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Core Switch packet forwarding anomaly

We have two core switches in our network.In one of them,we have assigned 2 ports to a same Vlan and each of the port is connected to two firewalls i.e firewall 1 and firewall 2.Firewall 1 and firewall 2 further connects to Router 1 and Router 2 respectively.The internal network is connected to internet via two ISP.Primary ISP is connected to router 1 and secondary ISP is connected to router 2.

We have assigned two default routes in the core switch for internet.The one with AD value 1 is through the primary ISP via firewall1 and another one with an increased AD value(100) is through secondary ISP via firewall 2.Naturally, we want the core switch to send packets via primary route until and unless it is down.In case its down it must forward via secondary route.

The problem is when both the ISP s are connected together, we are able to access internet but when are tracerouting the path it shows that core switch is forwarding its packets via primary route at one moment and via secondary route in the next moment.Its changing its path from time to time(in matter of seconds) but ultimately its reaching the web.Why such anomaly is occuring?what is the possible solution? Let me know if anything is required.

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Core Switch packet forwarding anomaly

**The core switch is of Cisco 6509 model and belong to 6500 series fyi

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