Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 277)

Multi-Service IronWare Switching Configuration Guide 247
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Chapter
10
IEEE 802.1ah Provider Backbone Bridging (PBB) Networks
for the Brocade NetIron CES and the Brocade NetIron CER
Overview
The IEEE 802.1ah Provider Backbone Bridges (PBB) standard was developed to address the
limitations of Provider Bridges (PB) and to add additional capabilities sought by Service Providers.
When compared to a PB network, a PBB network deployment offers simplified operations, lower
capital expenditures, and overall better scalability in terms of the number of supported customers.
PBB also provides advantages when used in conjunction with VPLS, since PBB reduces the overall
MAC address learning requirements. This section provides an overview of PBB, describes its
advantages over PB, examines common PBB deployment scenarios, and examines its many
benefits when deployed in combination with a core MPLS network supporting VPLS.
Provider Backbone Bridges
The Provider Backbone Bridges (PBB) standard, (IEEE 802.1ah), was developed to address the
limitations of the Provider Bridges (PB) standard, (IEEE 802.1ad), and to add additional capabilities
sought by Service Providers.
PB allows Service Providers to use a V-LAN identifier (VID) space separate from the customer VID
(C-VID) space. PB adds a Service Provider VLAN Tag (S-TAG) containing a Service Provider VID
(S-VID) to Ethernet frames (
Figure 29 ). Because PB stacks a second VLAN tag to Ethernet frames, it
is also known as “Q-in-Q,” as a reference to the standard that originally defined VLAN tags, in other
words, IEEE 802.1Q, which is known as defining “Q” frames.
The S-VID field of the S-TAG is 12 bits long, which is the same length of a C-VID field of a customer
VLAN Tag (C-TAG). Even though 12 bits can address up to 4096 distinct values, two values have
special meaning and are reserved. Therefore, the Service Provider is limited to at most 4090
distinct S-VID values to identify service instances, that is, services or customers in a PB network.
Another drawback is that PB frames are addressed by customer Media Access Control (MAC)
addresses. This means that core Ethernet switches in a PB network have to learn all the source
MAC addresses of all the customer frames traversing the core of the PB network. Thus, the size of
the MAC address tables of core PB switches ultimately limits the number of customers that can be
supported by a PB network.
To address the above described PB shortcomings, PBB adds a hierarchy view to Ethernet by
encapsulating PB frames with a PBB header (which becomes the equivalent of a “Service Provider
MAC header”) containing a Backbone Destination MAC Address (B-DA), Backbone Source MAC
Address (B-SA), and two new tags (
Figure 29 ), which are described later in this document. What
makes the B-DA and B-SA “backbone” addresses is the fact that these are MAC addresses of
Service Provider's PBB edge switches. An edge PBB switch encapsulates an ingress PB frame with
a PBB header containing the destination MAC address of an appropriate egress edge PBB switch.
The egress edge PBB switch removes the PBB header and forwards the frame to an attached PB
network. Because PBB adds a PBB header containing new destination and source MAC addresses,
it is also known as “MAC-in-MAC.”