Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 279)

Multi-Service IronWare Switching Configuration Guide 249
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Overview
10
The semantics and the structure of the Backbone VLAN Tag (B-TAG) are identical to that of the PB
S-TAG. The B-TAG was designed this way so that core PBB switches do not need to be aware of PBB.
In fact, standard PB switches can be used in the core of a PBB network. Only the switches at the
edge of the Service Provider PBB network need to be aware PBB.
A PBB network uses two types of bridges ( Figure 30 ): Backbone Edge Bridges (BEB) and Backbone
Core Bridges (BCB). As explained above, the functionality required from a BCB is the same as a
standard IEEE 802.1ad PB bridge. A BEB is used at the boundary of a PBB network to add and
remove the PBB header.
FIGURE 30 Backbone Edge Bridge operation
As defined in IEEE 802.1ah, a BEB has two main components: an I-Component and a
B-Component. From left to right in
Figure 30 , the I-Component maps S-VIDs to I-SIDs and adds a
PBB header without a B-TAG, while the B-Component maps I-SIDs to B-VIDs and adds a B-TAG.
These actions are reverted in the opposite direction.
As shown in Figure 30 , a BEB containing an I-Component and a B-Component is called an IB-BEB.
The B-Component of an IB-BEB forwards frames towards the core of a PBB network based on
backbone MAC addresses (that is, it learns backbone MAC addresses), while the I-Component
forwards frames towards the PB network based on customer MAC addresses (that is, it learns
customer MAC addresses). The terms I-BEB and B-BEB refer to optional BEBs that support a single
component type, that is, either I-Component or B-Component, respectively. I-BEBs and B-BEBs
expose an I-Tagged service interface, which carries frames with a PBB header, but without a B-TAG.
PB
PB PB
PBB
Network
PB
BEB
BEB BEB
BEB
S-Tagged
service
interface
B-Tagged
service
interface
C-TAG
S-TAG
C-SA
C-DA
Ethertype
Ethertype
Ethertype
Payload LLC
MAC
MAC
Relay
Customer
Network
Port
Provider
Instance
Port
MAC
LLC
I-Component S-Tagged
service
interface
MAC
Relay
Provider
Network
Port
Customer
Backbone
Port
MAC MAC
LLC LLC
I-Tagged
service
interface
B-Tagged
service
interface
B-Component
S-TAG
C-TAG
I-TAG
B-TAG
C-SA
C-DA
B-SA
B-DA
Ethertype
Ethertype
Ethertype
Ethertype
Ethertype
Payload
Backbone Edge Bridge (IB-BEB)
(PBB
Header
no B-TAG)