Subject: [rbridge] IVLANs vs OVLANs

From: sgai at nuovasystems.com (Silvano Gai)
Date: Tue, 14 Nov 2006 17:10:48 -0800

During the last WG meeting I had an action item to send an email on the
VLAN issue. Here it is.

In a previous email I introduced the concept of IVLAN and OVLAN.

IVLAN refers to the VLAN present on the untagged frames, which must be
propagated by ISIS, VLAN pruning must be performed and so on. All the
IVLANs share the same core instance of ISIS.

OVLAN refers to the fact that the traffic between two RBridges can be
encapsulated into a VLAN.

If we look to the format of a TRILL encapsulated frame the position of
these two fields is as follow:

+--------------------------------+
|               DA               |
+----------------+---------------+
|       DA       |     SA        |
+----------------+---------------+
|               SA               |
+--------------------------------+
|    ET = 1Q     |   OVLAN ...   |
+--------------------------------+
|    ET = TRILL  | RES  |  TTL   |
+--------------------------------+
| Egress Add.    | Ingress Add.  |
+--------------------------------+
|               DA               |
+----------------+---------------+
|       DA       |     SA        |
+----------------+---------------+
|               SA               |
+--------------------------------+
|    ET = 1Q     |   IVLAN ...   |
+--------------------------------+
|             Payload            |

Now let's consider the following topology:

               +-----+
               | RB1 |
               +-----+
                  |
                  | 1,2
                  |
+-----+   1    +-----+    2     +-----+
| RB2 |--------| SW1 |----------| RB3 |
+-----+        +-----+          +-----+

RB1, RB2, RB3 are Rbridges, SW1 is a classical Ethernet switch.
The link between RB2 and SW1 is in OVLAN 1, the link between SW1 and RB3
is in OVLAN 2 and the links between SW1 and RB1 is a trunk and it
carries both OVLAN 1 and 2.

Now my question is: "do we have one CRED or two?"

a) possible answer: "we have one CRED". RB2 is adjacent to RB1 but not
to RB3. The same applies to RB3 that is adjacent to RB1, but not RB2.
RB1 is adjacent to both RB2 and RB3. There is a single core instance of
ISIS. RB2 may reach RB3 through RB1.

The TRILL equivalent topology is:

+-----+        +-----+          +-----+
| RB2 |--------| RB1 |----------| RB3 |
+-----+        +-----+          +-----+

b) another possible answer: "we have two CREDs". One is composed by RB2
and RB1, the other by RB3 and RB1. RB2 may not reach RB3, since they are
in different CREDs.

The TRILL equivalent topologies are:

+-----+        +-----+
| RB2 |--------| RB1 |
+-----+        +-----+

+-----+          +-----+
| RB1 |----------| RB3 |
+-----+          +-----+

I like a) and I hope we are in agreement that the right answer is a),
but I haven't seen it explained in any of the documents.

-- Silvano