EIGRP Stub Explained

Lessons Discussion
63

Hello Görgen

I understand the confusion. In the lesson, Rene has enabled debugging only for EIGRP queries. There are other messages being sent in the meantime. When R2 is configured as a stub and the Loopback interface is shut down, this is what happens:

  1. EIGRP Route Removal: When you shut down the Lo0 interface on R2, the route to the 2.2.2.2 network (associated with Lo0) is removed from R2’s routing table.
  2. EIGRP Update Messages: R2 will send an EIGRP update message to its EIGRP neighbor, R1, indicating that the route to 2.2.2.2 is no longer available (because the interface associated with this route is down). This message is not shown in the lesson, but this is how R1 is informed.
  3. R1 Route Removal: Upon receiving this update, R1 will remove the 2.2.2.2 route from its own EIGRP topology table and routing table.
  4. R1 Sends Query: As shown in the lesson, R1 sends a query out to find out if it can find another route to the 2.2.2.2 destination. This query is sent to R2 (and to any other EIGRP neighbors it may have.)5.
  5. R2 does not send queries: Since R2 is configured as a stub router it will not send out queries. This is a key feature of the EIGRP stub configuration. The purpose of a stub router in EIGRP is to limit the queries and reduce the amount of routing information exchange, improving network stability and convergence time. Does that make sense?

I hope this has been helpful!

Laz

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64

Hello Rene,

i labbed this in Eveng with an image version 15.8(3)M2 , but its not showing the connected and summay by default when i enabled eigrp stub redistributed.

Router#sh run | s r e
router eigrp 100
 network 0.0.0.0
 redistribute static
 eigrp stub redistributed

Router#sh running-config all | i stub
 eigrp stub redistributed
Router#
Router#
65

Hello Sathish

I went into the lab for this one and could not reproduce your results. I am using:

Cisco IOS Software, IOSv Software (VIOS-ADVENTERPRISEK9-M), Version 15.9(3)M6, RELEASE SOFTWARE (fc1)

When I issue the eigrp stub command and then take a look at the running config, I get this as expected, with the connected and summary:

R1#show run | section router eigrp
router eigrp 123
 network 192.168.12.0
 eigrp stub connected summary

The only way to reproduce your results is if I issue the eigrp stub redistributed command. If you want to have connected, redistributed, and summary, you must issue all of the commands explicitly. Does that make sense?

I hope this has been helpful!

Laz

66

Hello, Why in the following the 1.1.1.1 loopback network ins not learned on R3 if it is not directly connected to R2

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I’m having a hard time understanding what the following mean on the stub router when its configured

Receive-only: The stub router will not advertise any network.
Connected: allows the stub router to advertise directly connected networks.
Static: allows the stub router to advertise static routes (you have to redistribute them).
Summary: allows the stub router to advertise summary routes.
Redistribute: allows the stub router to advertise redistributed routes.

Thanks

67

Hello Marcus

The purpose of the stub feature in EIGRP is to minimize the routing information a router propagates. By default, configuring an EIGRP router as a stub router will advertise only connected and summary routes. However, the eigrp stub command has some additional keywords that can modify this behavior somewhat.

Specifically, the eigrp stub connected command makes the EIGRP router on which it is configured advertise only directly connected routes. So, in the example you pointed out, R2 will advertise to R3 only its connected routes (192.168.12.0/24 and 2.2.2.2/24). It will not advertise the route it learned from R1 (1.1.1.1/24). This is, by definition, what the command does.

Similarly, the eigrp stub static command will cause the stub router to advertise only local static routes that have been redistributed into EIGRP.

The eigrp stub summary command allows stubs to advertise only summary routes. Only summary routes, which are either automatically or manually configured depending upon your implementation, will be advertised using EIGRP.

The eigrp stub redistribute command allows a stub router to advertise only redistributed routes, that is, routes that have been injected into EIGRP from any other source (other routing protocols, static routes, or directly connected routes).

Now the important thing to note here is that the eigrp stub command can be used with multiple keywords simultaneously. This gives you more flexibility in the type of stub you want to create. That way, you can ensure that only the type of advertisements you want will be shared.

The idea is that you have a lot of flexibility to filter out unwanted advertisements from stub areas, that is, for areas that don’t have another exit point. That way, you can make your routing tables smaller and more efficient. Does that make sense?

I hope this has been helpful!

Laz

68

Yes, Thanks, Laz. The more I read it, it started to make sense. Your explanation confirmed it.

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