Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 308)

278 Multi-Service IronWare Switching Configuration Guide
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Overview
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By adding the PBB header, PBB isolates the Service Provider and customer address spaces. This
means that Ethernet switches in the core of the Service Provider network will no longer learn
customer MAC addresses or use customer MAC addresses to forward customer frames to their
destinations. This improves the scaling of the Service Provider network in terms of the number of
supported customers, since the number of supported customers is no longer directly tied to the
size of the MAC address tables of the core Ethernet switches. In addition, the Service Provider
network is now protected from customer network failures, since frame forwarding is now based on
its own PBB header. Moreover, customers benefit from added security, since the customer's MAC
addresses are no longer learned or used for frame forwarding decisions in the core of the Service
Provider network.
As additional benefits to the Service Provider, PBB has the potential to simplify operations, e.g., by
separating the customer and Service Provider addressing spaces, and to lower capital
expenditures by reducing the cost of Ethernet switches used in the core of the network, since
memory and processing power requirements are reduced by limiting MAC address learning to
backbone MAC addresses.
FIGURE 43 Customer, PB, and PBB frame formats
The Backbone Service Instance Tag (I-TAG) contains a Backbone Service Instance Identifier (I-SID),
which is 24 bits long. The I-SID field allows a Service Provider to identify up to 2 to the power of 24,
that is, over 16 million, service instances. In other words, over 16 million services or customers can
be uniquely identified using the I-SID field. Therefore, PBB's I-TAG allows for highly scalable services
by eliminating the 4090 service instances limitation of PB.
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.