system set up as a bridge

[john guardian]

Is there a simple/obvious method for determining if a system is set up to operate as a bridge?

Thx.

[Torsten.]

If I understand your question correctly you can assume each multi-homed system can be a "bridge" between the connected networks as a gateway if configured.

[Matti_Kurkela]

There's a risk of terminology-related misunderstandings here, I believe.

For network technology, the word "bridge" has a specific meaning, and what Torsten said sounds to me more like a gateway/router than a bridge.

I was taught this:

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1.) When a network device just retransmits an incoming signal, it is a repeater.

A repeater works in Layer-1 (the physical layer) of the OSI model only.

A 2-port repeater might be used as a distance extender in some network technologies, but is more often seen as a media changer (for example, between fiber-optic and copper cabling when both are using Ethernet). A multi-port repeater in Ethernet networks is known as a hub.

Using any kind of a general-use computer as just a repeater would be a massive overkill.

2.) If a network device pays attention to hardware-level addresses (MAC addresses in Ethernet) to decide whether or not to forward an incoming packet from one of its interfaces out of another interface, it is a bridge. A bridge operates in Layer-2 of the OSI model.

In the time of 10Mbps Ethernet, a bridge could be made by running a special software in a PC with two (or more) NICs, but dedicated hardware bridges also existed, probably because they offered better performance in terms of latency and/or throughput.

In these days, a multi-port bridge is more commonly known as a network switch. Since switches are cheap and common, there is no reason to use general-purpose computers for this role any more, unless some special processing is required.

An enterprise-level Ethernet bridge would be expected to have the option to use some version of Spanning Tree Protocol to detect and eliminate network loops while maintaining the possibility to have redundant connections.

As far as I know, HP-UX does not include L2 bridge functionality of any kind. Perhaps some third-party software could offer such functionality (as a side effect of a load balancing solution, perhaps?), but off the top of my head, I can't think of one.

3.) If a network device can be used to pass traffic between different IP subnets, it operates on Layer-3 of the OSI model and is known as a gateway or a router; the terminology is somewhat ambiguous here.

(As I understand, a gateway implies only an access point from one IP subnet to another, while a router may imply multiple ports and more complicated routing tables. It might run a routing protocol to construct a routing table dynamically, although this is not an absolute requirement.)

This is what HP-UX can do easily, if configured with multiple NICs.

Use this command to see if a HP-UX (11.00 or newer) system is configured to route packets between its NICs:
ndd -get /dev/ip ip_forwarding

If the response is "0", the gateway/router functionality is disabled. In 11.23 and newer, I think this is disabled by default.

MK