CCNP Student lab manual -> question about EIGRP

Hello,

Attached, you have topology that was designed and tested under GNS3.

I want to clarify one thing regarding network convergence (last topic):
When we ping from R3 to R1's loopback interface, best route is through 10.1.1001 (R1's Fa0/0). If I shutdown this interface, it's possible to see that network converges and that packets go to R2 and then through serial link to R1's loopback.

Anyway, "show ip eigrp topology all-links" on R3's does not show this route (R3 - R2 - R1) to R1's loopback interface. It wasn't supposed to appear here?

R3#show ip eigrp topology all-links
IP-EIGRP Topology Table for AS(1)/ID(192.168.100.5)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 10.1.3.0/24, 1 successors, FD is 128256, serno 2
via Connected, Loopback3
P 10.1.2.0/24, 1 successors, FD is 156160, serno 5
via 10.1.100.2 (156160/128256), FastEthernet0/0
P 10.1.1.0/24, 1 successors, FD is 156160, serno 14
via 10.1.100.1 (156160/128256), FastEthernet0/0

Thanks,
AF

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Comments

hitmen

Isnt it this?

P 10.1.1.0/24, 1 successors, FD is 156160, serno 14
via 10.1.100.1 (156160/128256), FastEthernet0/0

The switch has no ip address.
Perhaps the eigrp doesnt run on a switch?

I am not sure as I am studying for ccnp too.
Correct me as I could be wrong.

aocferreira

Hi hitmen,

That route has next hop IP address 10.1.100.1 (the R1's fa0/0) interface.. that means that packets would go R3 -> R1.

Anyway, I thought that, when running "show ip eigrp topology all-links", we should something like:

P 10.1.1.0/24, 1 successors, FD is 156160, serno 14
via 10.1.100.1 (156160/128256), FastEthernet0/0
via 10.1.100.2 (FD/AD), FastEthernet0/0

Because when R1's fa0/0 fails, I could see that packets go through R2 and then from R2 serial interface to R1. Maybe some misunderstanding here?

Br,
AF

Jackace

One thing to remember with EIGRP is it is a distance vector protocol. It only sees routes that are advertised to it by it's neighbors. Another thing to remember is EIGRP by default has split horizon enabled.

aocferreira

Hi Jackace,

Yes, but "show ip eigrp neighbors" command on R3 shows that it has established neighbor relationships both with R1 and R2, so it should be receiving routes from both routers..

Regarding split horizon, I believe that this is only a problem if we are facing frame relay topologies with multipoint subinterfaces. I guess here there's no problem with that...

AF

Danielh22185

The command 'sh ip eigrp topology all-links' will only display the current links being used in the routing table. If you use the 'show ip eigrp topology' you can see the current routes AND the backup feasible successor routes.

aocferreira

But if the route through R2 is not a feasible successor, it should be available in show ip eigrp topology all-links

Jackace

Just as I thought. It is a split horizon issue. I have that lab saved from my Route studies so I pulled it up and saw the same thing you did. If you check R2 however you see that it knows both routes. I then turned off split horizon on all the FastEthernet links on R1, R2, and R3 in the switched network and I now see the routes in the Eigrp topology all-links database. My R3 output is below after the split horizon changes. I also found another point about split horizon that I can pass along.

Split Horizon is not just for loop prevention in eigrp; it is also used as an efficiency mechanism.

R3#sh ip eigrp topology all-links
IP-EIGRP Topology Table for AS(1)/ID(192.168.100.5)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 10.1.3.0/24, 1 successors, FD is 128256, serno 2
via Connected, Loopback3
via 10.1.100.1 (158720/156160), FastEthernet1/0
via 10.1.100.2 (158720/156160), FastEthernet1/0
P 10.1.2.0/24, 1 successors, FD is 156160, serno 6
via 10.1.100.2 (156160/128256), FastEthernet1/0
via 10.1.100.1 (158720/156160), FastEthernet1/0
P 10.1.1.0/24, 1 successors, FD is 156160, serno 17
via 10.1.100.1 (156160/128256), FastEthernet1/0
via 10.1.100.2 (158720/156160), FastEthernet1/0
P 192.168.100.0/30, 1 successors, FD is 128256, serno 3
via Connected, Loopback11
via 10.1.100.1 (158720/156160), FastEthernet1/0
via 10.1.100.2 (158720/156160), FastEthernet1/0
P 10.1.100.0/24, 1 successors, FD is 28160, serno 1
via Connected, FastEthernet1/0
via 10.1.100.1 (30720/28160), FastEthernet1/0
via 10.1.100.2 (30720/28160), FastEthernet1/0
P 10.1.200.0/24, 2 successors, FD is 40514560, serno 12
via 10.1.100.1 (40514560/40512000), FastEthernet1/0
via 10.1.100.2 (40514560/40512000), FastEthernet1/0

Edit - Just checked to make sure, but if you configure no ip split-horizon eigrp 1 on R2 alone you see 2 paths to R1's loopback.

aocferreira

Thanks a lot!

Now it's explained