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Other Large Scale Cores


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Other Large Scale Cores






Case Study: NHRP in an ATM Network







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Advanced IP Network Design (CCIE Professional Development series)

From: Advanced IP Network Design (CCIE Professional Development series)
Author: Russ White; Alvaro Retana; Don Slice
Publisher: Cisco Press (53)
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9. Other Large Scale Cores

Scalability of full mesh, Layer 3 designs is a major issue when building very large networks. Chapter 3, “Redundancy,” discussed problems with Layer 2, full mesh designs. Layer 3, full mesh designs have many of the same problems.

At some point, the number of possible neighbors and paths becomes overwhelming, and you need to reduce the amount of work that needs to be done by the core routers.

Two possible solutions to this problem are Next Hop Resolution Protocol (NHRP) and Multiprotocol Label Switching (MPLS).


One possible solution to the Layer 3 meshing problem is the Next Hop Resolution Protocol (NHRP). NHRP is technically a routing protocol rather than a new Layer 2/3 switching mechanism. Figure 9-1 provides an example network for discussion.

Figure 9-1. Full Mesh Neighbors

Now, as you know from Chapter 3, the full mesh design in Figure 9-1 results in 15=>6(6-1)/2 paths through the network. Suppose that you want to reduce the paths through the network by making it a hub and spoke design. It could look like the one illustrated in Figure 9-2.

Figure 9-2. Hub and Spoke Network Design

The real difficulty with this design (other than the single point of failure) is the amount of traffic that must pass through the hub router. If all of these links are 2.4 Gbps, the hub router needs to switch traffic at 12 Gbps or faster. There must be some way to spread this work out a bit.

If you are using a lower layer media that supports switched virtual circuits (SVCs), such as ATM (or Frame Relay SVCs), you should be able to take advantage of them to make direct connections between the spoke routers when needed.

The problem with this is routing. How does the router know that a given destination is reachable through some other means than the hub router? How does it know which SVC to use (what number to dial, so to speak) to reach this destination?

This is where NHRP comes in. In NHRP, a number of routers are configured as route servers. Each router advertises its reachable destinations to this route server along with a SVC to use to reach them.

When a router wants to reach a given destination, it queries the route server to find out if there is a direct path through the cloud. If there is, it will bring up a SVC to the next hop and pass the traffic along.

This effectively provides the advantages of a full mesh topology while also providing the scalability of a partial mesh topology.


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