Hi all ¡¡
I have a virtual environment mounted with Gns3.
I have an environment of several L3 switches 3600 Software (C3640-IK9O3S-M), Version 12.4(25b) connected to area 0 OSPF.
Specifically, a switch connects through 2 different networks to area 0. It learns a network twice (172.25.3.0/24) from 2 different neighbors, each of them on a different network. I don't know how I can see why it chooses one or the other route, when adding it to the route table. He always chooses the same destination in to the routing table and only if that neighbor is down, does he put the other one. Could you tell me why it takes one route and not the other? How can I see it?
"SW1#show ip route 172.25.3.0
Routing entry for 172.25.3.0/24
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 1
Last update from 10.127.7.6 on Vlan404, 00:20:28 ago
Routing Descriptor Blocks:
* 10.127.7.6, from 172.25.3.2, 00:20:28 ago, via Vlan404
Route metric is 20, traffic share count is 1"
CORE#show ip ospf database
172.25.3.0 172.25.3.1 1368 0x80000001 0x00DE2F 0
172.25.3.0 172.25.3.2 1367 0x80000001 0x00D834 0
I understand that if I don't have the route twice in the route table, is it because one has better costs than the other? Or maybe because you learn on different networks in the same area?
Solved! Go to Solution.
The issue start when you
redistribute connect subnet in R2 & R3 for subnet 188.8.131.52
even so this subnet connect two ospf router neighbor
this make R2 and R3 advertise E2 of 184.108.40.206/24 to R1
R1 will advertise both E2 but
R4 will process one because this prefix is pass through same Neighbor so R4 will only process one E2 not both.
OSPF chooses routes the following way:
So even at a higher cost it could still choose Intra-area routes over Inter area routes.
Can you provide the metrics of how both routes are learned from SW1 perspective ? I diagram would be helpful as well.
Also OSFP only does ECMP which means it only installs routes in the routing table of multiple paths if their metrics match.