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New Member

EIGRP summarization

Hello!

Is there any way to avoid the router to generate the route to the null 0 interface when configuring summarization using EIGRP?

Example:

int e0

ip summary-address eigrp 11 200.X.X.X 255.255.255.0

sh ip route

D 200.X.X.X/24 is a summary, 00:00:00, Null0 (I would like to avoid this route in its routing table)

Thanks in advance,

Marcelle.

6 REPLIES
Cisco Employee

Re: EIGRP summarization

You can disable the creation of a summary route with OSPF using the discard-route knob. As far as I know, there is no way to do that with eigrp. What problem are you trying to solve?

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: EIGRP summarization

Hello hritter,

Would you please provide more info on the discard-route knob. I couldn't find an discard-route command in IOS?

Many Thanks

Cisco Employee

Re: EIGRP summarization

I didn't found any ducumentation either but found a DDTs underlaying the fact that there was no documentation surronding that command. This command is only available with OSPF. By default, "discard-route internal" and "discard-route external" are on. If you don't want the summary route to be generated when using the "area-range" command, just configure "no discard-route internal". If you don't want the summary when using the "summary-address" (summarization of external routes) then configure "no discard-route external".

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: EIGRP summarization

Thank you hritter, that is very useful information.

VIP Purple

Re: EIGRP summarization

Hello,

AFAIK, the Null route only disappears if there is no other, more specific route in your routing table for the summarized network. The route to null0 ensures that if your router receives a packet for which it has no route (e.g., 200.2.1.1), it will route the packet using the null interface, thereby dropping the packet, rather than using some other route for the packet (such as a default route).

Try and remove all other IP addresses from the router that are more specific than your summarized route, and you will not see the Null route anymore.

HTH,

Georg

Gold

Re: EIGRP summarization

You can put an administrative distance after the summary route in later releases:

ip summary-address eigrp xxx x.x.x.x x.x.x.x <240>

You can set this to 255, and make the route not appear in the routing table, but you are opening yourself to routing loops by doing so (what the discard route is supposed to prevent). It's better to make the admin distance high enough to allow another route in the table, and let it come back into the table if that other route doesn't exist.

There has been some discussion about removing this feature, but I think it's here to stay for the moment.

:-)

Russ.W

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