EIGRP Stub Explained

Let’s see if there are any differences when I shut the loopback 0 interface…

R2(config)#interface loopback 0
R2(config-if)#no shutdown

I think it should be shutdown and not “no shut”

Thanks for letting me know, just fixed it.

HI Rene,

Nice explanation.

what is the difference between stub static and stub redistribute.
for me , it seems both are same as in both the cases only redistributed routes will be advertised as per the example you have provided.

Thanks, Praveen

Hi Praveen,

stub static will redistribute only static routes (you still need “redistribute static”) but it won’t redistribute other prefixes from OSPF, RIP or BGP. Stub redistribute will redistribute anything…static routes, RIP, OSPF, BGP, etc.

Rene

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I am curious…what is the difference in using stub and passive-interface for EIGRP? Having a bit of trouble wrapping my mind around separating these 2.

Sincerely,
Rob

Hi Rob,

There’s a big difference between the two. An EIGRP router that is configured as a stub does not receive queries when one of its neighbors loses a network.

Passive-interface is about sending hello packets on an interface. Normally when you use the network command to advertise something then it does two things:

  1. It advertises all networks that fall within the range of your network commmand on all active interfaces.
  2. It sends hello packets on all active interfaces that have networks that fall within the range of your network command.

When you enable passive-interface, it will stop sending the hello packets. This is useful on interfaces that connect to end-devices like phones, workstations, servers, etc. You probably don’t want to send EIGRP hello’s in their direction…there are no EIGRP routers there.

Hope this helps!

Rene

I have a question, you said Stub Redistributed includes Connected and Summary routes.
So if you didn’t take away the “ip summary-address eigrp 12 1.1.0.0 255.255.0.0” on Fa0/1,
R3 will also learn the 1.1.0.0 network?

Thanks!

That’s right John, in fact if you configure stub redistributed then it will enable connected and summary as well:

R1(config-router)#router eigrp 1
R1(config-router)#eigrp stub redistributed 

R1#show run | incl stub
 eigrp stub connected summary redistributed

Rene, i just labbed this one, i configure eigrp stub redistributed on the R2 and the ip summary-address. R3 didn’t learn the 1.1.0.0 network.

also i have another question, you said when i configure eigrp stub redistributed, it will also enable “Connected and Summary” right? so in short, Redistributed, Connected and Summary routes will be advertised.

Since you created a Loopback interface on R2, it is now directly connected on your R2, so even if you dont configure “redistribute connected” it will advertise the Loopback since it is directly connected and under stub redistributed, Connected and Summary is also enable on this one.

Thank you.

i see the problem now, im using 7200 ios15 on GNS3, when i typed “sh run | include stub” the result is only eigrp stub redistributed, no connected and summary. thats why R3 is not learning the summary address.

anyways, what im really intrigue about is, on your device, stub redistributed also enables connected and summary, if you create the loopback interface, R3 will immediately learn the loopback of R2 since it is directly connected, there’s no need to redistribute it.

Hi John,

That’s interesting, I just tried it on a 1841 with IOS 15 and it does the same thing. When I configure EIGRP redistributed then that’s the only thing that shows up in the config. On a 3725 running 12.4T it does add connected and summary automatically as well.

Rene

so Rene, is my understanding correct? lets say, im using 3725 with 12.4 ios. when i configure stub redistributed, it will also enable Connected and Summary. so when i add the loopback interface, it doesn’t need to be redistributed, since connected interface will be advertise as well.

thanks

Hi John,

That’s right, when you configure “eigrp stub redistributed” then it automatically adds connected and summary. This is how it works on 12.4 so maybe they changed it for 15.x

Rene

As always buddy, your explanation are easy to follow. I appreciate it.

Hello Rene,
I have got some confusion here. I am going to use the below topology to ask my questions.

  1. In this topology, let’s say Stub is not configured. If I shutdown the loopback 0 on R2, who would send out Query? Is it only R1 or only R2 or both routers?

  2. Here If R1 is configured as Stub and Loopback 0 gets shutdown on R2, R2 will not send any Query to R1. However, R1 will send Query to R2. Is it correct?

Thanks a lot.

Azm

Hello Azm

R2 will query R1 to find an alternate route to 2.2.2.0/24. R1 will not query any routers because it is not connected to any. Queries will not be sent back to the original router from which it was received, so R1 will not “re-query” R2. R1 will only send queries IF one of its connected networks go down.

If R1 is configured as a Stub and loopback 0 goes down, R2 will NOT send a query to R1 since it “knows” that because it is a stub, it cannot possibly have an alternate route to 2.2.2.0/24. Again, R1 will not send a query to R2 unless one of its connected networks go down. If that happens, then yes, R1 will send a query to R2.

I hope this has been helpful!

Laz

Hi Rene,

In which scenario we need to use eigrp stub receive-only option, If you do not mind I need an example of this ???

Hello Hussein

There are several specialised cases where the receive-only option is useful. Some of these include:

  1. When applying it to a dual homed stub router with more than one connection to the same EIGRP AS. In a Dual-homed scenario “receive-only” would prevent the stub router from becoming a transit router incase of link failure in your core and still have redundancy from the stubs perspective.
  2. When using a route server connected to an eBGP network
  3. When using DMVPN spokes.

I hope this has been helpful!

Laz

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Hi Rene,

I know this is an old post but I believe your statement to be incorrect as EIGRP stub routers do receive queries as queries are sent as multicast in the first instance. They reply to the majority of queries with an infinite metric, unless of course they are networks the stub router is allowed to advertise (summary, connected, static, redistributed or a leak-map - depending on configuration).

Let me know if i’m not correct here.

Hello Kevin

Thanks for bringing this up. It seems that it is an issue of terminology. You are right that a stub router will indeed receive queries from neighboring routers that lose a network. However, an EIGRP stub router will not process these queries. This, I believe, was Rene’s meaning as well. Specifically, Cisco states the following:

The stub device responds to all queries for summaries, connected routes, redistributed static routes, external routes, and internal routes with the message “inaccessible.”

This was taken from the following Cisco Documentation that contains additional information about EIGRP stub networks.


I hope this has been helpful!

Laz