Last Updated on July 31, 2021 by Admin 1
Examine the exhibit.
What additional EIGRP configuration is required to ensure that all destination networks are reachable if all routers are running pre- 15.0 versions of the IOS?
- The eigrp stub receive only command should be executed on routers A and B.
- A static route to 10.10.0.0/16 via the interface to router D should be configured at router C.
- The no auto-summary router configuration command should be executed on router C.
- The passive interface command should be executed on routers A and B.
- The no auto-summary command should be executed on routers A and B.
To ensure the full network is reachable, routers A and B must advertise their networks without first summarizing them to the class B 172.31.0.0/16 address in updates to router C. Otherwise, router C would incorrectly assume that it has two paths to the 172.31.0.0 classful network: one via router A and the other via router B. Therefore, routers A and B should be configured with the no auto-summary command so that they advertise 172.31.16.0/24 and 172.31.17.0/24, respectively. Starting with version 15, EIGRP auto summarization is disabled by default
Summarization is beneficial in most cases. It reduces the number of routes in the neighboring router tables and effectively contains EIGRP queries. The problem with discontiguous networks (or subnets) using EIGRP is that EIGRP will automatically summarize on the classful network boundary. By configuring the router to disable automatic summarization with the no auto-summary command, the routers will be able to see all of the individual subnets, not just a summary. The no auto-summary command must be issued from router configuration mode as shown below:
router(config-router)# no auto-summary
Note that auto summarization is effective only on directly connected routes. For example, in the scenario exhibit, router C does not need to have auto summarization disabled in order to advertise the subnets to routers D and E. Since those subnet routes were learned via a route advertisement, they will be advertised to routers D and E without summarization.
In some situations, it may be necessary to turn off auto summarization globally while still summarizing specific networks. If you need to manually summarize a set of networks, the following command when executed in EIGRP configuration mode can summarize those specific networks while auto summarization is disabled:
ip summary-address [eigrp as-number] [address] [mask]
For example: router10(config)# int Ethernet0/0 router10(config-if)# ip summary-address eigrp
Layer 3 Technologies
Configure and verify manual and autosummarization with any routing protocol