Stub routing is an EIGRP feature primarily designed to conserve local router resources, such as memory and CPU, and improve network stability. The stub routing feature is most commonly used in hub-and-spoke networks. This feature is configured only on the spoke routers. When configured on the spoke router, the router announces its stub router status using a new TLV in the EIGRP Hello messages. When the hub router receives the Hello packet from the spoke router, one of two things happens:

1. If the hub router is running a newer version of software, upon receiving the Hello packet with the new TLV, the router will not query the stub router about the status of any prefixes. This is the default mode of operation.
2. If the hub router is running a version of software less than 12.0(7)T, upon receiving the Hello with the new TLV, the router will ignore this field because it does not understand it. The router will send Query packets the stub router if it needs information about a route or routes. However, the stub router will respond with a message of inaccessible to any queries received from the hub router. This method allows for backward compatibility with older versions of software while retaining stub routing functionality.

When the stub routing feature is enabled on the spoke router, the router only advertises specified routes to the hub router. The router will not advertise routes received from other EIGRP neighbors to the hub router. Cisco IOS software allows administrators to select the type of routes that the stub router should advertise to the hub router. The EIGRP stub routing feature provides four advantages when implemented in hub-and-spoke networks;

1. It prevents sub-optimal routing from occurring within hub-and-spoke EIGRP networks
2. It prevents stub routers with low-speed links from being used as transit routers
3. It eliminates EIGRP Query storms, allowing the EIGRP network to convergence faster
4. It reduces the required amount of configuration commands on the stub routers

The stub routing feature prevents sub-optimal routing in typical hub-and-spoke networks by preventing stub routers from being used as transit routers;

When a neighbor changes a metric, or when a topology change occurs, and the Successor route is removed or changes, DUAL checks for feasible successors for the route and if one is found, DUAL uses it to avoid re-computing the route unnecessarily. However, if no feasible successor for the destination network exists, the router will send a Query to neighboring routers asking if they have information to the destination network. In hub-and-spoke networks, this may lead to a Query storm;

If we had a network with 50 or even 100 branch office routers for example. In that case, given that each neighbor is resides on its own point-to-point subnet, we can quickly see why this would be referred to as a Query storm because of the sheer amount of packets that would be sent to the neighbors. Sending the packets would be a waste of bandwidth and processing resources because all spoke routers would simply respond with an unreachable message, stating that there is no path for this route through this neighbor. The unnecessary Query storm can be prevented by using the EIGRP stub routing feature which prevents the hub router from querying the spoke router.

Stub routing is enable with the following router process command;

Router1(config-router)#eigrp stub ?

connected      Do advertise connected routes

leak-map       Allow dynamic prefixes based on the leak-map

receive-only   Set IP-EIGRP as receive only neighbor

redistributed  Do advertise redistributed routes

static         Do advertise static routes

summary        Do advertise summary routes

The receive-only keyword configures the router as a receive-only router. In other words, when this keyword is used, the stub router does not advertise any prefixes. Instead, it only receives, or accepts, prefixes advertised to it by its neighbor. This keyword cannot be used in conjunction with any other keywords when configuring stub routing.

The leak-map <name> keyword configures the EIGRP stub router to advertise those routes that would previously not been advertised based on the default operation of the stub routing feature. The leak map references a route map that matches one or more ACLs that permit the matched subnets or addresses to be leaked.

The connected keyword configures the stub router to advertise connected subnets. These are subnets on any interface directly connected to the router.

The static keyword configures the stub router to advertise static routes.

The summary keyword configures the stub router to advertise summary routes.

The redistributed keyword configures the stub router to advertise routes that have been redistributed into EIGRP from other route sources.

When the eigrp stub is used by itself the following are its default;

By default, when this command is enabled, the following is true on the stub router:

• The stub router advertises all connected subnets
• The stub router advertises all summary routes

When the stub router feature is enabled, the EIGRP neighbor relationship is reset. This allows the spoke router to send the Hello packet(s) with the new TLV. Another important point pertaining to the stub router feature is that while it prevents the router from being queried by neighbor routers, this feature does not prevent the hub router from advertising its complete routing table to the spoke router.  Instead, this must be manually configured on the hub router;

• Advertising a default route to the spoke router and filtering all other specific routes
• Summarizing the routes advertised to the spoke router

The hub router can be configured to advertise the default route to the spoke router(s) and then filter all other specific route entries, effectively reducing the routing table on the spoke to a single default route entry.

Simply issuing the eigrp stub static command, for example, does not automatically mean that the stub router will advertise static routes. When the network command was is used prior to using the eigrp stub command, static routes must be known to EIGRP before the stub routing feature will advertise them. This may be performed in one of two ways:

• Using the network command to enable EIGRP routing for the configured static routes
• Redistributing the static routes into EIGRP using the redistribute command

To verify what type of stub routes your router is advertising use the show ip protocols command as show below;

Router1#sh ip protocols

Routing Protocol is “eigrp 100″

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

EIGRP maximum hopcount 100

EIGRP maximum metric variance 6

EIGRP stub, static

Redistributing: eigrp 100

EIGRP NSF-aware route hold timer is 240s

Automatic network summarization is not in effect

Maximum path: 4

Routing for Networks:

0.0.0.0

Routing Information Sources:

Gateway         Distance      Last Update

10.1.13.3             90      00:00:09

10.1.12.2             90      00:00:08

10.1.23.2             90      00:00:08

Distance: internal 90 external 170

Stub routing feature provides great flexibility by allowing you to use any combination of keywords, with the exception of the receive-only keyword. However, this does not necessarily mean that two or more keywords must be used. Instead, any one of the keywords may be used on their own, with the most common configuration on the stub router being simply just the eigrp stub connected command on all the spoke routers.

via EIGRP -Stub Routing