Hello,
Please find the info below,, I have searches 12.2 SR family IOS, you may search it for the ios version you are using...
Route Summarization
You can configure EIGRP to perform automatic summarization of subnet routes into network-level routes. For example, you can configure subnet 172.16.1.0 to be advertised as 172.16.0.0 over interfaces that have subnets of 192.168.7.0 configured. Automatic summarization is performed when two or more network (EIGRP) router configuration or address family configuration commands are configured for the EIGRP process. By default, this feature is enabled.
Route summarization works in conjunction with the ip summary-address eigrp command available in interface configuration mode for autonomous system configurations and with the summary-address (EIGRP) command for named configurations in which additional summarization can be performed. If automatic summarization is in effect, there usually is no need to configure network-level summaries using the ip summary-address eigrp command.
Summary Aggregate Addresses
You can configure a summary aggregate address for a specified interface. If any more specific routes are in the routing table, EIGRP will advertise the summary address out the interface with a metric equal to the minimum of all more specific routes.
Floating Summary Routes
You can use a floating summary route when configuring the ip summary-address eigrp command for autonomous system configurations or the summary-address (EIGRP) command for named configurations. The floating summary route is created by applying a default route and administrative distance at the interface level, or address family interface level. The following scenarios illustrate the behavior of floating summary routes.
The figure below shows a network with three routers, Router-A, Router-B, and Router-C. Router-A learns a default route from elsewhere in the network and then advertises this route to Router-B. Router-B is configured so that only a default summary route is advertised to Router-C. The default summary route is applied to serial interface 0/1 on Router-B with the following configuration for an autonomous system configuration:
Router(config)# interface Serial 0/1 Router(config-if)# ip summary-address eigrp 100 0.0.0.0 0.0.0.0
The default summary route is applied to serial interface 0/1 on Router-B with the following configuration for a named configuration:
Router(config-router-af)# af-interface serial0/1 Router(config-router-af-interface)# summary-address 192.168.0.0 255.255.0.0 95
Figure 1. Floating Summary Route Applied to Router-B
The configuration of the default summary route on Router-B sends a 0.0.0.0/0 summary route to Router-C and blocks all other routes, including the 10.1.1.0/24 route, from being advertised to Router-C. However, this configuration also generates a local discard route on Router-B, a route for 0.0.0.0/0 to the null 0 interface with an administrative distance of 5. When this route is created, it overrides the EIGRP learned default route. Router-B will no longer be able to reach destinations that it would normally reach through the 0.0.0.0.0/0 route.
This problem is resolved by applying a floating summary route to the interface on Router-B that connects to Router-C. The floating summary route is applied by configuring an administrative distance for the default summary route on the interface of Router-B with the following statement for an autonomous system configuration:
Router(config-if)# ip summary-address eigrp 100 0.0.0.0 0.0.0.0 250
The floating summary route is applied by configuring an administrative distance for the default summary route on the interface of Router-B with the following statement for a named configuration:
Router(config-router-af-interface)# summary-address eigrp 100 0.0.0.0 0.0.0.0 250
The administrative distance of 250, applied in the summary-address command, is now assigned to the discard route generated on Router-B. The 0.0.0.0/0, from Router-A, is learned through EIGRP and installed in the local routing table. Routing to Router-C is restored.
If Router-A loses the connection to Router-B, Router-B will continue to advertise a default route to Router-C, which allows traffic to continue to reach destinations attached to Router-B. However, traffic destined to networks to Router-A or behind Router-A will be dropped when it reaches Router-B.
The figure below shows a network with two connections from the core, Router-A and Router-D. Both Router-B and Router-E have floating summary routes configured on the interfaces connected to Router-C. If the connection between Router-E and Router-C fails, the network will continue to operate normally. All traffic will flow from Router-C through Router-B to the hosts attached to Router-A and Router-D.
Figure 2. Floating Summary Route Applied for Dual-Homed Remotes
However, if the link between Router-A and Router-B fails, the network may incorrectly direct traffic because Router-B will continue to advertise the default route (0.0.0.0/0) to Router-C. In this scenario, Router-C still forwards traffic to Router-B, but Router-B drops the traffic. To avoid this problem, you should configure the summary address with an administrative distance on only single-homed remote routers or areas where there is only one exit point between two segments of the network. If two or more exit points exist (from one segment of the network to another), configuring the floating default route can cause a black hole to be formed.
Regards
Thanveer
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