1. Discard Routes and Avoiding Routing Loops
CCNA/CCNP
Rick Graziani
Cabrillo College

2. Cisco IP Routing by Alex Zinin Addison-Wesley Pub Co ISBN: 0201604736
This book is highly recommended for instructors and CCNP students.
Special thanks to the author, Alex Zinin, for his help.
Rick Graziani

3. Topics Situation: Normal Link Down Solution #1 – no ip classless
Solution #2 – discard route
What about packets destined for /24?
Final Notes
Rick Graziani

4. Scenario
Rick Graziani

5. Scenario Customer Network is running a dynamic routing protocol.
All subnets within the network are contained in this “Customer Network.” (There are no discontiguous subnets via ISP.)
Remote Office has /24 and /24 networks.
All default traffic is sent to ISP, via /0 default route on RTA that is propagated to RTB and RTC.
ISP has static routes pointing to RTA for /16 and /24 networks.
Rick Graziani

6. Situation Normal Situation: Normal
As long as all the networks are up, everything should work fine.
Actually, we will see later that even when all of our links are up, there could be a problem.
Rick Graziani

7. Link Down X
Link down
What would happen if our link between RTB and RTC failed?
Obviously, the Remote Office networks would be cut-off from the Central Office.
After the routing tables are updated, where would RTA or RTB send packets with the destination IP address ?
Rick Graziani

8. Link Down X
After the routing tables are updated, where would RTA or RTB send packets with the destination IP address ?
If the router is running “ip classless,” it will forward the packet using the default route. (For information on the ip classless command, see the presentation on Routing Table Structure.)
These packets will eventually be sent from RTA to ISP.
Now what will the ISP router do with these packets for ?
Rick Graziani

9. Link Down X
Now what will the ISP router do with these packets for ?
Since it has a route for this network pointing back to RTA, it will send it back to RTA.
This is known as a “blackhole” in the network.
Now we have a routing loop!
Theses packets will eventually be dropped when the TTL (Time-to-live) field, in the IP headers, is decremented to 0.
Is there a solution?
Rick Graziani

10. Link down – Solution #1: no ip classless
One solution could be to change from classless routing behavior to classful routing behavior using the command: “no ip classless” on RTA and RTB.
The affect of this modification is:
The router would search its routing table for a best-match for
The router would find the “parent network,” , and search the known subnets, “child routes,” /24 and /24, but would not find the route /24.
The affect of the “no ip classless” command makes the router drop any packets within the network where there is no known subnet, but there is a parent network. (In this case /24 is not known.)
With the “no ip classless” command, the router does not use any supernet or default route when the there is at least one known subnet.
The packets for would be dropped by RTA and RTB.
Rick Graziani

11. Link down – Solution #1: no ip classless
Although this is a remedy, this method cannot always be used.
We will see that this does not solve the problem for packets destined for /24 when this link is down.
In some situations you may need to have classless routing enabled (ip classless):
May have discontiguous subnets and relying on default routing to reach them.
May be using route summarization and relying on supernet routes to reach those specific destinations.
In any case, modifying the route look-up process with “no ip classless” is not an ideal solution when you are only trying to solve a specific problem, as this might have other affects on the routing behavior in your network that you did not foresee.
Rick Graziani

12. Link down – Solution #2: Discard RouteX
Link down – Solution #2: Discard Route
A more elegant and scalable solution is to use a discard route.
A discard route is a route that sends packets to null0, the “bit-bucket,” when they do not have a route in the routing table and you do not want them to be sent using the default route.
Discard Route on RTA: ip route null0
This would cause RTA to drop all packets for subnets in the network, that do not have a specific route in the routing table.
Using our failed route example and still using classless routing (ip classless), any packets not matching /24 or /24, would be routed to null0, using the discard route.
Rick Graziani

13. Link down – Solution #2: Discard RouteX
Discard Route on RTA: ip route null0
Remember when we said earlier, that even when all of our links are up, there could still be a problem.
Well, this also fixes any blackholing of traffic we might have for packets that are sent to /16 subnets that do not exist, for example, any packets incorrectly sent to /24 subnet. (This is even the case when all the links are up.)
RTA’s discard route will drop these packets as well!
Rick Graziani

14. Discard Routes X What about packets destined for 192.168.1.0/24?
In this case, the “no ip classless” command would not help, as this is not a subnet of a parent network in the routing tables of RTA or RTB.
This route would be removed from the routing tables of RTA and RTB and all packets would be sent to ISP.
Again, the ISP would send those packets back to RTA, causing another blackhole.
Rick Graziani

15. Discard Routes X What about packets destined for 192.168.1.0/24?
In many cases your networks can be summarized within a single supernet.
In these cases, your discard route should cover the entire range of your network, without including those routes outside your network.
“The discard route should cover all destinations in the central and remote offices, but on the other hand should be as specific as possible to prevent blackholing of traffic going to other subnets of the same summary.” Alex Zinin, Cisco IP Routing
Rick Graziani

16. Discard Routes X
What about packets destined for /24? (Continued)
However, in this case the there are networks between /16 and /24, so a single discard route will not work.
What about adding a second discard route on RTA?
2nd Discard Route: ip route null0
Unfortunately, this would cause RTA to drop all packets for /24 whether the link was up or not, because this static route would replace any dynamic route for this network in the routing table. (Lower administrative distance)
Rick Graziani

17. Discard Routes X
What about packets destined for /24? (Continued)
What about modifying the administrative distance?
2nd Discard Route: ip route null0 200
Now, this route would only enter the routing table for RTA, when the dynamic route to /24 is deleted.
Rick Graziani

18. Discard Routes ip route 172.16.0.0 255.255.0.0 null0
So, to protect our network from blackholing traffic for routes that are in our network but no longer reachable ( /24 and /24 examples), or for routes (subnets) that our ISP is routing back to us ( /24 example), discard routes can be used.
This also is independent of the ip classless or no ip classless configuration.
RTA discard routes:
ip route null0
ip route null0 200
Rick Graziani

19. Discard Routes Final Notes
Discard routes can be especially useful for networks that use static routes instead of dynamic routing, in order to prevent routing loops.
“Configuring discard routes is the only way to prevent routing loops when several major networks are summarized into a supernet.” Alex Zinin, Cisco IP Routing
For examples of these and other scenarios, read Cisco IP Routing, by Alex Zinin.
For more information on the affect of the “ip classless” and “no ip classless” commands, read my presentation on “The Routing Table -Structure, Lookups, and the ip classless command.”
Rick Graziani

20. Cisco IP Routing by Alex Zinin Addison-Wesley Pub Co ISBN: 0201604736
Looking for more?
This is the book to get!
Rick Graziani

21. Questions?
Instructors: If you have any questions or comments, or if you find any mistakes in this presentation, please contact me:
Rick Graziani
Rick Graziani