EIGRP Questions and Answers
1. What Does The EIGRP Stuck In Active Message Mean?
Answer: When EIGRP returns a "stuck in active" (SIA) message, it means the router has not received a reply from a query. This happens when:
A route is lost, and no feasible alternate route exists in the topology table.
A query is sent to neighbors, but no reply is received.
The SIA occurs due to:
The reported route being unavailable.
A neighbor failing to respond to the query.
When this happens, the router clears the non-responsive neighbor. This neighbor could be multiple hops away.
2. What Is The Difference In Metric Calculation Between EIGRP And IGRP?
Answer:
EIGRP's metric is derived by multiplying the IGRP metric by 256.
IGRP uses a 24-bit metric field in its updates, while EIGRP uses a 32-bit metric field.
Example:
If the IGRP metric to a network is 8586, the EIGRP metric will be 8586 x 256 = 2,198,016.
EIGRP uses a composite metric based on:
K1 (Bandwidth) and K3 (Delay) by default.
Additional variables: K2 (Load), K4 (Reliability), and K5 (MTU) can be configured.
3. Should I Always Use The EIGRP Log-Neighbor-Changes Command When I Configure EIGRP?
Answer: Yes, using this command simplifies troubleshooting by identifying the reason an EIGRP neighbor was reset, reducing troubleshooting time.
4. What Debugging Capabilities Does EIGRP Have?
Answer: EIGRP provides both protocol-independent and protocol-dependent debug commands. Additionally, various show commands help monitor EIGRP status:
show ip eigrp neighbors
show ip eigrp interfaces
show ip eigrp topology
show ip eigrp traffic
5. What Percent Of Bandwidth And Processor Resources Does EIGRP Use?
Answer:
EIGRP is designed to use no more than 50% of the configured bandwidth on any link during convergence.
You can adjust this using the ip bandwidth-percent eigrp command.
EIGRP optimizes resource usage with features like:
Partial and incremental updates—Routing information is sent only during topology changes.
Feasible successors—Minimizes route recomputation by limiting it to affected routes.
6. Two EIGRP Processes Run And Two Equal Paths Are Learned. Will Both Be Installed?
Answer: No, only one route is installed. The route from the EIGRP process with the lower Autonomous System (AS) number is preferred.
In older Cisco IOS releases (pre-12.2(7)T), the route with the latest timestamp was installed.
7. How Can I Use Only One Path When A Router Has Two Equal Cost Paths?
Answer: To prevent equal-cost paths:
Configure default bandwidth values on the interfaces.
Increase the delay on the backup interface to differentiate path costs.
8. What Are Different Route Types In EIGRP?
Answer: EIGRP has three route types:
Internal Route: Originates within the Autonomous System (AS).
Summary Route: Created by summarizing internal routes.
External Route: Introduced into EIGRP through redistribution from other protocols.
9. How Many Packet Types Are In EIGRP?
Answer:
Hello Messages:
Used for neighbor discovery and keepalive.
Multicast at 224.0.0.10.
Acknowledgement for unicast.
NULL Update:
Calculates SRTT (Smooth Round Trip Timer) and RTO (Retransmission Timeout).
SRTT: Time taken for a packet to reach a neighbor and receive an acknowledgment.
RTO: Time the router waits for an acknowledgment before resending the packet.
Let me know if you'd like further clarifications or additional questions!