I have two catyst 6509 with sup1 and msfc (referer Cat-A e Cat-B) with Layer 3 feature. There is a ospf process configured on Cat-a e cat-b in standard area 51. Every Cat has a netwotk p2p betwen Cat (svi vlan94) and a netwtork p2p to the same core (svi vlan 990 cat-a and svi vlan 989 cat-b) . There is a network (external type 2) that the two Cat receiving but:
show ip route
O E2 18.104.22.168 [110/100] via 22.214.171.124, 00:10:45, Vlan990
[110/100] via 126.96.36.199, 00:10:45, Vlan74
O E2 188.8.131.52 [110/100] via 184.108.40.206, 00:03:38, Vlan989
Why is there only one entry in cat-b?
Is there an algorithm in ospf to avaoid routing loop?
First the easy part of your question: yes OSPF does have an algorithm to avoid routing loops. OSPF maintains the Link State Data Base where it records the LSAs for the links that it has learned. It processes the Dykstra algorithm to determine loop free paths in the network to put into the routing table.
In your explanation you told us about VLAN 989 and 990 to the core. And this is how each switch is learning a path to the network. And cat-a is also learning a path to the network via VLAN 74. But you have not told us what is VLAN 74.
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