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I came across a practice question of true/false answer type . The statement is:

"A router has single network layer that will interact with 2 Data link layers" .

Ans:True (no explanation given)

enter image description here .I get that N/w layer is common for all interfaces on router(Nw layer is implemented by os).

DLL at router receives the incoming packet,pass it to N/W layer above,N/W layer finds the appropriate interface using routing table and forwards the packet to that interface . For this , the packet has to pass again through the DLL in the router .

My question is does the router have multiple DLLs(multiple NICs) with different mac addr(since Mac addr is based on NIC), each associated with their corresponding interface such that the receiving DLL passes the packet to NL above and during outgoing of same packet NL passes it to DLL of the appropiate interface which in turn passes it next hop?

OR

Router has only single DLL that handles all interfaces on router,(DLL->NL->DLL) and the packet passes through the same DLL in router twice (during incoming and during outgoing)?can ignore the distinction shown in DLL in router in the image above?

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  • A poorly worded question IMO. The “layers” are abstract concepts. They don’t really exist. – Ron Trunk Jun 9 at 10:41
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Yes - potentially, a router talks to a different data link layer network (ie. L2 segment) on each of its (sub)interfaces. On different physical interfaces you can even have different L2 protocols (very common in practice).

My question is does the router have multiple DLLs(multiple NICs) with different mac addr(since Mac addr is based on NIC), each associated with their corresponding interface such that the receiving DLL passes the packet to NL above and during outgoing of same packet NL passes it to DLL of the appropiate interface which in turn passes it next hop?

Yes. A router generally has multiple interfaces (not necessarily true NICs), each usually connecting into a different L2 segment (which I think you mean by DLL).

A router forwards a received packet to the next hop towards the destination (the next hop can also be the destination itself). For that, the router uses the underlying L2 that provides data framing and often requires addressing, possibly by MAC.

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