Cisco ASR 920 series, Руководство по настройке Ethernet-маршрутизатора

Страница: 423 / 438

The VLAN in the S-tag is used for forwarding the traffic in the service provider network. Different customers
use different S-VLANs, which results in isolation of traffic of each customer. In the S-tag, provider bridges
do not understand the standard Ethertype. Hence, they use an Ethertype value that is different from the standard
802.1Q Ethertype value. This difference makes customer traffic that is tagged with the standard Ethertype
appear as untagged in the provider network. The customer traffic is tunneled in the port VLAN of the provider
port. 802.1ad service provider user network interfaces (S-UNIs) and network-network interfaces (NNIs)
implement the S-bridge component.
For example, a VLAN tag has a VLAN ID of 1, the C-tag Ethertype has a value of 8100 0001, the S-tag
Ethertype has a value of 88A8 0001, and the class of service (CoS) has a value of zero.
C-tag S-tag
------------------------------------------------------- --------------------------------------------------
0x8100 | Priority bits | CFI | C-VLAN-ID 0x88A8 | Priority bits | 0 | S-VLAN-ID
------------------------------------------------------- --------------------------------------------------
C-Bridge Component
All customer VLANs (C-VLANs) that enter a user network interface (UNI) port in an S-bridge component
receive the same service (marked with the same S-VLAN). C-VLAN components are not supported, but a
customer may want to tag a particular C-VLAN packet separately to differentiate between services. Provider
bridges allow C-VLAN packet tagging with a provider edge bridge, called the C-bridge component of the
provider bridge. C-bridge components are C-VLAN aware and can insert or remove a C-VLAN 802.1Q tag.
The C-bridge UNI port is capable of identifying the customer 802.1Q tag and inserting or removing an S-tag
on the packet on a per-service instance or C-VLAN basis. A C-VLAN tagged service instance allows service
instance selection and identification by C-VLAN. The 801.1ad customer user network interfaces (C-UNIs)
implement the C-component.
NNI Port
Dot1ad NNI port are core facing ports. On this port dot1ad (0x88A8) ethertype is used. The customer facing
S-bridge port is identified by using the ethernet dot1ad nni command. The frames forwarded on this port are
double tagged with the S-Tag ethertype set at. 0x88a8.
MAC Addresses for Layer 2 Protocols
Layer 2 protocol data units (PDUs) of customers that are received by a provider bridge are not forwarded.
Hence, Layer 2 protocols running at customer sites do not know the complete network topology. By using
different set of addresses for the Layer 2 protocols running on provider bridges, IEEE 802.1ad causes Layer
2 PDUs of the customers device that enter the provider bridge to appear as unknown multicast traffic and
forwards it on customer ports (on the same service provider VLAN (S-VLAN)). Layer 2 protocols of customer
device can then run transparently.
The table below shows Layer 2 MAC addresses that are reserved for the C-VLAN component.
Table 27: Reserved Layer 2 MAC Addresses for the C-VLAN Component
Value Assignment
01-80-C2-00-00-00 Bridge Group Address
Carrier Ethernet Configuration Guide (Cisco ASR 920 Series)
405
Configuring IEEE 802.1ad
Service Provider Bridges