PART 14: EIGRP Route Summarization

Enhanced Interior Gateway Routing Protocol

EIGRP supports two methods of route summarization:

1. Manual Summarization

2. Auto-summarization (legacy feature)

Manual Summarization

Concept and Configuration

Manual summarization allows you to create aggregate routes that consolidate multiple more-specific prefixes into a single advertisement.

Process:

1. Disable auto-summarization within the EIGRP process (if not already disabled)

2. Identify the interface from which you want to advertise the summary

3. Configure the summary address command on that interface
   

Configuration Syntax:

For IOS Classic Mode:

Router(config-if)# ip summary-address eigrp <AS-number> <summary-address> <subnet-mask> [leak-map <route-map-name>]

For IOS Named Mode:

Router(config-router-af-interface)# summary-address <summary-address> <subnet-mask> [leak-map <route-map-name>]

For IOS XR:

Router(config)# router eigrp <process-name>
Router(config-eigrp)# address-family {ipv4 | ipv6}
Router(config-eigrp-af)# interface <type> <number>
Router(config-eigrp-af-if)# summary-address <prefix>/<prefix-length> [leak-map <route-policy-name>]

Effects of Manual Summarization:

• The configured summary route is advertised to EIGRP neighbors

• More specific routes covered by the summary are suppressed from advertisements

• A discard route (pointing to Null0) is automatically created with an administrative distance of 5

• The Null0 route acts as a loop prevention mechanism

• When external EIGRP routes (AD 170) are summarized, they become internal routes (AD 90) on downstream routers
  

Advanced Features:

• Leak Maps: Allow specific routes to be advertised alongside the summary route

  • Requires configuration of a route map to match the specific routes

  • Useful when you need exceptions to the summarization
    

Flexibility:

• No restrictions on what can be summarized as long as the component routes exist in the routing table

• Can summarize both internal and external EIGRP routes
  

Auto-Summarization

Concept and Behavior

Auto-summarization automatically summarizes routes to their classful boundaries when advertised across different major networks.

Key Points:

• Disabled by default in newer IOS versions (enabled by default in older versions)

• Only applies to locally originated routes that are:

  • Not redistributed from other routing protocols

  • Explicitly advertised via the network command in EIGRP configuration
    

Process Logic:

1. EIGRP examines locally connected routes advertised in the EIGRP process

2. Summarizes these routes to their classful boundaries (A:/8, B:/16, C:/24)

3. Creates a Null0 route for loop prevention

4. Determines advertisement eligibility:

   • If the outgoing interface's network differs from the summarized route's network → Advertise summary

   • If the outgoing interface's network matches the summarized route's network → Additional checks:

     • If summary mask is more specific than interface subnet mask → Do not advertise summary

     • If summary mask is less specific than interface subnet mask → Advertise summary
       

Limitations:

• Not suitable for discontiguous networks: Can cause routing problems with non-contiguous subnets

• Limited to classful boundaries only

• Lacks the flexibility of manual summarization
  

Best Practices

• Use manual summarization for more control and flexibility

• Disable auto-summarization in production networks unless specifically needed

• Create summary addresses that encompass all relevant prefixes to prevent routing loops

• Use leak maps when specific routes must be advertised alongside summaries

• Test summarization implementations thoroughly before deploying in production

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