Re: Feasible to implement a router on a system on a chip?
- From: David Brown <david.brown@xxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 28 Mar 2008 18:57:58 +0100
Keith M wrote:
David Brown wrote:
As far as I can tell, he is thinking of a routing packets as fast as possible between networks, not handling the packets on the system itself (except as needed for routing protocols).
Between networks? You mean between local LANs? Or between a LAN and WAN? 99%+ of the routers out there bridge(ahem, route) LAN to WAN. I know these terms start getting diluted when we start cable and DSL, so let's not. :)
There's a big difference between Layer 3 support on a switch (that happens to support BGP) and a router.
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.
mvh.,
David
.
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