EIGRP Hub and Spoke – EIGRP is the best protocol in Hub and Spoke topologies because of many reasons. In this article, I will explain the design consideration for EIGRP in a Hub and Spoke topology, also known as star topologies.
Figure – 1 EIGRP Hub and Spoke
In Figure – 1, EIGRP runs over the Hub and Spoke topology. Router A and Router B are the Hubs and Router C, D and E are the Spoke routers. If 192.168.0.0/24 network fails, Router A sends EIGRP query.
EIGRP query is not sent if the feasible successor is already placed in the topology table of EIGRP.
But since this is a stub network (Network connected to only Router A), there is no feasible successor.
Router A tries to find an alternate path for 192.168.0.0/24 network even if it is connected to itself. And since there is no other paths to 192.168.0.0/24 network, Router A still sends an EIGRP query. This summary is sent to all Router A’s neighbors, including stub nodes.
Best practice for EIGRP Hub and Spoke topology is to enable EIGRP Stub feature on the Spokes. When EIGRP Stub is enabled, Spokes are not used as transit nodes by other Hubs if there is no connection between the Hubs.
To ensure better design, always a put a link between the Hubs. Another benefit of enabling EIGRP Stub on the Spoke routers is that Hub doesn’t send query if there is a link failure.
In this network, if Router C, D and E are configured with the EIGRP Stub feature, Router A sends an EIGRP Query to only Router B. EIGRP query is sent to learn the alternate route for the failed subnet. EIGRP works this way and of course its mode of operations is very different from link state protocols.
Link state protocols, OSPF and IS-IS, do not send a query to learn an alternate route; instead, the nodes, which are locally connected to the failure, send a notification to their neighbors about the failure. EIGRP query slows down the convergence process.
And that’s why when you design an EIGRP network, always try to keep EIGRP domain as small as possible (Use filtering, summarisation as much as possible) and always enable EIGRP Stub at the edge of the network.