BDI spanning-tree issue

[Paheeradan Nagulan]

Experts,

We have a Cisco 8300 router(C8300-1N1S-6T Chassis) that is configured with a BDI interface(gi0/0/1 and Gi0/0/2). Those two interfaces are connecting a VPC switch on the other side(Nexus 9k).

1)From the Nexus 9k point of view, does that think the BDI interface as a single interface in terms of Spanning-tree(Was wondering whether both the links will be in forwarding or one link will be blocked state)?

2)Instead of configuring BDI interface, can I configure a Layer 3 port-channel on the Cisco 8300 router and have two links to the Nexus 9k VPC switch?

Thanks 

[Junkerstein]

Hallo Nagulan

1) each physical link with BDI still appears separately to the Nexus (Spanning Tree will block one port unless you form an EtherChannel).

2)yes, you can make a Layer3 port‑channel on the 8300 and vPC on the Nexus to have both links active at L3 with no STP involvement.

[Paheeradan Nagulan]

Thanks @Junkerstein , We have 2 8300 series routers(R1 and R2) that are using BDI. Both of them are connected to the same Nexus 9k VPC switches with two links from each of the routers going to the Nexus 9k switches. Interestingly, both the links from R1 to the Nexus 9k VPC switches are up and running(Spanning tree is forwarding from the Nexus side) but on R2 only single link is up and running , other one is mac flapping so we had to disable it. The configuration between R1 and R2 are identical except the BDI IP address. Not sure what's wrong?