no auto-summary
Disables automatic summarization of EIGRP routes at classful boundaries, allowing the router to advertise subnets without summarizing to the classful network address.
Definition: no auto-summary is a Cisco IOS router config command. Disables automatic summarization of EIGRP routes at classful boundaries, allowing the router to advertise subnets without summarizing to the classful network address.
Overview
The `no auto-summary` command is used in EIGRP router configuration mode to disable automatic summarization of routes at classful network boundaries. By default, EIGRP automatically summarizes subnet routes to their classful network address when advertising across different major networks. For example, if a router has subnets 10.1.1.0/24 and 10.1.2.0/24, without this command, EIGRP would advertise only the classful network 10.0.0.0/8 to neighbors in a different major network.
This behavior can lead to routing inefficiencies, suboptimal path selection, and potential routing loops in discontiguous networks. Disabling auto-summary ensures that all subnet prefixes are advertised individually, allowing for precise route selection and proper operation in modern network designs that often use Variable Length Subnet Masking (VLSM) and discontiguous addressing. This command is essential when networks are not contiguous or when you need to advertise specific subnets rather than summarized blocks.
It is typically used in enterprise networks where subnets are spread across multiple locations and need to be advertised without summarization. The command is applied in router configuration mode and takes effect immediately, updating the running configuration. It does not require a reload and can be verified with `show ip eigrp topology` or `show ip route eigrp`.
The command is available in all IOS versions and is a standard part of EIGRP configuration. Understanding when to use `no auto-summary` versus manual summarization with `ip summary-address eigrp` is crucial: use `no auto-summary` to disable automatic summarization globally, and use manual summarization to create specific summary routes on desired interfaces. This command is a fundamental tool for network engineers to control route advertisement and ensure optimal routing behavior in complex EIGRP deployments.
no auto-summaryWhen to Use This Command
- When you have discontiguous subnets of the same classful network (e.g., 10.1.1.0/24 and 10.2.2.0/24) and need them to be reachable across the network.
- In a network with VLSM where you want to advertise specific subnet prefixes rather than a summarized classful route.
- When configuring EIGRP for a multi-area or complex topology where automatic summarization could cause routing black holes.
- When you need to ensure that all subnets are visible in the routing table for optimal path selection.
Command Examples
Disabling auto-summary on a router with discontiguous subnets
Router(config-router)# no auto-summaryRouter(config-router)#
The command is entered in router configuration mode. No output is displayed upon successful execution. The router will now advertise subnets without summarizing to the classful network.
Verifying auto-summary is disabled
Router# show ip protocols | include auto-summaryAutomatic summarization is disabled
The output confirms that automatic summarization is disabled. If it were enabled, the output would show 'Automatic summarization is enabled'.
Understanding the Output
The 'no auto-summary' command does not produce direct output when entered. To verify its effect, use 'show ip protocols' and look for the line 'Automatic summarization is disabled'. In the routing table (show ip route), you will see individual subnet routes (e.g., 10.1.1.0/24) instead of a single classful route (e.g., 10.0.0.0/8).
This is critical in networks with discontiguous subnets to avoid routing black holes. If auto-summary were enabled, the router would only advertise the classful network, potentially causing packets to be dropped if the subnet is not directly connected.
Configuration Scenarios
Disable Auto-Summary in a Multi-Area EIGRP Network with Discontiguous Subnets
A company has two branch offices connected via a WAN link. Each branch uses subnets from the 10.0.0.0/8 range but in different locations, creating a discontiguous network. Without disabling auto-summary, EIGRP would summarize routes to 10.0.0.0/8, causing routing issues.
Topology
R1(Gi0/0)---10.0.12.0/30---(Gi0/0)R2
R1 has loopbacks 10.1.1.0/24 and 10.2.2.0/24
R2 has loopbacks 10.3.3.0/24 and 10.4.4.0/24Steps
- 1.Step 1: Enter global configuration mode: configure terminal
- 2.Step 2: Enter EIGRP router configuration mode: router eigrp 100
- 3.Step 3: Disable automatic summarization: no auto-summary
- 4.Step 4: Exit configuration mode: end
- 5.Step 5: Verify the change: show ip protocols | include auto-summary
! router eigrp 100 network 10.0.0.0 no auto-summary !
Verify: Use `show ip protocols` and look for 'Automatic summarization: disabled' in the output. Also use `show ip route eigrp` to see individual subnet routes (e.g., 10.1.1.0/24, 10.2.2.0/24) instead of a single summary route.
Watch out: Forgetting to apply `no auto-summary` on all routers in the EIGRP domain; if one router still has auto-summary enabled, it may advertise summarized routes causing inconsistencies.
Enabling Subnet Advertisement for VLSM in a Redundant EIGRP Topology
A network uses VLSM with subnets like 172.16.1.0/25 and 172.16.1.128/25. Without disabling auto-summary, EIGRP would summarize these to 172.16.0.0/16, losing the subnet granularity needed for load balancing and path selection.
