LAB: EIGRP Poisoned Floating Summarization

R5:
interface FastEthernet0/0
ip summary-address eigrp 100 150.1.4.0 255.255.254.0 255


Routes with an administrative distance of 255 are not candidate to be installed in the routing table. By poisoning the interface level summary on R5 with a distance of 255, the route to Null0 cannot be installed locally in the routing table, but the summary itself can be advertised out the interface. The use of this configuration design-wise is in cases where you want the router to forward traffic for destinations inside the summary that it does not have a longer match for. In this case we can see that SW2 has the route 150.1.4.0/23 to reach 150.1.4.4. However since R4 is only advertising a default route to R5, R5 has no longer match for 150.1.4.4. In the previous case R5’s longer match to 150.1.4.4 was its own summary to Null0, meaning that all traffic going to 150.1.4.4 was dropped. By poisoning the summary with a distance of 255 R5 can now use the default route to reach 150.1.4.4.