Working of router in Bridge mode

Hai

Can You explain the working of router in "bridge mode". ( suppose my router have three ethernet interface ) . Which circumstance we use router in bridge mode .

Regards
Aneesh

A linksys wrt54g is "default" a router with 3 interfaces (vlans).
The WiFi vlan is bridged to the inside vlan , to make the wireless clients appear as if they were connected with a wire to the inside (4.port switch).

A bridge is connecting interfaces on layer 2 , a router is connectng them on layer 3.

/Bingo

Aneesh a router bridging 3 ethernet interfaces is layer 2 switching and in this configuration is not routing at all; IP is routed via gateway device (router) to another network (different sub-net); layer 3 routing.

Btw you owe me 100€ for helping you with your last homework assignment; lol

tia


Bingo600 typically after the radio de-modulates the radio packet it then strips off the 802.11 header and sends it out on the erthernet interface, no vlan's or sub-netting required as all components in this scenario share the same sub-net. Only if you have multiple WLAN’s mapped to various vlan's (ethernet sub-interfaces) will the AP even add any .1q data to the 802.3 frame. That’s why typically in say a Cisco environment you config a BVI interface and not the radio or Ethernet port for layer 3 so the AP will do nothing but strip off the 802.11 header and sent it on the wire.

btw while I know what a wrt45g is I can’t speak about it technically as the only WLAN gear I see is real IT gear, Moto Cisco Aruba whatever?

ks1009 said:

Bingo600 typically after the radio de-modulates the radio packet it then strips off the 802.11 header and sends it out on the erthernet interface, no vlan's or sub-netting required as all components in this scenario share the same sub-net. Only if you have multiple WLAN’s mapped to various vlan's (ethernet sub-interfaces) will the AP even add any .1q data to the 802.3 frame.

Click to expand...

I agree.
But how many people , not knowing the difference between layer2/layer3 , has ever heard of (ethernet sub-interfaces)

Thats why i chose a "Box" the OP might have seen.

ks1009 said:

That’s why typically in say a Cisco environment you config a BVI interface and not the radio or Ethernet port for layer 3 so the AP will do nothing but strip off the 802.11 header and sent it on the wire.

Click to expand...

Welll .....
In a simple Cisco config (aka. single SSID) you are prob correct , all you need to configre the BVI , and map it to/on the FE.
This also applies to most multi-"home" SSID installations i have met , where the users have a "private ssid" , and a "guest ssid". The meaning of that setup was to to be able to set/change the guest WPA static key (for guests), without having to change the key on all their own pc's. Not to separate the traffic.


In corp. i have normally always seen a 1:1 mapping between SSID & a different vlan on the "wire".

I know my security depp would get a fit if , i FSCK'ed up the ssid->vlan mapping , or even worse forgot it.
And the 802.11 packets ended up in "Native vlan"

And in our setup Cisco-lwapp's with 5500 controllers , the wireless data tunneled home via capwap.
And all your WiFi-Clients appear as if they were connected directly to/on the 5500 Controller.

As yoou prob. know , that makes seamless roaming between ap's easy. As the controller automatically (gre)tunnels the enduser's WiFi connection to(via) the ap he's connected to.

ks1009 said:

btw while I know what a wrt45g is I can’t speak about it technically as the only WLAN gear I see is real IT gear, Moto Cisco Aruba whatever?

Click to expand...

I have been writing/modifying DD-WRT to suit my own needs , and thats why i suggested the wrt54g example.
As the wrt54g behaves excactly as i describe above.

You could even make a 5-port/IF firewall from it , as each of the 4 "inside switchports" are vlan mappable , and therefore can be. a new/different layer3 IF.

Edit:
Would you care to explain why a wrt54g isn't real IT Gear ?
The wrt54G is an excellent box (allthough it maxes out when routing/forwarding approx 26Mb)
I can't seem to understand why a user should use a $1000 Cisco-1142 , if a $60 Linksys fits the bill.

/Bingo

One more question. When router working in Bridge mode, is it broadcast arp request to all of its interfaces like a Layer 2 switch ?

A bridge is a layer2 connection , and layer2 connections are always member of the same broadcastdomain.
There's a few exceptions , but they are uncommon (an in non-standard)
You can actually have some firewalls working in layer2 mode (transparent firewalls) , and they would be able to block. Linux (wrt54g) have ebtables (look it up)

But a normal bridge is a layer2 connection , and must therefore forward all mac-layer packets.

Edit:
The router won't forward broadcasts to all it's interfaces.
Only the ones in the same layer2 broadcastdomain (IF's bridged together)

So the linksys-wan IF (layer 3) won't see the arp request.
This makes sense , as it won't be able to resolve the arp request it's member of another subnet.

/Bingo

You mean when router in bridge mode ,it won't forward broadcasts to all it's interfaces.I have one more doubt , Does router reformat the incoming packet when it is in bridge mode ( I mean whether source mac and ip will change ) ?


regards
Aneesh

Aneesh stop saying router in bridge mode its making me crazy, just think of it as a layer 2 switch.
A layer 2 switch will forward a broadcast packet out of all its interfaces without any change in the IP header.


Edit:
Would you care to explain why a wrt54g isn't real IT Gear ?
The wrt54G is an excellent box (allthough it maxes out when routing/forwarding approx 26Mb)
I can't seem to understand why a user should use a $1000 Cisco-1142 , if a $60 Linksys fits the bill.

No I’m not saying that a home user should spend $1000 on a Moto 7131 vs $50 for some consumer grade AP; just that professionally I never see anything but Cisco ect; even at home I have several wifi options from a Moto 6532 to a Cisco 1162. Oh I almost forgot I’ve got controllers too, WS5100’s, and both 2100 and 4400 Cisco controllers.
By asking me that question are you saying that wrt45g is a enterprise class piece of gear?

ks1009 said:

By asking me that question are you saying that wrt45g is a enterprise class piece of gear?

Click to expand...

Certainly not enterprise , but i do consider it real IT-Gear.
Just destined for another user segment.

I have a Linksys@home that has been running for 6+ years , without any hw-errors.

I have a lot of Cisco's with PoE Injectors , where the PSU lasted 3 years.
The AP is stable (super stable) , but their 48v PSU's die.
So lately we have begun to install PoE switches , even if there's only 2 AP's on the site.

/Bingo