bridge domain questions

[sarahr202]

Hi everybody.

At work , I have seen a lot of bridge domains configured on a single switch. My question is what is bridge domain and why we use them. An example with configurations will be great.

Thanks and have a great day.

[Richard Burts · Accepted solution]

I have not used this feature. So I can not speak from experience or provide much in the way of config examples.

I did find a page which discusses configuring bridge-domain on ASR routers as a way to connect layer 2 switching/bridging to a layer 3 service. See this page for details

http://www.cisco.com/en/US/docs/routers/asr1000/configuration/guide/chassis/bdi.html

but this is on a router and your question was about switches. So I looked a bit further.

I did find this page which discusses using bridge-domain when connecting switched networks to ATM PVCs or to Frame Relay DLCIs. Here is the link if you want to look more closely at it.

http://www.cisco.com/en/US/docs/ios/lanswitch/command/reference/lsw_b1.html

Both of those are pretty old technologies and I wonder if that is what is going on in the switches that you have seen. Perhaps if you could provide partial configs from those switches we might determine better what is going on.

HTH

Rick

[sarahr202]

Thanks Rick.

I will post the config tomorrow.

Thanks.

[Marvin Rhoads · Accepted solution]

Rick, your first suggestion was going down the right path.

Bridge domains are used on Metro Ethernet switches that interface with backbone networks running VPLS. A good overview can be found in the VPLS Configuration Guide for the ASR 9k. See Figure 20 in this document:

http://www.cisco.com/en/US/docs/routers/asr9000/software/mpls/configuration/guide/gcasr9kvpls.html

There you will see as good explanation of why and how we use bridge domains. Basically it is mapping local broadcast domains across a WAN. In the case of VPLS, the Metro Ethernet switch at the provider edge has functionality roughly analogous to the PE router in a more commonly seen MPLS network. I most commonly see this used on a Cisco  ME3600 (or equivalent Juniper switch).

[sarahr202]

Thanks Marvin

[sarahr202]

Hi Marvin and Rick

Please consider the following config and questions:

R1#  show platform

Interrupt Throttling:

  Throttle Count   = 00052552   Timer Count      = 00039372

  Netint usec      = 00000800   Netint Mask usec = 00000240

  Active           =        0   Configured       =        1

  Longest IRQ(usec)= 00003999

MSFC CPU IDPROM:

IDPROM image:

  (FRU is 'C7600 MSFC4 Daughterboard')

+++++++++++++++++++++++++++++++++++++++++

R1#show running-config interface gigabitEthernet 9/7

service instance 1251 ethernet

  encapsulation dot1q 1251

  rewrite ingress tacg pop 1 symmetri

  bridge-domain 440

service instance 2001 ethernet

  encapsulation dot1q 2001

  rewrite ingress tag pop 1 symmetric

  bridge-domain 440

+++++++++++++++++++++++++++++++++++++++++

Let say R1 receives a packet with vlan tag 1251 on g9/7. What will happen next? will R1 update its MAc table with source mac?

1)Will there be a mac table for bridge -domain 440?  Do we have one-to one correspondence between the two i.e each  bridge-domain has its own mac table.?

2)Let say R1 receives a packet with vlan tag 1251 on g9/7. What will happen next? will R1 update its MAc table with source mac?

3) Let say R1 receives a frame with vlan tag 1251 with destination mac ff:ff:ff:ff

What will R1 do next?

4)Will R1 forward it to all service instance in bridge-domain 440 except the one R1 receives the broadcast frame? 

( in our case we have two service instances under  bridge -domain 440 i.e service instance 1251 ethernet,service instance  2001 ethernet)

5) Will R1 change the vlan tag 1251 to 2001 when forwarding the broadcast frame out of instance 2001?

Appreciate your help.

Thanks