OSPF - "discard-route"

I came across a nice little command to remember in OSPF. In fact I would have to admit that I knew this command but it completely flew off my mind, hence, today when one of my friends preparing for his CCIE R&S reminded me of this command, I decided to put it down on my blog so that I never forget it again

So here is a little explanation from the command reference on Doc-cd:

To reinstall an external or internal discard route that was previously removed, use the discard-route command in router address family topology or router configuration mode. To remove an external or internal discard route, use the no form of this command.

discard-route [external [distance]] [internal [distance]]

no discard-route [external [distance]] [internal [distance]]

External and internal discard-route entries are installed in routing tables by default. During route summarization, routing loops may occur when data is sent to a nonexisting network that appears to be a part of the summary, and the router that is performing the summarization has a less specific route (pointing back to the sending router) for this network in its routing table. To prevent the routing loop, a discard route entry is installed in the routing table of the ABR or ASBR.

If for any reason you do not want to use the external or internal discard route, remove the discard route by entering the no discard-route command with the external or internal keyword.

Examples

The following display shows the discard-route functionality installed by default. When external or internal routes are summarized, a summary route to Null0 will appear in the router output from the show ip route command. See the router output lines that appear in bold font:

Router# show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

172.16.0.0/24 is variably subnetted, 3 subnets, 2 masks
C 172.16.0.128/25 is directly connected, Loopback1
O 172.16.0.0/24 is a summary, 00:00:14, Null0
C 172.16.0.0/25 is directly connected, Loopback0
172.31.0.0/24 is variably subnetted, 3 subnets, 2 masks
C 172.31.0.128/25 is directly connected, Loopback3
O 172.31.0.0/24 is a summary, 00:00:02, Null0
C 172.31.0.0/25 is directly connected, Loopback2
C 192.168.0.0/24 is directly connected, Ethernet0/0

Router# show ip route ospf

172.16.0.0/24 is variably subnetted, 3 subnets, 2 masks
O 172.16.0.0/24 is a summary, 00:00:29, Null0
172.16.0.0/24 is variably subnetted, 3 subnets, 2 masks
O 172.16.0.0/24 is a summary, 00:00:17, Null0

When the no discard-route command with the internal keyword is entered, notice the following route change, indicated by the router output lines that appear in bold font:

Router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# router ospf 1
Router(config-router)# no discard-route internal
Router(config-router)# end

Router# show ip route ospf


172.31.0.0/24 is variably subnetted, 3 subnets, 2 masks
O 172.16.0.0/24 is a summary, 00:04:14, Null0

Next, the no discard-route command with the external keyword is entered to remove the external discard route entry:
Router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# router ospf 1
Router(config-router)# no discard-route external
Router(config-router)# end

The following router output from the show running-config command confirms that both the external and internal discard routes have been removed from the routing table of the router. See the router output lines that appear in bold font.

Router# show running-config

Building configuration...

Current configuration : 1114 bytes
!
version 12.2
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Router
.
.
.
router ospf 1
log-adjacency-changes
no discard-route external
no discard-route internal
area 1 range 172.16.0.0 255.255.255.0
summary-address 172.31.0.0 255.255.255.0
redistribute rip subnets
network 192.168.0.0 0.0.0.255 area 0
network 172.16.0.0 0.0.0.255 area 1
!

So in simpler terms if you want the Null0 routes not to be inserted in the OSPF routing table of the router that is summarizing an area or external routes, you can use this command.

Nice one to remember!