Specific differences between AP and bridge modes?

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Jan 21st, 2009
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I've RTFM'd everywhere but cannot find anything definitive regarding the differences between AP+client versus bridge+bridge modes.


Presume you have a pair of AP1200's. First test case: One is in AP mode and the other is in WGB mode. Second test case: One is in root-bridge mode and the other is in non-root-bridge mode.


In both cases traffic will pass bidirectionally from one Ethernet port to the other. What, then, are the technical differences between these two situations? Do the "bridges" place less load on the AP1200's CPU? Does AP-based operation yield more diagnostic data? What are the advantages and disadvantages of each mode?


I can't find anything that digs into the details. Any help appreciated. Thanks!

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dsweeny Tue, 01/27/2009 - 10:07
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# Advantages:


* Looks to the AP just like a standard client, so it interoperates with all APs

* No configuration required


# Disadvantages:


* Supports only a single wired client

* WGB itself is not manageable

richardhartman Tue, 01/27/2009 - 10:18
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Thanks for your response. A few comments, and then a question:


* Looks to the AP just like a standard client, so it interoperates with all APs


What does? A 1200 in WGB mode? If so, then yes it looks like a standard client to the AP but Cisco explicitly states it will only associate with other Cisco AP's. It is not compatible with "all APs".


If you mean a non-root-bridge looks like a standard client to an AP, that's true but the AP must be in "AP with wireless clients" mode which is not one of the test cases I was asking about.


* No configuration required


Of the WGB? Mine need configuration: SSID, encryption keys, etc. It's just like setting it up as an AP except for the mode you select.


Of a (non-)root-bridge? Mine have needed configuration in that mode, too. Same as above.


Not sure what you mean by "no configuration required".


# Disadvantages:


* Supports only a single wired client


Nope, in WGB mode pre-1200 units supported up to eight wired devices. 1200's and later support up to 250 clients. Cisco docs are clear on this.


In bridge mode I'm not aware of any wired client limits. The device is simply passing traffic between the two interfaces.


* WGB itself is not manageable


Not sure what you mean by "not managable".


Back to my question: What is the difference between AP+client mode vs. root-bridge+non-root-bridge mode? Does the AP+client mode make more decisions about traffic routing or prioritization, while bridge-based operation is less "intelligent"? Something else?


My tests show no discernable difference in throughput, even with multiple clients/nonroot bridges. When multiple users consume bandwidth simultaneously, it is shared among them just as you'd expect.


So... what ARE the differences, as far as the decisions IOS is making, between the different modes?


Thanks!

scottmac Tue, 01/27/2009 - 11:42
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IMO, the primary difference is that "bridge mode" is not standardized, and is implemented differently per vendor, and not likely to be interoperable.


What makes "bridge mode" different is that some of the timing parameters need to be extended beyond what is permissible in an AP mode (the distances are longer, the transmission latency is longer, the SNR is (should be) radically different than an AP/Client in teh same normal-sized space.


Some wireless bridges also use proprietary signaling, or proprietary encapsulations (to reduce overhead), proprietary error correction, eliminate some signaling (since it's talking to a known peer 100% of the time it doesn't have to do a lot of the administrative signaling) ... stuff like that.


APs are using a standard (802.11) ... bridges might be based on 802.11 signaling, but it's a mutant, mangled version.


Good Luck

Scott


richardhartman Tue, 01/27/2009 - 12:50
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OK, that's good info.


Any idea of the differences in the routing/bridging portion of IOS? For example, when in bridge mode does the 1200 series act like a true (transparent) bridge between the two media, or is there still some semi-intelligent routing decisionmaking happening - and if so, what?


Is bridging any less consumptive of CPU resources? Any more efficient? That sort of thing.

scottmac Tue, 01/27/2009 - 17:41
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There is no routing functionality at all. If you had multiple VLANs/SSIDs, you'd need an external L3 device to do any inter-VLAN traffic movement (the Fast Ethernet interface does support 802.1q encaps).


I never measured it, but I am inclined to believe that a simple p2p bridge would be somewhat less CPU / resource intensive because the device does not have to poll or track multiple clients ... it only needs to encaps and forward the traffic.


FWIW


richardhartman Tue, 01/27/2009 - 20:53
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I don't mean "routing" in the L3 sense, but the versions of IOS that run on these boxes do support the "bridge irb" command. That's Integrated Routing and Bridging. There's plenty of IOS commands they *don't* support, so the fact that IRB is there means something is going on....

mirek.tichy_2 Wed, 01/28/2009 - 05:54
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Hi,

1)

WGB can connect several (not only one) Ethernet clients to the central AP. I am just not sure about the limit I think it is up to 8 Ethernet clients. As WiFi client the WGB can connect only to one SSID and one VLAN. WGB has no STP functionality.

2)

Wireless bridge can carry multiple VLAN and acts as a standard bridge with Spanning Tree functionality.

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