PART 12: EIGRP STUB

Enhanced Interior Gateway Routing Protocol

EIGRP stub routers play a critical role in optimizing network convergence and resource utilization in hierarchical network designs. This feature helps limit query propagation and reduce unnecessary control plane traffic in specific network topologies.

Purpose and Benefits of EIGRP Stub Routers

EIGRP stub configuration is designed primarily to:

1. Limit EIGRP query scope during route failures

2. Reduce bandwidth consumption on WAN links

3. Accelerate network convergence

4. Minimize processing requirements on edge devices

5. Protect remote routers from participating in complex route calculations
   

Common Deployment Scenarios

Remote Office/Branch Office

In a typical enterprise network, small remote offices connect to headquarters via limited-bandwidth WAN links. Without stub configuration, these remote routers would:

• Receive all query messages for any route failure in the core network

• Consume valuable WAN bandwidth with query/reply traffic

• Process queries for routes they have no alternate path to reach
  

Hub-and-Spoke Topologies

In hub-and-spoke designs, spoke routers only have a single path back to the network core. Configuring these as stubs prevents unnecessary query propagation while maintaining full connectivity.

EIGRP Stub Operation

When a router is configured as a stub:

1. It sends special Hello packets containing a Stub TLV (Type-Length-Value) field

2. This TLV identifies the router as a stub and indicates its capabilities

3. Upstream neighbors will not send queries to the stub router for route information

4. The stub router will only advertise routes according to its configured stub options

The upstream neighbor effectively acts as a shield, answering queries on behalf of the stub router by immediately replying "unreachable" for routes not in its own table.

Stub Configuration Options

TLV Flag Behavior

As shown in the image, the stub capabilities are encoded in a bit field within the Stub TLV:

• Connected: Set (1)

• Static: Not set (0)

• Summary: Set (1)

• Redistributed: Set (1)

• Leak-Map: Not set (0)

• Receive-Only: Not set (0)

Notably, the "redistributed" bit is set by default in the TLV, even if not explicitly configured, though it will only take effect for connected routes unless explicit redistribution is configured.

Configuration Commands

Cisco IOS/IOS XE (Numbered Mode)

Router(config-router)# eigrp stub [connected] [static] [summary] [redistributed] [leak-map name] [receive-only]

Cisco IOS XE (Named Mode)

Router(config-router-af)# eigrp stub [connected] [static] [summary] [redistributed] [leak-map name] [receive-only]

Important Considerations

1. Static routes require either network statements or redistribution to be advertised

2. Default advertisement includes connected and summary routes only

3. Neighbor reestablishment occurs when stub configuration changes

4. Redistributed bit appears set in packet captures by default, but only applies to connected routes unless redistribution is configured

5. Receive-only option cannot be combined with any other stub options

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