router and broadcast transparently bridging

[Zillah]

While I was reading through some documentation I found this :

" When a router is routing IP, an IP broadcast will never
be forwarded from one interface to another by default.

An IP broadcast can be forwarded between interfaces if the router is transparently bridging, but will not be forwarded out the same interface it was received on.

Therefore, assuming that the hub of the network is routing IP, spoke devices on the NBMA network can neither send nor receive broadcast or multicast packets between each other ".

I have got couple questions:

1- How can a router be transparently bridging ?
2- Is the concept here similar to the concept in the link below:

[URL unfurl="true"]http://www.freesoft.org/CIE/Topics/30.htm[/URL]

3- Bold word "Therefore" means; this is conclision which depends on what has been mentioned before therfore,,,Am I right ?,,,,but I can not see what has been mentioned before, related to the paragraph after word "Therefore",,,Any comment ?

 

[jneiberger]

If a router is routing IP, broadcasts are not forwarded from one interface to another by default. If a router is bridging IP then broadcasts are forwarded.

The "therefore" is referring back to "When a router is routing IP, an IP broadcast will never be forwarded from one interface to another by default."

Are you familiar with Ethernet switches? They are transparent bridges. They will forward broadcasts, correct? In the same way, a router that is configured for transparent bridging will forward broadcasts.

Broadcasts are constrained by what is called a broadcast domain. Routers are usually at the edge of broadcast domains, meaning that they do not normally allow broadcasts to propagate. However, if you configure a router for bridging instead of routing then it is no longer performing a routing function and is now acting as a layer two device.

Does that make sense?

 

[Zillah]

Thanks jneiberger for this effort, actually I am aware of what you have mentioned (but may be my question was not so clear)

I was looking for this:
Configuring Bridging

[URL unfurl="true"]http://www.cisco.com/univercd/cc/td/doc/product/l3sw/4908g_l3/ios_12/7w515d/config/bridging.htm#wp1019871[/URL]

The link shows me how can a router be transparently bridging (i.e. means configuring a router)

if you configure a router for bridging instead of routing then it is no longer performing a routing function and is now acting as a layer two device

This is part of what I am looking for,,,thanks again

 

[jneiberger]

Oh, I see. You were asking how to configure a router for transparent bridging. My apologies. I did misunderstand your question.

 

[Zillah]

Thanks jneiberger, I really appreciate your response.