FREE CCNP LAB

The purpose of this CCNP GNS3 lab is to gain a better understand of how to configure EIGRP Stub Routing on Cisco IOS routers. In this lab we will use GNS3 to model a simple lab to demonstrate how to configure and verify EIGRP Stub implementation. Additionally we will learn to configure Key-Chain authentication.

What is EIGRP stub routing and what do we need it for? Stub routing is an EIGRP feature primarily designed to conserve local router resources, such as memory and CPU, and improve network stability, and simplifies stub router configuration.

The stub routing feature is most commonly used in hub-and-spoke networks, and is configured only on the spoke routers. When configured 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 older 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 requires information about a route or routes. However, the stub router will answer with a message of inaccessible. This technique allows for backward compatibility with older IOS versions while retaining stub routing functionality.

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

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

Stub routing 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;

In hub-and-spoke environments, spoke routers (branch routers) often do not need to maintain complete routing tables of the network, as the paths to other portions of the network are always through the hub routers (head quarter routers). Additionally, the massive query-reply processes between the hub and spoke routers upon a loss route would affect the stability of the network.

A typical connection between a hub router and a spoke router has much less bandwidth than a connection in the network core. Therefore attempting to use such connections as transit paths would typically results in excessive congestion. EIGRP stub routing is an efficient method for limiting the query range, therefore conserves bandwidth due to unnecessary queries, prevents SIA events, and improves network stability.

An EIGRP stub router would inform its stub status to upstream routers via the Hello packets. Any neighboring upstream router that receives such Hello packets will not query the stub router for lost routes, as the stub router has no downstream EIGRP neighbors and hence would not have alternative paths for a lost route. The upstream routers which connected to the stub router would answer any query on behalf of the stub router, which results in improved convergence time.

The eigrp stub [receive-only | connected , static , summary , redistributed] router subcommand configures an EIGRP router as an EIGRP stub router. An EIGRP stub router would advertise its connected and summary routes to other neighboring routers by default. Below describes the optional keywords that can be used to modify this behavior:

• Receive-only: Restricts an EIGRP stub router from advertising any route to other routers.
• Connected: Allows an EIGRP stub router to advertise its connected routes.
• Static: Allows an EIGRP stub router to advertise its static routes.
• Summary: Allows an EIGRP stub router to advertise its summary routes.

EIGRP Message Authentication:

Configuring authentication on your routers’ EIGRP messages ensures that your routers only accept routing messages from other routers that recognize the same pre-shared key. Without this authentication configured, if someone introduces another router with different or conflicting route information on to the network, the routing tables on your routers could become corrupt and may well result in a denial of service attack. Thus, when you add authentication to the EIGRP messages sent between your routers, it stops someone from purposely or accidentally adding another router to the network and causing a problem.

Learning Objectives:

• Review basic router configuration.
• Enable EIGRP on Cisco IOS routers.
• Advertise network using EIGRP.
• Configure EIGRP stub routing
• Configure EIGRP authentication.
• Verify EIGRP configuration using Cisco IOS commands.

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  CCNP GNS3 Lab2 (102.8 KiB, 2,380 hits)

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