delay [tens-of-us]
Sets the delay value (in tens of microseconds) for an interface, used by routing protocols like EIGRP to calculate metric.
Definition: delay [tens-of-us] is a Cisco IOS interface config command. Sets the delay value (in tens of microseconds) for an interface, used by routing protocols like EIGRP to calculate metric.
Overview
The `delay` command in Cisco IOS is used to set the delay value for an interface, expressed in tens of microseconds. This delay value is a key metric used by routing protocols, most notably Enhanced Interior Gateway Routing Protocol (EIGRP), to calculate the composite metric for path selection. In EIGRP, the default metric calculation uses bandwidth and delay, where delay is the sum of the delays along the path.
By modifying the delay on an interface, a network engineer can influence the routing decision without altering the actual physical characteristics of the link. This is particularly useful for traffic engineering, where you want to prefer one path over another even if the bandwidth is the same. For example, you might increase the delay on a backup link to make it less preferred, or decrease it on a primary link to ensure it is chosen.
The command is applied in interface configuration mode and takes a single argument: the delay value in tens of microseconds. The default delay for most interfaces is based on the interface type (e.g., 100 microseconds for Gigabit Ethernet, 1000 microseconds for Fast Ethernet). It is important to note that the delay command affects only the routing protocol metric; it does not change the actual propagation delay of the interface.
This command is often used in conjunction with other metric manipulation tools like the `bandwidth` command. When troubleshooting routing issues, checking the delay value can help explain why a particular path is being preferred or ignored. The command's effect is immediate and is stored in the running configuration.
It requires privileged EXEC mode to configure, and the change is saved to the startup configuration with `copy running-config startup-config`. Understanding the delay command is essential for CCNA and CCNP candidates as it appears in both configuration and troubleshooting scenarios, especially in EIGRP environments.
delay [tens-of-us]When to Use This Command
- Adjust EIGRP metric on a slow WAN link to prefer an alternative path
- Influence routing path selection by increasing delay on a backup interface
- Simulate higher latency for testing routing protocol behavior
- Fine-tune EIGRP composite metric without changing bandwidth
Parameters
| Parameter | Syntax | Description |
|---|---|---|
| tens-of-us | <1-16777215> | The delay value in tens of microseconds. Valid range is 1 to 16,777,215 (which corresponds to 0.1 microseconds to 167.77215 seconds). Common values: 1 (10 microseconds) for very low delay, 100 (1 millisecond) for typical LAN, 1000 (10 milliseconds) for WAN. A common mistake is setting the value too low or too high, causing unexpected routing behavior. The value must be an integer. |
Command Examples
Setting delay on a serial interface
Router(config-if)# delay 20000Router(config-if)# end
Router# show interfaces serial 0/0/0
Serial0/0/0 is up, line protocol is up
Hardware is HD64570
Internet address is 192.168.1.1/24
MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255The 'delay 20000' command sets the delay to 20000 tens of microseconds (i.e., 200,000 microseconds). In the output, 'DLY 20000 usec' shows the configured delay in microseconds. This value is used by EIGRP in its metric calculation.
Verifying delay with show interface
Router# show interfaces gigabitethernet 0/1GigabitEthernet0/1 is up, line protocol is up
Hardware is CN Gigabit Ethernet, address is aabb.cc00.0201 (bia aabb.cc00.0201)
Internet address is 10.0.0.1/24
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255The default delay for GigabitEthernet is 10 microseconds (displayed as 'DLY 10 usec'). This is a low delay value, indicating a high-speed link. The delay is used by EIGRP; lower delay results in a better (lower) metric.
Understanding the Output
The 'show interfaces' output includes the 'DLY' field which shows the interface delay in microseconds. This value is a key component in EIGRP metric calculation (metric = bandwidth + delay). A lower delay value indicates a faster link and results in a lower (more preferred) EIGRP metric.
The default delay varies by interface type (e.g., 10 usec for GigabitEthernet, 20000 usec for serial). When troubleshooting routing issues, verify that the delay values are consistent with expected path selection. If a path is not being preferred, check if the delay has been manually changed or if defaults are appropriate.
The delay is also used by other routing protocols like OSPF for cost calculation on certain platforms, but primarily it's an EIGRP metric component.
Configuration Scenarios
Influence EIGRP path selection by adjusting delay on a backup link
In a dual-homed network, you want the primary link to be preferred over the backup link for all traffic. By increasing the delay on the backup link, EIGRP will calculate a higher metric for that path, making the primary link more attractive.
