RE: IRB bridge ethernet interfaces in different VLANS

Maarten,

When running IRB the whole purpose is to span the two VLAN's together, to
make them into one VLAN. But are you saying that you are using two routers
to SPAN the same subnets together? If that is the case then one of them is
going to be blocking on one side which will cause the BVI of one of the
routers to not be reachable from one side. That is the nature of
spanning-tree. If you want to make sure you can reach the BVI of one of
them from one side or the other then control where your root bridge is.

HTH

Regards,
 
Tyson Scott - CCIE #13513 R&S, Security, and SP
Technical Instructor - IPexpert, Inc.
Mailto: tscott_at_ipexpert.com
Telephone: +1.810.326.1444, ext. 208
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-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
Maarten Vervoorn
Sent: Tuesday, January 26, 2010 8:59 AM
To: ccielab_at_groupstudy.com
Subject: IRB bridge ethernet interfaces in different VLANS

Hey,

I'm currently working on the CCIE 360 program from cisco. I have some
questions about the IRB techologies used in the workbook labs.Some tasks
want to connect 2 different VLANs into one IP subnet.
So for example 2 routers have configured a interfaces with 2 sub interfaces
with dot1q tagging for 2 different vlans. Those interfaces belong to a
bridge group with a BVI interface with one IP address and 2 routers 1 in the
first vlan and one in the second but all in the same ip subnet.
It seems that the router is running one spanning-tree instance for its
brigde group so it creates a loop between two VLANs one port is going to be
blocked even if logic tells there is no loop in your network. This is
because the router forwards the BPDU form one VLAN into another.
Is there a way to prevent this behaviour or to configure the router so it is
running a STP instance for each VLAN? Because one ip adres is always
unreachable due to a spanning-tree block

Kind regards,

Maarten

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Received on Tue Jan 26 2010 - 09:17:12 ART