In most cases, when we summarise a route, Cisco IOS will inject the summary route pointing to the Null0 interface. This is what is called a discard-route – i.e. all packets sent to the summary route will be discarded. Cisco IOS also provides the option of disabling this default behaviour.
It is all nice to understand what Cisco IOS does. However, we ought to ask the question: Why? This blog answers this question.
WHY DOES IOS INJECT THE DISCARD ROUTE?
To answer this question, let’s look at the following scenario:
Here the router to the left (R1) is connected to eight /24 networks configured on different routers in the network. These are summarised and sent over to R1 so that it can only “see” two /22 networks – 220.127.116.11/22 and 18.104.22.168/22. R1 is also sending a summary down to R2 (router on the right) – 22.214.171.124/29.
Now let’s assume that there is an outage on the network and all 126.96.36.199/22 routes are lost. Since we are sending a summary to R2, it will have no visibility of such event. As far as router R2 is concerned, route 188.8.131.52/29 is healthy! R1 however, will loose the 184.108.40.206/22 network.
In this case, when R2 sends a packet to any of the component networks (say for example, to IP 220.127.116.11), R1 gets the packet and since 18.104.22.168/22 is not in the routing table anymore, it uses it’s default gateway – so it sends the packet back to R2…. and the process repeats into a L3 loop until TTL reaches 0.
Now, let’s look at the same scenario, only this time, a discard route is injected by R1:
In this case, R2 will also forward the packet to R1. This time though, R1 has a route to Null0 for that subnet which includes 22.214.171.124 /24 prefix. Since this is the longest match, it will drop the packet.
Below are the two respective traces:
Rafael A. Couto Cabral • LinkedIn Profile
Cisco | F5 | VMware Certified • PRINCE2 Practitioner
Originally posted 2017-11-20 20:40:16.