Topology
R1(Gi0/0)---10.0.12.0/30---(Gi0/0)R2
R1(Gi0/1)---10.0.13.0/30---(Gi0/0)R3
R2 and R3 are connected to the same remote network 192.168.1.0/24.Steps
- 1.Step 1: Enter global configuration mode: configure terminal
- 2.Step 2: Enter interface configuration for the backup link on R1: interface GigabitEthernet0/1
- 3.Step 3: Set the delay to a higher value (e.g., 2000 tens of microseconds = 20 ms): delay 2000
- 4.Step 4: Exit and verify: end, show interfaces GigabitEthernet0/1 | include delay
! On R1 interface GigabitEthernet0/1 delay 2000
Verify: Use 'show interfaces GigabitEthernet0/1' and look for 'Delay' in the output. Expected: 'Delay 20000 usec' (since IOS displays in microseconds, multiply by 10). Also use 'show ip eigrp topology' to see the metric change.
Watch out: The delay value is in tens of microseconds, but 'show interfaces' displays it in microseconds. So if you set delay 2000, the output shows 20000 usec. Also, ensure EIGRP is enabled on the interface; otherwise, the delay change has no effect.
Equalize paths in EIGRP by adjusting delay on a faster link
You have two links between routers with different bandwidths, but you want EIGRP to load-balance equally. By reducing the delay on the slower link, you can make the composite metric equal to that of the faster link.
Topology
R1(Gi0/0)---10.0.12.0/30---(Gi0/0)R2 (100 Mbps)
R1(Gi0/1)---10.0.13.0/30---(Gi0/1)R2 (10 Mbps)Steps
- 1.Step 1: On R1, enter interface configuration for the slower link: interface GigabitEthernet0/1
- 2.Step 2: Check current delay: do show interfaces GigabitEthernet0/1 | include delay
- 3.Step 3: Set a lower delay to compensate for lower bandwidth (e.g., reduce from default 1000 to 100): delay 100
- 4.Step 4: Verify the metric using 'show ip eigrp topology 192.168.1.0/24'
! On R1 interface GigabitEthernet0/1 delay 100
Verify: Use 'show ip eigrp topology 192.168.1.0/24' to see the metric for both paths. They should be equal if the delay adjustment is correct. Also check 'show interfaces GigabitEthernet0/1' for the delay value.
Watch out: Reducing delay too much can make the path appear better than the faster link, causing all traffic to use the slower link. Also, remember that EIGRP metric also includes bandwidth; you may need to adjust both. The delay command affects the composite metric directly as a coefficient.
Troubleshooting with This Command
When troubleshooting routing issues related to the delay command, the primary tool is the 'show interfaces' command. Look for the 'Delay' field in the output. A healthy interface will have a delay value that matches the configured value (or default if not configured).
For example, a GigabitEthernet interface typically has a default delay of 100 microseconds (10 tens of microseconds). If you have configured a delay, ensure it appears correctly. A common problem is that the delay value is set incorrectly, causing unexpected routing behavior.
For instance, if you set delay 1 (10 microseconds) on a WAN link, EIGRP might prefer that path over a LAN link, leading to suboptimal routing. To diagnose, compare the delay values on all interfaces participating in the routing protocol. Use 'show ip eigrp topology' to see the computed metric and identify which path is being used.
The metric is displayed as a composite value; you can break it down by looking at the 'show ip eigrp topology' detail output, which shows the individual components (bandwidth, delay, reliability, load). Focus on the delay component. If the delay is unexpectedly high or low, check the interface configuration.
Another useful command is 'show ip route' to see which path is installed. If a path is not being used despite having a lower metric, verify that the delay is not causing the metric to exceed the variance or that the path is not feasible. Also, check for any 'delay' configuration on the remote end; the delay is cumulative along the path.
Use 'traceroute' to see the actual path taken. If you suspect a delay misconfiguration, use 'debug eigrp packets' cautiously to see metric advertisements. However, be aware that changing delay can cause routing loops if not done consistently.
Always verify the impact on the entire network. In summary, the delay command is a powerful tool for metric manipulation, but misconfiguration can lead to routing anomalies. Systematic verification using show commands and understanding the metric calculation is key.
