Discard Routes

Discard Routes and Avoiding Routing Loops

CCNA/CCNP

Rick Graziani

Cabrillo College

Rick Graziani [email protected] 2

Note to instructors

• If you have downloaded this presentation from the Cisco Networking Academy Community FTP Center, this may not be my latest version of this PowerPoint.

• For the latest PowerPoints for all my CCNA, CCNP, and Wireless classes, please go to my web site:

http://www.cabrillo.cc.ca.us/~rgraziani/• The username is cisco and the password is perlman for all of

my materials.

• If you have any questions on any of my materials or the curriculum, please feel free to email me at [email protected] (I really don’t mind helping.) Also, if you run across any typos or errors in my presentations, please let me know.

• I will add “(Updated – date)” next to each presentation on my web site that has been updated since these have been uploaded to the FTP center.

Thanks! Rick


• This book is highly recommended for instructors and CCNP students.

• Special thanks to the author, Alex Zinin, for his help.

Cisco IP Routing

by Alex Zinin

Addison-Wesley Pub Co

ISBN: 0201604736


Topics

• Situation: Normal

• Link Down

• Solution #1 – no ip classless

• Solution #2 – discard route

• What about packets destined for 192.168.1.0/24?

• Final Notes


ISP Network

Central Office Remote Office

Customer Network172.16.0.0/16 and 192.168.1.0/24

192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24

RTA has a static defaultroute 0.0.0.0/0 to ISP

RTA

RTB RTC

ISPISP has static routes for

172.16.0.0/16 and192.168.1.0/24 to RTA

Scenario


• Customer Network is running a dynamic routing protocol.

• All subnets within the 172.16.0.0 network are contained in this “Customer Network.” (There are no discontiguous 172.16.0.0 subnets via ISP.)

• Remote Office has 172.16.4.0/24 and 192.168.1.0/24 networks.

• All default traffic is sent to ISP, via 0.0.0.0/0 default route on RTA that is propagated to RTB and RTC.

• ISP has static routes pointing to RTA for 172.16.0.0/16 and 192.168.1.0/24 networks.

Scenario

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA


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.

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

Situation Normal


ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

Situation Normal

RouterA#show ip route

172.16.0.0/24 is subnetted, 3 subnets

C 172.16.1.0 is directly connected, Ethernet0


R 172.16.3.0 [120/1] via 172.16.2.1, 00:00:20, Ethernet1

R 172.16.4.0 [120/2] via 172.16.2.1, 00:00:20, Ethernet1

R 192.168.1.0/24 [120/2] via 172.16.2.1, 00:00:20, Ethernet1

S* 0.0.0.0/0 is directly connected, Serial1


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 172.16.4.10?

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

X

Link Down


• After the routing tables are updated, where would RTA or RTB send packets with the destination IP address 172.16.4.10?

• 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 172.16.4.1?

Link Down







• Now what will the ISP router do with these packets for 172.16.4.10?• 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?

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

X

Link Down


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

172.16.4.10. – The router would find the “parent network,” 172.16.0.0, and search

the known subnets, “child routes,” 172.16.1.0/24 and 172.16.2.0/24, but would not find the route 172.16.4.0/24.

– The affect of the “no ip classless” command makes the router drop any packets within the 172.16.0.0 network where there is no known subnet, but there is a parent network. (In this case 172.16.4.0/24 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 172.16.4.10 would be dropped by RTA and RTB.

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

192.168.1.0/24 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.

Link down – Solution #1: no ip classless







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 172.16.0.0 255.255.0.0 null0• This would cause RTA to drop all packets for subnets in the 172.16.0.0

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 172.16.0.0 packets not matching 172.16.1.0/24 or 172.16.2.0/24, would be routed to null0, using the discard route.

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

X

Link down – Solution #2: Discard Route


• Discard Route on RTA: ip route 172.16.0.0 255.255.0.0 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 172.16.0.0/16 subnets that do not exist, for example, any packets incorrectly sent to 172.16.5.0/24 subnet. (This is even the case when all the links are up.)

• RTA’s discard route will drop these packets as well!



172.16.0.0/16 is subnetted, 3 subnets, 2 masks

S 172.16.0.0/16 is directly connected, Null0

C 172.16.1.0/24 is directly connected, Ethernet0




• When all links are up…













R 172.16.3.0/24 [120/1] via 172.16.2.1, 00:00:20, Ethernet1

R 172.16.4.0/24 [120/2] via 172.16.2.1, 00:00:20, Ethernet1

R 192.168.1.0/24 [120/2] via 172.16.2.1, 00:00:20, Ethernet1



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.

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

X

Discard Routes


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

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

X

Discard Routes


What about packets destined for 192.168.1.0/24? (Continued)• However, in this case the there are networks between 172.16.0.0/16 and

192.168.1.0/24, so a single discard route will not work.• What about adding a second discard route on RTA?• 2nd Discard Route: ip route 192.168.1.0 255.255.255.0 null0• Unfortunately, this would cause RTA to drop all packets for 192.168.1.0/24

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)

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

X

Discard Routes


What about packets destined for 192.168.1.0/24? (Continued)

• What about modifying the administrative distance?

• 2nd Discard Route: ip route 192.168.1.0 255.255.255.0 null0 200

• Now, this route would only enter the routing table for RTA, when the dynamic route to 192.168.1.0/24 is deleted.

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

X

Discard Routes


Discard Routes






R 172.16.3.0/24 [120/1] via 172.16.2.1, 00:00:20, Ethernet1

R 172.16.4.0/24 [120/2] via 172.16.2.1, 00:00:20, Ethernet1

R 192.168.1.0/24 [120/2] via 172.16.2.1, 00:00:20, Ethernet1







R 172.16.3.0/24 [120/1] via 172.16.2.1, 00:00:20, Ethernet1

R 172.16.4.0/24 [120/2] via 172.16.2.1, 00:00:20, Ethernet1

S 192.168.1.0/24 is directly connected, null0


If RIP with AD=120 goes down, replaced with static route with AD=200

Discard route for 172.16.0.0 network


• So, to protect our network from blackholing traffic for routes that are in our network but no longer reachable (172.16.4.0/24 and 192.168.1.0/24 examples), or for routes (subnets) that our ISP is routing back to us (172.16.5.0/24 example), discard routes can be used.

• This also is independent of the ip classless or no ip classless configuration.• RTA discard routes:

ip route 172.16.0.0 255.255.0.0 null0ip route 192.168.1.0 255.255.255.0 null0 200

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

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.”

ISP Network



192.168.1.0/24

172.16.1.0/24

172.16.2.0/24

172.16.3.0/24

172.16.4.0/24


RTA

RTB RTC


172.16.0.0/16 and192.168.1.0/24 to RTA

Discard Routes


• Looking for more?

• This is the book to get!

Cisco IP Routing

by Alex Zinin

Addison-Wesley Pub Co

ISBN: 0201604736


Instructors: If you have any questions or comments, or if you find any mistakes in this presentation, please contact me:

Rick Graziani

[email protected]

www.cabrillo.edu/~rgraziani

Questions?

Documents

Discard Routes