EIGRP Metric Wrong Due to Incorrect Bandwidth Statement
Presenting Symptom
A directly connected EIGRP neighbor is not forming adjacency, or routes are missing from the routing table despite correct network statements.
Network Context
Two routers (R1 and R2) are connected via a serial link (Serial0/0/0) in a small branch office. Both run Cisco IOS 15.x. The link is configured with EIGRP AS 100, and both routers advertise their connected networks. The bandwidth on the serial interface is manually set to a value that does not match the actual link speed, causing EIGRP metric calculation to differ from expected.
Diagnostic Steps
Check EIGRP neighbors
show ip eigrp neighborsR1# show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 192.168.12.2 Se0/0/0 13 00:00:15 1 100 0 1If neighbor is listed, adjacency is up. If not, check for mismatched AS, authentication, or passive interface. In this scenario, neighbor may be missing or flapping.
Verify EIGRP interfaces and metrics
show ip eigrp interfaces detailR1# show ip eigrp interfaces detail
EIGRP-IPv4 Interfaces for AS(100)
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Se0/0/0 1 0/0 1 0/10 50 0
Hello interval: 5 sec, Hold time: 15 sec
Bandwidth: 64 kbps, Delay: 20000 usec
Reliability: 255/255, Load: 1/255
...Look at the Bandwidth field. If it is incorrect (e.g., 64 kbps instead of 1544 kbps for a T1), that will affect EIGRP metric calculation and may cause route preference issues or failure to form adjacency if K-values are mismatched.
Check interface bandwidth configuration
show running-config interface serial0/0/0interface Serial0/0/0 ip address 192.168.12.1 255.255.255.0 bandwidth 64 no shutdown
The bandwidth statement shows 64 kbps. This is likely incorrect for the actual link speed (e.g., T1 is 1544 kbps). EIGRP uses bandwidth in metric calculation; a low bandwidth can cause high metric and potential route suppression.
Compare with neighbor's bandwidth
show ip eigrp topology all-linksR1# show ip eigrp topology all-links
EIGRP-IPv4 Topology Table for AS(100)/ID(192.168.12.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 192.168.12.0/24, 1 successors, FD is 28160
via Connected, Serial0/0/0
P 10.1.1.0/24, 1 successors, FD is 40537600
via 192.168.12.2 (40537600/28160), Serial0/0/0If the feasible distance (FD) is unusually high for a directly connected route, it may be due to incorrect bandwidth. Compare with expected metric based on actual link speed.
Root Cause
The bandwidth statement on the serial interface is set to 64 kbps, which does not match the actual link speed (e.g., T1 at 1544 kbps). EIGRP uses bandwidth in its composite metric; an incorrect low bandwidth inflates the metric, potentially causing routes to be suppressed or not installed, and may prevent neighbor adjacency if K-values are mismatched.
Resolution
Verification
Run 'show ip eigrp interfaces detail' to confirm bandwidth is now 1544 kbps. Also run 'show ip route eigrp' to verify that all expected routes are present with correct metrics.
Prevention
1. Always configure the bandwidth statement to match the actual link speed, especially on serial interfaces. 2. Use network documentation to verify correct bandwidth values. 3. Consider using 'auto-cost reference-bandwidth' if using OSPF, but for EIGRP, ensure bandwidth is set correctly.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why an EIGRP neighbor is not forming or why routes are missing. The exam tests understanding that EIGRP uses bandwidth and delay in metric calculation, and that the bandwidth command is a configurable parameter that can affect routing. Candidates must know the 'show ip eigrp interfaces detail' command and how to interpret the bandwidth field.
Exam Tips
Remember that EIGRP uses the lowest bandwidth along the path in its metric calculation; an incorrect bandwidth on any interface can affect the entire path metric.
The 'bandwidth' command is a Cisco IOS parameter that does not change the actual line speed; it only affects routing protocols and QoS.
Be able to identify a mismatched bandwidth by comparing the 'show ip eigrp interfaces detail' output between neighbors.
Commands Used in This Scenario
show ip eigrp neighbors
Displays the neighbor table for EIGRP, showing all directly connected EIGRP routers and their status, used to verify EIGRP adjacencies and troubleshoot neighbor relationships.
show ip eigrp topology
Displays the EIGRP topology table, showing all learned routes and their feasible successors, used to verify EIGRP convergence and path selection.
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Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
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