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OSPF no discard route scenario

Answered Question
Nov 22nd, 2013
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Can anyone give me a scenario where "no discard router internal/external" route command be used, i am having hard time to some up with a useful scenario.


Thanks in adavance..            




Siddhartha       

Correct Answer by Rolf Fischer about 3 years 8 months ago

Maybe it's easier with some practice.


Before summarization, the DMZ server is reachable via the default route:

R6#ping 172.16.100.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.16.100.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 20/40/76 ms


R6#show ip route 172.16.100.1

% Subnet not in table


R6#show ip cef 172.16.100.1

0.0.0.0/0, version 9, epoch 0, cached adjacency to Serial0/0


Now we configure the summary:

R6(config-router)#area 0 range 172.16.0.0 255.255.0.0


By default, a discard-route is installed.


R6(config-router)#do ping 172.16.100.1 repeat 2

..

Success rate is 0 percent (0/2)


R6(config-router)#do show ip route 172.16.100.1

Routing entry for 172.16.0.0/16

Known via "ospf 1", distance 110, metric 0, type intra area

Routing Descriptor Blocks:

* directly connected, via Null0



Now we remove the automatically installed discard-route:


R6(config-router)#no discard-route internal


R6(config-router)#do ping 172.16.100.1 repeat 2

!!

Success rate is 100 percent (2/2), round-trip min/avg/max = 16/50/84 ms


Now the default route is the best match again.   

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Rolf Fischer Fri, 11/22/2013 - 11:00
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Hi,


Probably this option is rarely used indeed.

Generally speaking, you would want to use it when some networks within the summary are only reachable through routes with shorter prefix-lengths, e.g. the default-route.


An example:

Let's say you summarize 172.16/16 on an ABR for the backbone area but you have some DMZ-networks within that range but outside the OSPF domain which are only reachable through the default-route. Now with a discard-route for 172.16/16 in area 0 pointing to Null0, you would have to add static routes to those DMZ networks or enable a routing-protocol.


Hope that makes sense.

Rolf

siddhartham Fri, 11/22/2013 - 11:32
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Thanks Rolf-


In the above example, like you mentiond- if we want to reach 172.16 subnets then we will configure static routes on the ABR but we won't delete the discard route.


So do you know of any production scenario where we use no discard route command and take out the route to null0?


Thanks,

Siddhartha

Rolf Fischer Fri, 11/22/2013 - 12:12
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Hi  Siddhartha,


In the above example, like you mentiond- if we want to reach 172.16 subnets then we will configure static routes on the ABR but we won't delete the discard route.


what I wanted to point out is that the automatic discard route cuts the reachability to the DMZ-networks (e.g. 172.16.100.0/24), which were reached through the default route before the summary was configured. Instead of configuring static routes, you could use the "no discard-route internal" command to solve the connectivity problem.

(The discard-route has a prefix-length of /16 which is longer than the default-route's /0)


HTH

Rolf

Correct Answer
Rolf Fischer Fri, 11/22/2013 - 12:45
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  • Blue, 1500 points or more

Maybe it's easier with some practice.


Before summarization, the DMZ server is reachable via the default route:

R6#ping 172.16.100.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.16.100.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 20/40/76 ms


R6#show ip route 172.16.100.1

% Subnet not in table


R6#show ip cef 172.16.100.1

0.0.0.0/0, version 9, epoch 0, cached adjacency to Serial0/0


Now we configure the summary:

R6(config-router)#area 0 range 172.16.0.0 255.255.0.0


By default, a discard-route is installed.


R6(config-router)#do ping 172.16.100.1 repeat 2

..

Success rate is 0 percent (0/2)


R6(config-router)#do show ip route 172.16.100.1

Routing entry for 172.16.0.0/16

Known via "ospf 1", distance 110, metric 0, type intra area

Routing Descriptor Blocks:

* directly connected, via Null0



Now we remove the automatically installed discard-route:


R6(config-router)#no discard-route internal


R6(config-router)#do ping 172.16.100.1 repeat 2

!!

Success rate is 100 percent (2/2), round-trip min/avg/max = 16/50/84 ms


Now the default route is the best match again.   

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