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.
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.
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.
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 current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.
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