In Cisco networking, the interface bandwidth value plays a crucial role in routing decisions without affecting the actual link speed. For CCNA students, this is key to understanding basic router configurations and why routing protocols might choose suboptimal paths. CCNP learners will appreciate its ties to advanced topics like OSPF cost calculations and QoS policies. This guide explains defaults, configuration, verification, and real-world applications, with labs to practice.
All Cisco interfaces have default bandwidth values assigned during startup. If the default is much higher than the actual link bandwidth, it can lead to suboptimal routing decisions. For instance, in OSPF or EIGRP, the protocol might select a path assuming 100 Mbps availability when it’s only 50 Mbps, ignoring a truly faster alternative. This is because bandwidth influences metric calculations, for example, OSPF cost = reference bandwidth / interface bandwidth.
The interface bandwidth value does not affect the actual speed or capacity of the link. It is only used for routing metric calculation. The bandwidth value must reflect the actual speed of the link so that the routing table has accurate best-path information.
Differences from Clock Rate and Speed Commands
Unlike the bandwidth command, which is purely informational for routing protocols:
- Clock rate: Used on serial interfaces (DCE side only) to set the actual bit transmission rate, for example
clock rate 64000for 64 Kbps. Without a clock rate configured on the DCE end, a back-to-back serial link stays down entirely, regardless of any bandwidth value configured. - Speed: Sets the operational speed on Ethernet interfaces, for example
speed 100for 100 Mbps. Unlike bandwidth, this enforces an actual hardware-negotiated limit.
Example clock rate configuration:
interface Serial0/0/0
clock rate 128000Ethernet interfaces usually report a bandwidth value matching the negotiated link speed, but serial interfaces commonly default to 1544 kbps (T1 speed). The bandwidth command informs Cisco IOS of the usable bandwidth for metric purposes only, not for enforcing any actual transmission rate.
Default Bandwidth Values
| Interface Type | Default Bandwidth (kbps) |
|---|---|
| Ethernet | Matches negotiated link speed |
| FastEthernet | 100000 |
| GigabitEthernet | 1000000 |
| Serial | 1544 (T1 speed) |
Note: the specific default value shown for a serial interface can vary in some cases depending on platform and IOS version; always confirm the actual configured value with show interfaces on your specific hardware rather than assuming a single universal default applies to every serial interface you encounter.
Example Topology
Refer to Figure 1 for the network diagram:
- Router1 to Router2: 10 Mbps
- Router2 to Router3: 1544 kbps
- Router3 to Router4: 100 Mbps
- Router1 to Router4: 128 kbps
Lab Setup Tip for CCNA/CCNP
Replicate this topology in GNS3 or Packet Tracer. Configure OSPF and observe route changes before and after bandwidth adjustments.
Use show ip route to verify the routing table’s chosen path, and show interfaces to view the currently configured bandwidth on an interface (for example, Figure 2 shows BW 128 Kbit for Serial0/3/1). If the reported bandwidth is higher than the actual link speed, adjust it to match reality.
Additional verification:
show ip ospf interface— shows the OSPF cost calculated from the configured bandwidthshow interfaces | include BW— quick filter to see the bandwidth line across all interfaces at once
Adjusting the Interface Bandwidth
To adjust an interface’s bandwidth value for routing metric purposes, use the bandwidth [kilobits] command in interface configuration mode. To restore the interface’s default bandwidth, use no bandwidth.
interface Serial0/3/1
bandwidth 128This sets the interface’s bandwidth value to 128 kbps for routing metric calculations only, without changing the interface’s actual physical transmission rate. Figure 3 illustrates this bandwidth configuration; the serial interface on Router-3 in the example topology requires the same bandwidth configuration to accurately reflect its actual 1544 kbps link speed for correct routing metric calculations.
QoS Integration (CCNP Level)
In Modular QoS CLI (MQC) policy maps, bandwidth percent 20 guarantees 20% of the interface’s configured bandwidth for a specific traffic class. Use bandwidth remaining percent instead for relative allocation among remaining traffic classes after any priority queues have already been serviced.
Impact on Routing Protocols
For CCNA students, bandwidth is essential in dynamic routing. In OSPF, the default reference bandwidth is 100 Mbps, so cost = 100,000,000 / interface bandwidth (in bps). For example:
- A 10 Mbps link (
bandwidth 10000) has cost = 100,000,000 / 10,000,000 = 10. - If the configured bandwidth is mismatched from the actual link speed, OSPF might prefer a slower path simply because it calculates a lower cost.
