IP accounting with IRB (bridging)

Unanswered Question
Oct 13th, 2007


I have configured a dual ethernet Cisco router to go inline between a WAN CPE and a LAN so that I can capture some WAN usage statistics. I have no access to the CPE and need the device to be transparent and just "drop in" to the existing infrastructure. This is working well with the following configuration;


bridge irb

interface Ethernet0

no ip address

bridge-group 1


interface Ethernet1

no ip address

bridge-group 1


interface BVI1

ip address 10.x.x.x


bridge 1 protocol ieee

bridge 1 route ip


Now I want to enable IP accounting so I add the following line;


ip accounting output-packets


to either of the physical interfaces no statistics are generated. If I apply it to the BVI interface I only capture broadcast traffic.

I have done some investigation but cannot find any in-depth information to determine if what I want to do is possible and if so what I am missing. Any help would be greatly appreciated.



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Edison Ortiz Sun, 10/14/2007 - 07:16

IP Accounting will only work on interfaces where you have an IP address. Your physical interfaces are missing that information.

As your BVI interface, the only way to see accounting is if your clients are using that interface as a gateway to a next hop resource. Since your router is bridging, I don't think that's the case.

stephenamstutz Mon, 10/15/2007 - 04:38

Hmm - that makes sense I guess - thanks for the insight.

I tried with NetFlow instead, but seem to be having the same issue. I thought as NetFlow works on switches it would not be subject to the same restrictions, however I can only gather statistics when adding "ip route-cache flow" to the BVI interface, leaving me in the same position.

Is this not working for the same reason?

Edison Ortiz Mon, 10/15/2007 - 15:50

You are right, NetFlow works in switches. However, it can only capture data as it enters and exits the SVI. Your BVI is only used for management access and the clients aren't using it as gateway to a next-hop destination.


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