EIGRP Configuration for CCNA Students

lagapidis · December 31, 2021, 9:10am

Hello Chris

Thanks for adding your understanding to the discussion! It really gets you thinking.

Yes, that is exactly the case. But remember, this potentially suboptimal routing is only for the FS which is a backup path that is used only in the event of a failure. The original best path is indeed the best path. But also keep in mind that you have to really contrive your metrics to actually get a situation where you can potentially have a suboptimal FS. It’s not likely, but it is possible.

Yes, that is true as well. It is always preferable to have a suboptimal path as an FS rather than a better FS that is a potential routing loop.

But since you know these things, you can always design your network such that EIGRP will not choose a suboptimal FS, by making sure your metrics are what they should be so that won’t happen.

Thanks for your input, and I hope this too has been helpful!

Laz

boubou.souley · December 4, 2022, 11:20pm

Hi All,
I am newbie here and do not know if i can ask a question here.
But any way.
Can i connect one of 2 eigrp neighbour to another router who has only static routes?

lagapidis · December 7, 2022, 7:08am

Hello Boubou

In order for two routers to become EIGRP neighbors, you must have the following:

The routers must be directly connected
The interfaces through which they are connected must be on the same subnet
The IP addresses on the interfaces must be participating in EIGRP. That means that they must be included in a network command under the EIGRP configuration.

If these prerequisites are met, then the neighborship can exist. From there on, you can configure any static routes you like within each of those routers, and if you choose to, you can also advertise those static routes to EIGRP using redistribution.

For more information on troubleshooting EIGRP neighbor adjacencies, take a look at this lesson:

I hope this has been helpful!

Laz

Both routers must be Yes, it is possible to create an EIGRP neighbor adjacency between two routers, where one or both of those routers include static routes.

psf1575 · February 21, 2024, 5:25am

Hello Rene,

If we issue the “no auto-summary” command under our router eigrp 1 configuration (or if its already enabled by default), am I correct in understanding that we do not have to also specify a wildcard mask in order to enable eigrp on the appropriate interfaces?

For example in R1:

Router eigrp 1
network 192.168.13.0

Since we disabled classful eigrp, how would the router know what interfaces to enable eigrp on if we dont use a wildcard mask alongside the statement such as:

router eigrp 1
network 192.168.13.1  0.0.0.0

or

router eigrp 1
network 192.168.13.0  0.0.0.255

I am assuming that, even if we configure the no auto-summary, that if we dont specify wildcard masks that it still acts classful, and thus enables eigrp on any interfaces that have IP addresses that fall within the 192.168.13.0/24 network, is this correct?

My final question, does the network command in eigrp work the same way as in ospf? meaning, any interface with an IP address that falls within the network command range will be enabled for eigrp like how it would be in ospf?

Thank You!

lagapidis · February 22, 2024, 8:47am

Hello Paul

When you specify a network under EIGRP without a wildcard mask, it defaults to classful behavior. So, if you put “network 172.16.2.0” under EIGRP, it would enable EIGRP on any interface within the 172.16.0.0/16 network, not just the 172.16.2.0/24 network. The following configuration and verification commands confirm this:

R1(config)#router eigrp 1
R1(config-router)#network 172.16.2.0
R1(config-router)#exit
R1(config)#exit
R1#show run | section eigrp
router eigrp 1
 network 172.16.0.0
R1#

The no auto-summary command tells EIGRP not to summarize routes at classful boundaries, but it doesn’t affect how the network statement is interpreted. So, even with no auto-summary, you would still need to specify a wildcard mask if you want to enable EIGRP on a more specific subnet than those specified by classful boundaries.

To enable EIGRP on the 172.16.2.0/24 network, you would use:

network 172.16.2.0 0.0.0.255

As for your final question, yes, the network command in EIGRP works similarly to OSPF. Any interface with an IP address that falls within the range specified by the network command (and wildcard mask) will be enabled for EIGRP.

I hope this has been helpful!

Laz

psf1575 · February 22, 2024, 5:24pm

Hello Laz,
Thank you for the response!

Im confused by your statements that mention that the subnet mask of the interfaces are also taken into account when using the network command to enable EIGRP (or OSPF) on certain interfaces.

I was under the impression that the network command only took into account an interfaces IP address, and not the subnet mask configured on them. Once an interface matches the network statement (via its IP), then the prefix thats actually advertised would 100% depend on the subnet mask configured on the interfaces.

Therefore I have not found it to be the case that I can only use the 0.0.0.0 wildcard statement to enable interfaces that are configured with a /32 mask, it should work as long as the interface has that IP address configured on it regardless of what mask its using.

I ran a quick lab on GNS3 with R1 and R2 being directly connected to eachother via Fa0/0 using a 10.1.1.0/24 network, then configured a loop0 in each using a /24 mask, and also configured Fa2/0 with a 192.168.1.1/24 on R1 and 192.168.2.1/24 on R2. I was able to enable EIGRP on the Loopback interfaces as well as on the Fa2/0 interfaces using a 0.0.0.0 statement even though they are not configured with a /32 mask and it worked fine. Each router was able to learn about eachothers networks via EIGRP:

R1:

interface Loopback0
 ip address 1.1.1.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 10.1.1.1 255.255.255.0
 duplex full
!
interface FastEthernet2/0
 ip address 192.168.1.1 255.255.255.0
 speed auto
 duplex auto
!
interface FastEthernet2/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
!
router eigrp 1
 network 1.1.1.1 0.0.0.0
 network 10.1.1.0 0.0.0.255
 network 192.168.1.1 0.0.0.0
!



R1#sh ip route eigrp
Codes: L - local, C - connected, S - static, 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
       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, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      2.0.0.0/24 is subnetted, 1 subnets
D        2.2.2.0 [90/156160] via 10.1.1.2, 00:22:10, FastEthernet0/0
D     192.168.2.0/24 [90/30720] via 10.1.1.2, 00:23:07, FastEthernet0/0

R2:

interface Loopback0
 ip address 2.2.2.2 255.255.255.0
!
interface FastEthernet0/0
 ip address 10.1.1.2 255.255.255.0
 duplex full
!
interface FastEthernet2/0
 ip address 192.168.2.1 255.255.255.0
 speed auto
 duplex auto
!
interface FastEthernet2/1
 no ip address
 shutdown
 speed auto
 duplex auto
!
!
router eigrp 1
 network 2.2.2.2 0.0.0.0
 network 10.1.1.0 0.0.0.255
 network 192.168.2.1 0.0.0.0
!

R2#sh ip route eigrp
Codes: L - local, C - connected, S - static, 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
       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, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      1.0.0.0/24 is subnetted, 1 subnets
D        1.1.1.0 [90/156160] via 10.1.1.1, 00:24:30, FastEthernet0/0
D     192.168.1.0/24 [90/30720] via 10.1.1.1, 00:24:30, FastEthernet0/0

Is there something I am not understanding right about the network command as it pertains to EIGRP/OSPF?

Thank You Laz

lagapidis · February 24, 2024, 7:04am

Hello Paul

I did a bit of experimentation myself and confirmed your lab as well. You’re absolutely right. The whole of the subnet that corresponds to the interface in question does not need to be contained within the network address and wildcard mask used in the EIGRP network command. Just the IP address of the interface. Once that’s done, it is the whole subnet of the interface that is advertised. I made a NetworkLessons note on the topic as well. Thanks for taking the time to test this out!

I hope this has been helpful!

Laz