CCNA Exam Tips
Remember that delay is configured in tens of microseconds, but 'show interfaces' displays it in microseconds (multiply by 10).
EIGRP metric formula: metric = (K1 * BW + K3 * DLY) * 256, with default K values K1=1, K3=1. Delay is the sum of delays along the path.
The 'delay' command does not affect actual interface latency; it only influences routing protocol metrics.
On CCNA exam, you may be asked to calculate EIGRP metric changes after modifying delay on an interface.
Common Mistakes
Confusing delay units: the command uses tens of microseconds, but output shows microseconds. Setting 'delay 100' results in 1000 microseconds displayed.
Forgetting that delay changes affect EIGRP metric only if the interface is participating in EIGRP; other protocols may ignore it.
Setting delay to an extremely high value (e.g., 4294967295) which can cause metric overflow or unexpected path selection.
delay [tens-of-us] vs bandwidth [kbps]
Both the delay and bandwidth commands influence routing protocol metrics on an interface, but they modify different attributes. They are often confused because adjusting either can alter EIGRP or OSPF path selection, yet they serve distinct purposes in metric calculation and have different default values.
| Aspect | delay [tens-of-us] | bandwidth [kbps] |
|---|---|---|
| Scope | Interface-specific delay value | Interface-specific bandwidth value |
| Effect on routing metric | Directly affects EIGRP composite metric (delay component); OSPF uses it for cost calculation only if configured | Directly affects EIGRP composite metric (bandwidth component) and OSPF cost (cost = reference bandwidth / bandwidth); also influences QoS |
| Persistence | Persists; not overridden by media autodetection | Persists; overrides default detected bandwidth |
| Precedence | User-set value takes precedence over default | User-set value takes precedence over default |
| Dependency on media type | Default delay is derived from interface type (e.g., 100 us for Ethernet); user can override | Default bandwidth is derived from interface type (e.g., 10,000 kbps for FastEthernet); user can override |
| Typical use | Used to influence path selection by increasing delay to discourage traffic | Used to match provisioned bandwidth for accurate OSPF cost or QoS shaping |
Use delay [tens-of-us] when you need to adjust the delay component of the EIGRP metric or manipulate OSPF cost on a per-interface basis without affecting actual physical bandwidth.
Use bandwidth [kbps] when you need to set the true provisioned bandwidth for accurate OSPF cost calculation, EIGRP metric, or to configure QoS parameters like shaping and policing.
Platform Notes
In IOS-XE, the `delay` command syntax and behavior are identical to classic IOS. The output of 'show interfaces' also displays the delay in microseconds. However, in IOS-XE, there is a newer command `delay` under interface configuration that accepts values in milliseconds or microseconds depending on the platform; but the standard is still tens of microseconds.
For NX-OS, the equivalent command is `delay` under interface configuration, but the value is in microseconds (not tens of microseconds). For example, to set a delay of 100 microseconds on NX-OS, you would use 'delay 100' (not 10). Also, NX-OS uses a different routing protocol implementation (EIGRP is not supported in NX-OS; instead, OSPF and EIGRP are replaced by OSPF and EIGRP?
Actually, NX-OS does not support EIGRP; it uses OSPF, ISIS, and BGP. So the delay command in NX-OS is primarily for OSPF metric calculation? Actually, OSPF does not use delay; it uses cost based on bandwidth.
So the delay command in NX-OS is less relevant. For ASA, the `delay` command is not available; ASA uses a different configuration paradigm. In IOS-XR, the `delay` command exists under interface configuration but the value is in microseconds.
Also, IOS-XR uses a different metric calculation for routing protocols. In terms of IOS versions, the delay command has been consistent from 12.x to 16.x. However, in newer versions, the default delay for certain interface types may have changed (e.g., 10 Gigabit Ethernet defaults to 10 microseconds).
Always verify the default delay for your interface type. The command is available in all IOS images that support interface configuration.
Related Commands
bandwidth [kbps]
Sets the bandwidth value (in kbps) on an interface for routing protocol metric calculations and QoS, overriding the default detected bandwidth.
show interfaces
Displays detailed Layer 1 and Layer 2 statistics for all interfaces or a specific interface: line protocol state, hardware type, MAC address, MTU, bandwidth, duplex, speed, input/output rates, error counters, and queue statistics.
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.
show ip route
Displays the IP routing table showing all known routes, their source protocols, administrative distances, metrics, next-hops, and outgoing interfaces.
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