There are many things to consider when thinking about the feasibility of replacing a WAN provider’s provider (P) and provider edge (PE) devices with routers.
The first problem with using routers within the WAN cloud is that in addition to IPv4, customers might also be using other routed protocols. With Layer-2 switches in the WAN cloud, this isn’t a problem, but if the PE and P devices are routers, they will need multiple routing tables, one for each routed protocol for which they’re forwarding traffic (multiprotocol routers). Since routing tables are kept in RAM, and RAM costs money, keeping multiple large routing tables on many routers could entail significant costs.
The second problem is that packet processing by a router takes longer than frame forwarding by a Layer-2 switch. A Layer-2 switch only has to find the match in its switching table, and forward the frame. If using “cutthrough” switching, this can happen at nearly line speed. An IPv4 router, on the other hand, must strip the frame header and trailer, do a longest-prefix match on the destination address, decrement the IP TTL, check that the TTL hasn’t reached zero, recalculate the IP header checksum, encapsulate the packet within a new frame, and forward the frame.
The third problem is that if two customers attempt to use the same address space, the routers within the WAN cloud will be confused as to which prefix lies where.
Continue reading to learn more about the problems of using routers within the WAN cloud and the solutions.