Re: Feasible to implement a router on a system on a chip?

On Mar 28, 12:57 pm, David Brown
<david.br...@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
I think it is pretty important to establish exactly what is meant by
"router", "bridge", and "switch", since I am not sure we have the same
definitions (or else I am misunderstanding what you wrote). So correct
me if you think I've got something wrong here.

A "bridge" is a device that has two or more network ports, and which
passes traffic between the ports (which may be of different types -
e.g., WLAN, 100 MBit and 1 Gb ports). It is a Layer 2 (e.g., Ethernet)
device, and has no concept of IP addresses. A bridge will typically
automatically learn which Ethernet MAC addresses are attached to its
ports, so that traffic is only passed to ports that have the destination
MAC address attached (broadcasts are passed to all ports). Normally, no
filtering or interpretation of the packets is done.

A "switch" is a type of bridge that is specialised for low-latency
bridging of Ethernet packets, allowing packets to pass through different
pairs of ports simultaneously. It is also possible to have purely
software bridges (such as bridges in Linux, which can also have
filtering tables).

A "router" has two or more network ports and passes packets between them
based on their IP addresses (Layer 3), or possibly other higher layers.
The router may also have virtual network ports for vpns, it may also
modify the packets (such as for NAT or some kinds of vpn), and it may
have filters (a firewall) for the packets.

A "layer 3 switch" is a sort of combined layer 2 switch and layer 3
router, normally with specialised hardware to do the routing as fast as
possible.

Given that the OP wants to support BGP, which is used to track layer 3
IP routes, I expect that it is a layer 3 router that he wants to make.
Whether it is between different LANs, or between LAN(s) and WAN(s), I
don't know - presumably it's for a complex setup, since a common tree
hierarchy does not need BGP as there are never alternative routes.

The typical DSL router device is actually a combination of a bridge, a
switch, and a router (and is therefore a "router"). You normally get
four LAN ports which are connected together by a switch, a WLAN
interface which is bridged to the switch, and a router (with NAT and a
firewall) connecting the LAN switch to the WAN port.


There is no difference between a switch and a bridge. The former is
the more fashionable marketing term. Some early switches where
characterised by having cut-through forwarding, but that's almost
impossible to do with mixed link speeds, and the reduced latency at
the faster speeds makes it a non-issue anyway.

A layer-3 switch is a router. Again, routers are (or were) passé, and
switches were fashionable, hence the brand extension for the marketing
term switch. "Layer-3 router" is redundant and repetitive.

Devices that route some protocols and bridge other were once called
brouters, but I haven't heard the term in years, and bridging is an
optional function of most medium and high end routers.

NAT and VPN functions are common on non-low-end routers, although they
don't necessarily require much in the way of routing function.

The typical DSL router is really several devices in one box, and is
both a bridge and a router in the sense described above. As you
described, it's really a switch connecting the Ethernet ports, router
(when then heads out to DSL port) and access point. That differs from
the prior scenario in that it cannot bridge a protocol out the DSL
port (which a proper bridge/router could).
.

Relevant Pages

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