New-generation Protocols 95
which the machines may be a very great distance apart. The objective
is to make this network operate as though all of the points were
geographically close to one another to form a local-area network.
A VLAN may contain several users. Each Ethernet frame is diffused
to all the machines in the VLAN. The tables which determine the
frame switching are fixed and may be viewed as switching tables in
which the addresses of the recipients are references.
When the VLAN has only two points, the VLAN defines a path.
This is the vision which was employed for Carrier-Grade Ethernet.
Paths are formed by determining VLANs. The path is unique and
simple if the VLAN has only two points. VLAN introduces a
multipoint if there are more than two points.
The problem with this solution stems from the limited size of the
VLAN field, which is only defined by 12 bits. This is perfectly
appropriate in the context of a company network with standard
Ethernet switching, but becomes completely insufficient for CarrierGrade Ethernet, which is aimed at operator networks. Therefore, the
size of the VLAN field has had to be increased.
Carrier-Grade Ethernet can be subdivided into various solutions
of extension of the VLAN zone, all of which are illustrated in
Figure 4.10. The most typical solution consists of using the
IEEE 802.1ad standard, which has a variety of names: PB (Provider
Bridge) Ethernet, QiQ (Q in Q) or cascading VLAN. IEEE 802.1ah
is also known as MiM (MAC-in-MAC) or PBB (Provider Backbone
Bridge). The most advanced solution is called PBT (Provider
Backbone Transport), or pseudo wire (PW) over PBT. A PseudoWire
is an MPLS-based tunnel whereby Ethernet frames can be transported
over an IP network with QoS guarantees.
The solutions described in the foregoing sections are illustrated in
Figure 4.10.
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which the machines may be a very great distance apart. The objective
is to make this network operate as though all of the points were
geographically close to one another to form a local-area network.
A VLAN may contain several users. Each Ethernet frame is diffused
to all the machines in the VLAN. The tables which determine the
frame switching are fixed and may be viewed as switching tables in
which the addresses of the recipients are references.
When the VLAN has only two points, the VLAN defines a path.
This is the vision which was employed for Carrier-Grade Ethernet.
Paths are formed by determining VLANs. The path is unique and
simple if the VLAN has only two points. VLAN introduces a
multipoint if there are more than two points.
The problem with this solution stems from the limited size of the
VLAN field, which is only defined by 12 bits. This is perfectly
appropriate in the context of a company network with standard
Ethernet switching, but becomes completely insufficient for CarrierGrade Ethernet, which is aimed at operator networks. Therefore, the
size of the VLAN field has had to be increased.
Carrier-Grade Ethernet can be subdivided into various solutions
of extension of the VLAN zone, all of which are illustrated in
Figure 4.10. The most typical solution consists of using the
IEEE 802.1ad standard, which has a variety of names: PB (Provider
Bridge) Ethernet, QiQ (Q in Q) or cascading VLAN. IEEE 802.1ah
is also known as MiM (MAC-in-MAC) or PBB (Provider Backbone
Bridge). The most advanced solution is called PBT (Provider
Backbone Transport), or pseudo wire (PW) over PBT. A PseudoWire
is an MPLS-based tunnel whereby Ethernet frames can be transported
over an IP network with QoS guarantees.
The solutions described in the foregoing sections are illustrated in
Figure 4.10.
www.it-ebooks.info