vb.net generate barcode image PART II in Visual C#.NET

Maker PDF 417 in Visual C#.NET PART II

PART II
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VPN Deployment
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Dynamic routing As the name implies, dynamic routing allows for the
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dynamic addition and deletion of routing in the routing table that reflect upto-the minute changes in the network, and it allows the network to converge itself in the event that there is a change in the network topology. An example of dynamic routing is shown in Figure 9-1.
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Traffic flow Router A Network A Router B Router C Network C
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Traffic flow Router A Network A Router B Router C Network C
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Figure 9-1. Dynamic routing operations
In Figure 9-1, we can see that the network has two ways for a user on Network A to reach Network C. If everything is working properly on the routing infrastructure, there is a direct link from Router A to Router C. Therefore, the fastest way for traffic to get to Network C is to use that link. Next we see that the physical circuit from Router A to Router C is lost. The topology change triggers a dynamic routing protocol update to all interested routers, and the routing protocol makes an assessment of the new topology. It finds that there is another path from Network A to Network C through Router B. The routing protocol updates all interested routers with the change, and now the path via Router B is the primary choice for communications. Next we see that the link between Router A and Router C is reestablished. Because that link is a faster route with fewer hops for the data to traverse than the Router A to Router B to Router C option, the routing protocol will automatically
9
Deploying Site-to-Site VPNs |
update all routers again with the new information and give precedence to the shortest hop path. Note We have used the term converge a few times, so it is appropriate that we define network convergence. Convergence is the process a network uses to change its topology combined with the routing infrastructure s process of miti gating that change in the topology. For instance, in the preceding example, when the link between Router A and Router C goes down, all three routers will exchange routing protocol information and recalculate their routing tables to account for the change. The recalculation process is called convergence. As the number of routers involved in the calculation process increases, so does the amount of time the network needs to converge on an agreed-upon topology for routing traffic.
Static routing As the name implies, this routing solution mandates that
the routes to various addresses or subnets be manually programmed onto each router and that they cannot be automatically changed. An example of static routing is shown in Figure 9-2.
Traffic flow Router A Network A Router B Router C Network C
Traffic blocked! Router A Network A Router B Router C Network C
Traffic flow Router A Network A Router B Router C Network C
Figure 9-2. Static routing operations
PART II
VPN Deployment
In this example, we see that Router A has a static route configured to Router C. There can be several reasons why the route is defined statically: operations, high-latency link, security, and so forth. We then see that the physical link between Router A and Router C is lost. Though there is an alternative physical path from Network A to Network C through Router B, no static routing exists that describes that route. Because we are using static routing in this configuration, the routing tables do not change all traffic between Network A and Network C is now blocked. Once the link is re-established, all traffic flows normally. This setup might seem undesirable on the surface, but there are definite instances where this behavior might be desirable, especially for site-to-site VPN connections.
Pros and cons of deploying routing protocols with VPNs When looking at the preceding examples, you would think that you d want to use dynamic routing whenever possible because it is self-configuring and static routing is less configurable and less flexible. So why would we ever want to use static routing There is one basic, but flawed, assumption administrators make when deploying a dynamic routing protocol: the network links are relatively stable and are always under the control of the administrator. Neither one of these conditions applies when routing traffic over the Internet! What s more, Internet links are susceptible to latency issues. One example is when there is a long delay on an Internet connection and there is no communication between nodes for a few seconds. In such a case, the link can be perceived as being down by the dynamic routing protocol, thus causing a reconvergence of the routing infrastructure when there was no real physical change in the topology.
In a well-designed VPN installation, there is a place and time to use both dynamic and static routing, such as OSPF and Routing Information Protocol (RIP). The way to deploy both for maximum performance and benefit is shown in Figure 9-3.
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