PART 13: EIGRP Route Filtering

Enhanced Interior Gateway Routing Protocol

Distribute lists in EIGRP provide a mechanism to filter routes being advertised or received. They act as a control mechanism determining which networks are permitted or denied in routing updates.

Matching Tools

Distribute lists can utilize the following matching tools:

1. Access Control Lists (ACLs)

   • Standard ACLs: Match a single network address

   • Extended ACLs: Match based on source (next-hop) and destination (network)

   • Named ACLs: Provide the same functionality with descriptive identifiers
     

2. Prefix Lists

   • More granular control over route filtering

   • Match multiple networks based on subnet mask specifications

   • Critical area requiring careful configuration to avoid filtering errors
     

3. Route Maps

   • Complex matching capability

   • Can reference ACLs or prefix lists

   • Can match based on EIGRP route type (internal vs. external)
     

4. Gateway Filtering

   • Filters based on next-hop addresses

   • Uses prefix lists for matching the next-hop
     

Direction Configuration

Distribute lists require a direction specification:

• IN: Filters incoming routes (received from neighbors)

• OUT: Filters outgoing routes (advertised to neighbors)
  

Interface Specification

The filter can be applied to:

• A single interface

• Multiple specific interfaces

• All interfaces (by omitting the interface parameter)
  

Configuration Syntax

For EIGRP in Classic Mode:

Router(config-router)# distribute-list {acl-number | prefix-list-name | route-map-name} {in | out} [interface-name]

For EIGRP in Named Mode:

Router(config-router-af-topology)# distribute-list {acl-number | prefix-list-name | route-map-name} {in | out} [interface-name]

For IOS XR:

Router(config)# router eigrp process-name
Router(config-eigrp)# address-family {ipv4 | ipv6}
Router(config-eigrp-af)# distribute-list {prefix-list | route-policy} name {in | out} [interface-type interface-number]

Special Considerations for Extended ACLs

When using extended ACLs with EIGRP distribute lists:

• Source field: Matches the next-hop IP address (advertising router)

• Destination field: Matches the network being advertised

• Cannot match based on subnet mask

• Primarily used to filter routes based on which neighbor sent them

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