Topology
R1(Gi0/0)---172.16.12.0/30---(Gi0/0)R2
R1 has loopback 172.16.1.0/25
R2 has loopback 172.16.1.128/25Steps
- 1.Step 1: Enter global configuration mode: configure terminal
- 2.Step 2: Enter EIGRP router configuration mode: router eigrp 200
- 3.Step 3: Disable automatic summarization: no auto-summary
- 4.Step 4: Exit configuration mode: end
- 5.Step 5: Verify that both subnets appear in the routing table: show ip route eigrp
! router eigrp 200 network 172.16.0.0 no auto-summary !
Verify: Use `show ip route eigrp` to confirm that both 172.16.1.0/25 and 172.16.1.128/25 are present as separate entries. Also use `show ip eigrp topology` to see the topology table entries.
Watch out: If you later add a manual summary route with `ip summary-address eigrp`, ensure that auto-summary is disabled to avoid conflicting summarization behaviors.
Troubleshooting with This Command
When troubleshooting EIGRP routing issues related to summarization, the `no auto-summary` command is often a key fix. A healthy output after disabling auto-summary shows individual subnet routes in the routing table rather than a single classful summary. For example, instead of seeing a single 10.0.0.0/8 route, you should see 10.1.1.0/24, 10.2.2.0/24, etc. Problem indicators include missing subnets, suboptimal routing (e.g., traffic going to a default route instead of a specific subnet), or routing loops.
Common symptoms that this command helps diagnose include: (1) A router receiving only a summary route and not the specific subnets, causing traffic to be black-holed or misrouted; (2) In discontiguous networks, routers may have multiple paths to the same summary route, leading to inconsistent forwarding; (3) When using VLSM, automatic summarization can hide the subnet mask information, causing routers to assume the default classful mask. A step-by-step diagnostic flow: 1) Check the routing table with `show ip route eigrp` to see if subnets are summarized. 2) Use `show ip protocols` to verify if auto-summary is enabled or disabled. 3) If auto-summary is enabled, disable it with `no auto-summary` in router config mode. 4) Clear the EIGRP neighbor adjacency or wait for updates to propagate. 5) Re-check the routing table to confirm individual subnets appear. 6) Use `debug eigrp packets` or `debug ip eigrp` to monitor route advertisements and verify that subnets are being sent without summarization. Correlate with `show ip eigrp topology` to see all learned routes.
If issues persist, check for manual summary statements with `show running-config | section router eigrp` and ensure they are not interfering. Also verify that all routers in the EIGRP domain have consistent summarization settings. The `no auto-summary` command is a quick fix for many EIGRP routing problems and should be one of the first checks when subnets are missing.
CCNA Exam Tips
CCNA exam tip: Auto-summary is enabled by default in EIGRP for IPv4. Remember to disable it when using VLSM or discontiguous subnets.
CCNA exam tip: The 'no auto-summary' command is configured under router configuration mode, not interface mode.
CCNA exam tip: In EIGRP for IPv6, auto-summary is not supported; this command only applies to IPv4.
CCNA exam tip: Disabling auto-summary allows EIGRP to advertise exact subnet prefixes, which is essential for proper routing in networks with variable-length subnet masks.
Common Mistakes
Mistake 1: Forgetting to disable auto-summary when using discontiguous subnets, causing routing black holes where some subnets become unreachable.
Mistake 2: Applying the command in interface configuration mode instead of router configuration mode, resulting in an error.
Mistake 3: Assuming auto-summary is disabled by default; it is enabled by default in EIGRP for IPv4.
no auto-summary vs show ip protocols
These two commands are commonly confused because 'show ip protocols' reveals whether auto-summary is enabled for EIGRP, while 'no auto-summary' is the configuration command to disable it. Both are essential for understanding and controlling EIGRP route summarization behavior.
| Aspect | no auto-summary | show ip protocols |
|---|---|---|
| Scope | Specific to EIGRP | All IP routing protocols |
| Configuration mode | Router configuration mode | Privileged EXEC mode |
| Persistence | Persists in running-config | One-time display only |
| Typical use | Discontiguous network design | Troubleshooting routing protocol state |
| Effect on routing | Stops classful summarization | Shows current summarization settings |
Use no auto-summary when you need to advertise EIGRP subnets without classful summarization in discontiguous networks.
Use show ip protocols when you need to verify the current state of routing protocols, including auto-summary status.
Platform Notes
In IOS-XE, the `no auto-summary` command behaves identically to classic IOS, with the same syntax and output. In NX-OS, the equivalent command is `no auto-summary` under `router eigrp` configuration mode, but note that NX-OS EIGRP implementation may have different default settings; always verify with `show ip eigrp` or `show running-config eigrp`. For ASA devices, EIGRP is not supported; instead, use OSPF or RIP.
In IOS-XR, EIGRP is not supported; the closest routing protocol is OSPF or ISIS. There are no significant behavior differences between IOS 12.x, 15.x, and 16.x for this command; it remains consistent. However, in newer IOS versions, the default for auto-summary may be disabled in some images, but it is good practice to explicitly configure it.
Always verify the current state with `show ip protocols`.
Related Commands
show ip protocols
Displays the current state of all IP routing protocols running on the router, including timers, filters, and network advertisements.
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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