PART 7: EIGRP Router ID

Enhanced Interior Gateway Routing Protocol

The Router ID (RID) serves as a unique identifier for routers in various routing protocols. While critically important in protocols like OSPF and BGP, EIGRP treats Router ID differently, which can lead to unexpected behavior in specific scenarios.

Router ID Across Different Protocols

Router IDs are implemented differently across routing protocols:

• OSPF: Strictly requires unique Router IDs; adjacencies won't form between routers with duplicate RIDs

• IS-IS: Uses Network Entity Title (NET) as a unique identifier for each router

• BGP: Requires unique Router IDs for proper operation

• RIP: Does not use the Router ID concept

• EIGRP: Uses Router ID but with less stringent uniqueness requirements
  

EIGRP Router ID Characteristics

The EIGRP Router ID has several notable characteristics:

1. It's represented as a 32-bit number in dotted-decimal notation (like an IP address)

2. Unlike other protocols, EIGRP generally allows duplicate Router IDs

3. Routers with duplicate Router IDs can still:

   • Form neighbor adjacencies

   • Exchange internal EIGRP routes successfully
     

The Duplicate Router ID Problem

While EIGRP tolerates duplicate Router IDs for most operations, this duplication creates specific issues:

• Routes with an originator Router ID matching your local Router ID will be ignored
  

Router ID Selection Process

EIGRP selects a Router ID using this priority sequence:

1. Manually configured Router ID (if specified)

2. Highest IP address of any loopback interface

3. Highest IP address of any active physical interface
   

Configuration Commands

To explicitly set the EIGRP Router ID and avoid potential conflicts:

Cisco IOS/IOS XE (Numbered Mode)

Router(config-router)# eigrp router-id A.B.C.D

Cisco IOS XE (Named Mode)

Router(config-router-af)# eigrp router-id A.B.C.D

Cisco IOS XR

Router(config)# router eigrp process-name
Router(config-eigrp)# address-family ipv4
Router(config-eigrp-af)# router-id A.B.C.D

Best Practices

To avoid the issues created by duplicate Router IDs:

1. Always explicitly configure unique Router IDs in EIGRP domains

2. Use loopback interfaces with unique IP addresses as a backup method

3. Document Router ID assignments in network documentation

4. Verify Router IDs when troubleshooting route exchange issues, especially with external routes

5. Include show ip eigrp events in your EIGRP troubleshooting routine

---