 6

How does Step 3 - Choose Best Path via DUAL work?

Answer 6

Once the topology is learned, DUAL runs to choose the loop-free best path to each destination.
Metric to choose the best path
-Administrative Weighting
-Bandwidth
-Delay
-Load
-Reliability
The path with the lowest metric is considered best and installed in the routing table.
One or more backup routes can also be calculated per destination.
Only the best route is advertised to other EIGRP neighbors.

Question 7

How does Step 4 - Neighbor and Topology Table Maintenance work?

Answer 7

EIGRP Hello packets contain Hold time
-if no hello is received within the hold time, the neighbor is declared unreachable
When the neighbor is lost paths are removed from the topology table
If backup routes exist, they become the new best path.
If no backup routes, then DUAL must run again.