The 24-bit I-SID field in the I-tag carries the EVC identifier information that was
in the 12-bit VLAN ID field in the S-tag. Thus, the number of customers has expanded
from 4094 to 16,777,216, which should be adequate for most providers. This solves the
problem of the number of available EVCs. The priority and DEI bits are the same as in
the S-tag, and there are 4 bits for future expansion.
The B-tag is simply an S-tag. Its name has changed, but its function relative to the
MAC addresses is the same in a backbone bridge as in a provider bridge.
396 Chapter 13
The I-Component The MAC addresses in the MAC-in-MAC format identify I-components.
An I-component is equivalent to a VLAN bridge with certain additional functionality.
It may be implemented as a separate system or, more likely, as an additional chip
(or chops) on a line card in a larger bridge. Figure 13.10 shows a simple backbone network
serving four 802.1ad provider bridge networks (S-clouds). In Figure 13.10, you
can see
?– Four S-clouds, 1A, 1B, 2, and 3. Each S-cloud is attached to up to 4094 EVCs.
?– Three provider edge bridges (PEBs), P, Q, and R. Each PEB has two I-components.
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