In EIGRP, bandwidth is part of the composite metric:
Metric = [K1×BW + (K2×BW)/(256−load) + K3×Delay] × [K5/(reliability+K4)]Where bandwidth is derived from the inverse of the lowest bandwidth along the path (scaled). By default, K5 is 0, in which case the entire second bracket term is treated as 1, simplifying the formula to just the first bracket.
CCNP expansion: In multi-area OSPF, bandwidth mismatches specifically affect cost-based path selection within and between areas; in BGP, bandwidth is not used directly as a route metric input the way it is in OSPF or EIGRP, though underlying link bandwidth mismatches can still contribute to broader traffic blackholing or suboptimal forwarding if paths are engineered without accounting for actual link capacity. Always verify with show ip ospf neighbor and show ip eigrp topology when diagnosing bandwidth-related routing issues.
Common Mistakes and Pitfalls
- Forgetting to configure bandwidth on both ends of a link, leading to asymmetric metric calculations between the two directions.
- Confusing kbps vs. Mbps in the command: the
bandwidthcommand uses kilobits, so 10 Mbps is entered asbandwidth 10000. - Ignoring QoS interactions: an inflated bandwidth value can cause queue oversubscription in policy maps that calculate percentages based on it.
- Serial links: without
clock rateconfigured on the DCE end, the interface stays administratively up but the line protocol remains down; check withshow controllersto confirm DCE/DTE role and clocking status.
Lab Exercises
Basic CCNA Lab: Verify and Adjust Bandwidth
- Boot routers in Packet Tracer with the Figure 1 topology.
- On Router1, run
show interfaces serial0/3/1and note the default bandwidth shown. - Configure the interface:
interface serial0/3/1
bandwidth 128- Verify the change with
show interfaces serial0/3/1. - Enable OSPF across all interfaces for this lab exercise:
router ospf 1
network 0.0.0.0 255.255.255.255 area 0- Check the resulting OSPF costs with
show ip ospf interface.
Troubleshooting Tips
- If routes appear suboptimal, check
show ip eigrp topologyto see the specific metrics being influenced by the current bandwidth configuration. - A common error is forgetting
clock rateon the serial DCE end, which causes the link to stay down regardless of anybandwidthvalue configured. - Exam tip: the
bandwidthcommand is entered in kilobits, so remember that 10 Mbps must be entered asbandwidth 10000, notbandwidth 10.
Conclusion
Mastering interface bandwidth configuration ensures accurate routing decisions across your network, since both OSPF and EIGRP rely directly on this value, not the interface’s actual physical speed, to calculate their respective metrics. Keeping the configured bandwidth value aligned with the real link speed, verifying with show interfaces and show ip ospf interface, and understanding the clear distinction between bandwidth, clock rate, and speed are foundational skills for both CCNA exam success and real-world network troubleshooting. Practice the lab exercises above to solidify this for exam day.
FAQs
What is the default bandwidth for Cisco serial interfaces?
The commonly documented default bandwidth for Cisco serial interfaces is 1544 kbps, matching T1 line speed, used for routing metric calculations without affecting the interface’s actual physical transmission rate. Always confirm the specific default on your actual hardware and IOS version with show interfaces, since defaults can vary by platform, rather than assuming a single universal value applies everywhere.
How does bandwidth affect OSPF routing decisions?
Bandwidth affects OSPF by directly determining the calculated cost for that interface, using the formula reference bandwidth (100 Mbps by default) divided by the interface’s configured bandwidth in bits per second. If the configured bandwidth value doesn’t match the actual physical link speed, OSPF can end up preferring a path that appears cheaper by its cost calculation but is actually slower in real-world throughput, making accurate bandwidth configuration essential for genuinely optimal path selection.
What is the difference between the bandwidth and clock rate commands?
The bandwidth command sets a value used purely for routing protocol metric calculations, such as OSPF cost or the EIGRP composite metric, and has no effect on the interface’s actual transmission rate. The clock rate command, configured only on the DCE end of a serial connection, sets the interface’s actual physical bit transmission rate; without it configured correctly, a serial link will not come up at all, regardless of what bandwidth value has been set.
How can I verify the bandwidth configuration on a Cisco router?
Use show interfaces to view the currently configured bandwidth value for a specific interface, shown in the output as a line like “BW 128 Kbit.” To see how that bandwidth value is actually being used in OSPF’s cost calculation specifically, use show ip ospf interface, which displays the resulting calculated cost directly. For EIGRP, show ip eigrp topology shows the composite metric values that bandwidth contributes to for each learned route.
