no auto-summary
Disables automatic summarization of routes at classful boundaries in RIP, allowing subnets to be advertised with their actual subnet masks.
no auto-summaryWhen to Use This Command
- When you have a discontiguous network (e.g., 10.1.0.0/16 and 10.2.0.0/16 separated by another network) and need RIP to advertise the specific subnets rather than summarizing to 10.0.0.0/8.
- When using Variable Length Subnet Masking (VLSM) and need to propagate all subnet routes without losing visibility of smaller subnets.
- When connecting to a non-Cisco device that does not support classful routing and expects classless advertisements.
- When troubleshooting routing issues caused by automatic summarization hiding more specific routes.
Command Examples
Disabling auto-summary on a RIP-enabled router
Router(config)# router rip
Router(config-router)# no auto-summaryRouter(config-router)#
The command is entered in router configuration mode. No output confirms the command was accepted. After this, RIP will advertise subnets with their actual masks.
Verifying auto-summary is disabled
Router# show ip protocols | include auto-summaryAutomatic network summarization is not in effect
The output confirms that auto-summary is disabled. If it were enabled, it would say 'Automatic network summarization is in effect'.
Understanding the Output
The 'no auto-summary' command itself produces no output when entered correctly. To verify its effect, use 'show ip protocols' and look for the line 'Automatic network summarization is not in effect'. This indicates that RIP will advertise routes with their original subnet masks rather than summarizing to the classful network boundary. In a real network, this is critical when using VLSM or discontiguous subnets; without it, routes may be incorrectly summarized, causing reachability issues. A good configuration shows 'not in effect' when classless routing is needed; a bad configuration shows 'in effect' when subnets are being hidden.
CCNA Exam Tips
CCNA exam tip: Remember that 'no auto-summary' is required for RIP to support VLSM and discontiguous networks; without it, RIP will summarize to classful boundaries.
CCNA exam tip: The exam may ask you to identify why a route is missing; if auto-summary is enabled, a more specific subnet may be hidden by a summary route.
CCNA exam tip: 'no auto-summary' is configured under router rip mode, not interface mode.
CCNA exam tip: EIGRP also uses 'no auto-summary' but it is enabled by default in older IOS versions; in newer versions, it is disabled by default.
Common Mistakes
Mistake 1: Forgetting to disable auto-summary when using VLSM, causing subnets to be summarized and some networks becoming unreachable.
Mistake 2: Applying 'no auto-summary' in global configuration mode instead of router rip mode, resulting in an error.
Mistake 3: Assuming auto-summary is off by default; in older IOS versions, it is on by default for RIP.
Related Commands
router rip
Enters RIP router configuration mode to enable and configure the Routing Information Protocol (RIP) on a Cisco router.
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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