Cisco ASR 920 series, Руководство по настройке Ethernet-маршрутизатора

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the packets egress the service provider network onto the customer network, the S-VLAN tag is decapsulated
and the original customer packet is restored.
Figure below shows the tag structures of the double-tagged packets.
In figure below, Customer A was assigned VLAN 30, and Customer B was assigned VLAN 40. Packets
entering the edge switches with 802.1Q tags are double-tagged when they enter the service-provider network,
with the outer tag containing VLAN ID 30 or 40, appropriately, and the inner tag containing the original
VLAN number, for example, VLAN 100. Even if both Customers A and B have VLAN 100 in their networks,
the traffic remains segregated within the service-provider network because the outer tag is different. Each
customer controls its own VLAN numbering space, which is independent of the VLAN numbering space used
by other customers and the VLAN numbering space used by the service-provider network. At the outbound
port, the original VLAN numbers on the customer's network are recovered.
Method 1
In this example, for Customer A, interface Gigabit Ethernet 0/1 is the customer-facing port, and Gigabit
Ethernet 0/2 is a trunk port facing the service provider network. For Customer B, Gigabit Ethernet 0/3 is the
customer-facing port, and Gigabit Ethernet 0/4 is the trunk port facing the service provider network.
Customer A
Router (config)# interface gigabitethernet0/1
Router (config-if)# service instance 1 Ethernet
Router (config-if-srv)# encapsulation dot1q 1-100
Router (config-if-srv)# bridge-domain 4000
Router (config)# interface gigabitethernet0/2
Router (config-if)# service instance 2 Ethernet
Router (config-if-srv)# encapsulation dot1q 30
Router (config-if-srv)# rewrite ingress tag pop 1 symmetric
Router (config-if-srv)# bridge-domain 4000
For Customer A, service instance 1 on Gigabit Ethernet 0/1 is configured with the VLAN encapsulations
used by the customer: C-VLANs 1
– 100. These are forwarded on bridge-domain 4000. The service provider
facing port is configured with a service instance on the same bridge-domain and with an
encapsulation dot1q
command matching the S-VLAN. The
rewrite ingress pop 1 symmetric
command also implies a push of
the configured encapsulation on egress packets. Therefore, the original packets with VLAN tags between 1
and 100 are encapsulated with another S-VLAN (VLAN 30) tag when exiting Gigabit Ethernet port 0/0/2.
Similarly, for double- tagged (S-VLAN = 30, C-VLAN = 1
–
100) packets coming from the provider network,
the rewrite ingress pop 1 symmetric command causes the outer S-VLAN tag to be popped and the original
C-VLAN tagged frame to be forwarded over bridge-domain 4000 out to Gigabit Ethernet 0/1.
The same scenario applies to Customer B.
Customer B
Router (config-if)# service instance 1 Ethernet
Router (config-if-srv)# encapsulation dot1q 1-200
Router (config-if-srv)# bridge-domain 4001
Router (config)# interface gigabitethernet0/4
Router (config-if)# service instance 2 Ethernet
Router (config-if-srv)# encapsulation dot1q 40
Router (config-if-srv)# rewrite ingress tag pop 1 symmetric
Router (config-if-srv)# bridge-domain 4001
Carrier Ethernet Configuration Guide (Cisco ASR 920 Series)
31
Ethernet Virtual Connections Configuration
Configuring IEEE 802.1Q Tunneling and Layer 2 Protocol Tunneling Using EFPs