Whether you can replace the 891 device with another device boils down to a single question: Do you need to run BGP with the Service Provider in order to use their service. If you need to run a routing protocol with your service provider, your service is likely a L3VPN (IP VPN) solution ( i.e. you inject your site's routes into the providers L3VPN session, they use MP-BGP+VRF for segmentation within their network).
If, however, they just drop you a L2 connection and provide L2 emulated services ( e.g. L2VPN or VPLS ) across their network, then your device can be whatever you want it to be.
From your device's perspective, it is not VRF aware. That is, it does not know about how the service provider segments your service from another customers. In the L3VPN case, your device is routing-protocol aware. In the L2VPN case, your device is not routing protocol aware and does not need to form adjacency with the service provider's equipment.
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