EIGRP topology issue

[MARCELO MATURO]

I have two 6500 interconected by a vlan interface. In the show ip eigrp topoloy for and espefic prefix, this one, is presents in both routers, learned by the same interface.

This is the ouput por both routers. The neighbors are 76.248.249.2 and 76.248.249.48 ( vlan 73). I don´t understand how this situation is posible. They are learning and publishing a prefix in the same interface? Thanks

RouterA#sh ip eigrp topology 76.0.88.0/23
EIGRP-IPv4 Topology Entry for AS(200)/ID(192.168.34.181) for 76.0.88.0/23
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2114048
Descriptor Blocks:
77.249.240.5 (Vlan545), from 77.249.240.5, Send flag is 0x0
Composite metric is (2114048/2113792), route is Internal
Vector metric:
Minimum bandwidth is 3072 Kbit
Total delay is 50030 microseconds
Reliability is 255/255
Load is 7/255
Minimum MTU is 1427
Hop count is 3
76.249.248.31 (Vlan73), from 76.249.248.31, Send flag is 0x0
Composite metric is (20512512/20512256), route is Internal
Vector metric:
Minimum bandwidth is 128 Kbit
Total delay is 20020 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1438
Hop count is 2
76.249.248.48 (Vlan73), from 76.249.248.48, Send flag is 0x0
Composite metric is (2114304/2114048), route is Internal
Vector metric:
Minimum bandwidth is 3072 Kbit
Total delay is 50040 microseconds
Reliability is 255/255
Load is 10/255
Minimum MTU is 1427
Hop count is 4

RouterB#sh ip eigrp topology 76.0.88.0/23
EIGRP-IPv4 Topology Entry for AS(200)/ID(77.245.0.5) for 76.0.88.0/23
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2114048
Descriptor Blocks:
77.249.240.1 (Vlan544), from 77.249.240.1, Send flag is 0x0
Composite metric is (2114048/2113792), route is Internal
Vector metric:
Minimum bandwidth is 3072 Kbit
Total delay is 50030 microseconds
Reliability is 255/255
Load is 10/255
Minimum MTU is 1427
Hop count is 3
76.249.248.31 (Vlan73), from 76.249.248.31, Send flag is 0x0
Composite metric is (20512512/20512256), route is Internal
Vector metric:
Minimum bandwidth is 128 Kbit
Total delay is 20020 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1438
Hop count is 2
76.249.248.2 (Vlan73), from 76.249.248.2, Send flag is 0x0
Composite metric is (2114304/2114048), route is Internal
Vector metric:
Minimum bandwidth is 3072 Kbit
Total delay is 50040 microseconds
Reliability is 255/255
Load is 7/255
Minimum MTU is 1427
Hop count is 4
76.251.252.2 (Vlan74), from 76.251.252.2, Send flag is 0x0
Composite metric is (2119168/2114048), route is Internal
Vector metric:
Minimum bandwidth is 3072 Kbit
Total delay is 50230 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1427
Hop count is 4

[Richard Burts]

There is not enough information here for us to be able to provide a good answer. We need more information about the topology of this network. Perhaps a simple diagram showing the relationships would be a starting point. It might also help us understand if you provide the output of show cdp neighbor from both switch, along with the output of show ip eigrp interface and of show ip eigrp neighbor, and the output of show ip interface brief.

HTH

Rick

[Peter Paluch]

Hi Marcelo,

I absolutely agree with Rick - we do not have enough information about your network and the placement of RouterA, RouterB, 76.248.249.2 and 76.248.249.48 (are these addresses RouterA and RouterB, or do they belong to different devices?).

However, one thing is worth looking at: Is it possible that 76.248.249.2 and 76.248.249.48 have Split Horizon deactivated on interface Vlan73? That would explain why they seem to receive and advertise the network over the same interface.

Best regards,
Peter

[MARCELO MATURO]

I uploaded a graphic with de complete topology. Thanks

[MARCELO MATURO]

Peter,

The split horizon hasn't deactivated in the vlan 73. I uploaded a graphic with the complete topology.

Thanks

[Richard Burts · Accepted solution]

Thank you for the graphic which does clarify what is going on. Based on the graphic here is what I believe is the case:

- network 76.0.88.0/23 is advertised to Router E.

- Router E advertises the network to both Router C and to Router D.

- Router C advertises the network to Router A on vlan 545 while Router D advertises the network to Router B on vlan 544.

- So Router A has a route to the network and advertises it to Router B on vlan 73. So Router B has a better route to the network through Router D and the less attractive route through Router A on vlan 73. And Router B advertises the route to Router A on vlan 73. So Router A has both the better route to the network through Router C and the less attractive route through Router B on vlan 73.

So it is legitimate that each of Router A and Router B will have learned the network via vlan 73 and will have it in the EIGRP topology table. And note that in both routers the route via vlan 73 is less attractive than the alternative learned via vlan 544/545.

HTH

Rick

[MARCELO MATURO]

Thank you Richard for your answer, but this situation doesn't violate de split-horizon feature?

[Jon Marshall]

No it doesn't and Rick's answer is spot on.

The split horizon rules states that you should not advertise a route out of the interface that is used to get to the network.

And as Rick says vlans 544/545 are the next hop interfaces to get to that network. So they can both advertise the same route to each other over vlan 73.

Jon

[MARCELO MATURO]

Thank you Jon. Thats was my main doubt about this topology