phamminhtuan
29-10-2008, 02:51 PM
When an entry in a router's EIGRP topology table changes for the worse (either the metric increases, or the successor is no longer accessible), if there is no feasible successor for the address, the entry goes into Active state, and the router sends query packets to all its neighbors. If Earhart's link to Yeager, in Figure 7-36 (http://vnpro.org/forum/#ch07fig36), goes down, Earhart sends queries to all its neighbors, including Johnson and Lindbergh, to find out if any neighbors have a path to Yeager. Earhart cannot modify its active entries in the topology table until it hears responses from all its queries regarding that entry. If a problem develops on the link to Lindbergh before Earhart has received a response to the query it sent about Yeager's addresses, Yeager's addresses will remain Active, even if the link between Earhart and Yeager comes back up.
Figure 7-36. Yeager is added to the network with a single link to Earhart.
http://img514.imageshack.us/img514/6315/23971875px8.jpg (http://img514.imageshack.us/my.php?image=23971875px8.jpg)
Johnson and Lindbergh, in Figure 7-36 (http://vnpro.org/forum/#ch07fig36), do not have back-door routes to any other site in the network. They are spoke routers in a hub-and-spoke design. The routers are not used to provide transit paths to any addresses in the network. When Lindbergh or Johnson need to forward a packet to an address that is not local to its site, the packet is forwarded to Earhart. Lindbergh knows of one path to 172.20.10.0, for instance, and that path is via Earhart. There is no need to send Johnson queries about addresses in other locations of the network and risk causing network instabilities. Johnson and Lindbergh can be configured with stub routing.
A router that has EIGRP Stub neighbors will not send queries to the stubs, thereby eliminating the chance that a stub-configured remote site will cause stuck in active conditions, and routing instabilities in other parts of the network.
Johnson is configured as an EIGRP stub router. Johnson's stub router configuration is displayed in Example 7-32 (http://vnpro.org/forum/#ch07ex32).
Example 7-32. Johnson's EIGRP stub router configuration.
router eigrp 15
eigrp stub
No configuration changes are required on Earhart, the hub router.
The command eigrp stub causes Johnson to send updates containing its connected and summary routes only. Johnson can be configured to include any combination of connected routes, summary routes, static routes, or routes that have been redistributed into EIGRP, with the command:
eigrp stub {connected | redistributed | static | summary | receive-only}
Johnson can also be configured to not send any route information in updates, with the receive-only option. With the receive-only option, the remote router will not include any addresses in an update. Addresses connected to the Johnson router would have to be advertised to the rest of the network in some other way to ensure that traffic can reach the site, perhaps with static routes configured on Earhart.
Figure 7-36. Yeager is added to the network with a single link to Earhart.
http://img514.imageshack.us/img514/6315/23971875px8.jpg (http://img514.imageshack.us/my.php?image=23971875px8.jpg)
Johnson and Lindbergh, in Figure 7-36 (http://vnpro.org/forum/#ch07fig36), do not have back-door routes to any other site in the network. They are spoke routers in a hub-and-spoke design. The routers are not used to provide transit paths to any addresses in the network. When Lindbergh or Johnson need to forward a packet to an address that is not local to its site, the packet is forwarded to Earhart. Lindbergh knows of one path to 172.20.10.0, for instance, and that path is via Earhart. There is no need to send Johnson queries about addresses in other locations of the network and risk causing network instabilities. Johnson and Lindbergh can be configured with stub routing.
A router that has EIGRP Stub neighbors will not send queries to the stubs, thereby eliminating the chance that a stub-configured remote site will cause stuck in active conditions, and routing instabilities in other parts of the network.
Johnson is configured as an EIGRP stub router. Johnson's stub router configuration is displayed in Example 7-32 (http://vnpro.org/forum/#ch07ex32).
Example 7-32. Johnson's EIGRP stub router configuration.
router eigrp 15
eigrp stub
No configuration changes are required on Earhart, the hub router.
The command eigrp stub causes Johnson to send updates containing its connected and summary routes only. Johnson can be configured to include any combination of connected routes, summary routes, static routes, or routes that have been redistributed into EIGRP, with the command:
eigrp stub {connected | redistributed | static | summary | receive-only}
Johnson can also be configured to not send any route information in updates, with the receive-only option. With the receive-only option, the remote router will not include any addresses in an update. Addresses connected to the Johnson router would have to be advertised to the rest of the network in some other way to ensure that traffic can reach the site, perhaps with static routes configured on Earhart.