Use of default routes in OSPF

Perhaps the reader is most interested in making o s p f automatically calculate the route cost, although this is a bit impossible. For example, the old vro may not support Automatic computing. In this case, all interfaces have the same o s p f cost. Therefore, T 3 will have the same cost as the lease line of 5 6 K B p s. Obviously, these two lines provide different performance levels, which should be the basis for Route decision.

However, some environments accept the cost of using the default route. For example, if a user's network is composed of similar transmission lines, the default value is acceptable. In addition, the administrator can manually modify the consumption measurement for a specific interface. This will enable the network administrator to reasonably plan the network traffic mode while still using the default route.

1. homogeneous network

In homogeneous networks, all transmission lines are the same. For example, all the interfaces of l a n are 1 0 M B p s Ethernet, and all WA N interfaces are T 1. In this case, the use of the default value will not cause Routing Problems. This is correct when there is almost no or few routing redundancy.
To display this, consider the network diagram in Figure 1 3-1 0.

Figure 13-10 use the default OSPF interface value in an acceptable manner
In Figure 1 3-1 0, the default value 1 768 is allocated to each interface. However, all the wa n links are T 1. Considering that all values are the same, it does not matter if the allocation value is 1 128, 1 768, or 1 000! The routes in homogeneous networks are determined by simple calculation and comparison of hops. This is correct no matter how many routes are redundant in the network.

Obviously, in a network with a lot of redundant routes and different transmission technologies, the default value will not select the optimal route to any destination.

2. manually set the value

In some networks, you want to accept the o s p f default route, and then manually set the special link values different from the default link values. For example, the default consumption value of the user network may be the calculated value of the 1 768-5 6 K B p s serial link.

If only one or two links in the network do not provide the same bandwidth, you can accept the default value and set other values for those special links.

It is not important for o s p f nodes to use automatically calculated route consumption, default consumption, or manual configuration consumption. They accept all such consumption values and calculate the Shortest Path Tree of the network.

The objective of various l s a replication is to enable the router to construct A network topology view. This topology is arranged as a tree. The o s p f router forms the root of the tree. This tree provides the complete path to the known destination address, although only the next hop is used to forward packets. The reason is simple. Record the complete path to the destination so that the redundant path can be compared and selected as the best path.

Possible. If there are multiple paths with the same consumption, they will be discovered and used by o s p f, and the traffic will be balanced in these available links.

1. View of vro3 3

To better understand the concept of the shortest path tree, consider the network in Figure 1 3-11. The Simple Network shown in the figure is a small o s p f network. The Network Administrator has started the automatic calculation of Route consumption. Note that the Ethernet between vrouters 5 and 6 constitute the network 1 9 3. 1. 5. 0 and 1 9 3. 1. 6. 0 passes through another route of vro2 2. Therefore, the cost of o s p f is calculated as 1 0, and similar consumption is not allocated to other Ethernet networks.

Figure 13-11 OSPF network with route consumption
The shortest path tree of the network (Figure 1 3-1 0) varies with the vro. Figure 1 3-1 2 shows the tree from the perspective of vro3 3.

From Figure 1 3-1 2, we can see that the tree structure simplifies the computing of Route consumption to a given destination. The root router (vro3 3-1 9) can quickly

The router interface consumption on the route of any destination is added up. From the perspective of vro3 3, the route consumption of any network is summarized in Table 1 3-2. For destinations with more than one hop, the interface is added in brackets. This allows the reader to track the path along the network in Figure 1 3-1 2.

Figure 13-12 Shortest Path Tree of vro3 3
Table 13-2 consumption from vro3 3 to a known destination

In this example, there are two routes to network 1. A path contains fewer hops, but it is much more expensive because of the existence of low-speed serial links between vrouters 2 and 6. The other route has more hops, But it consumes less. In this case, o s p f will discard high-cost routes and use low-cost routes. If the two redundant routes share the same total cost, o s p f maintains two independent table items in the routing table and balances the load between them as evenly as possible.

2. vro2 2 View

The view of each vro in the network is different. Although the view of each vro is a bit boring, it is necessary to give an example to illustrate the impact of different vrouters on the shortest path tree. Figure 1 3-1 3 shows the Shortest Path Tree of vro2 2.

Table 1 3-3 summarizes the route consumption from vro2 2 to all known destinations.

Figure 13-13 Shortest Path Tree of vro2 2
Table 13-3 consumption from vro2 2 to a known destination

Points are different. The view may indicate any problems. This is why the o s p f router uses the data updated by l s a from other routers to construct its own network view without directly using the information to update the route table.