H3C Technologies H3C S12500 Series Switches - Manual (Page 12)

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SPBM data packet
SPBM data packets use the IEEE 802.1ah MAC-in-MAC frame format, as shown in Figure 2 .
Figure 2 IEEE 802.1ah-compliant MAC-in-MAC frame format
Table 1 IEEE 802.1ah encapsulated frame header fields
Field Description
B-DA Destination B-MAC that identifies the destination BEB.
B-SA
Source B-MAC. It is a known MAC address of the BEB that encapsulates the MAC-in-MAC
frame.
B-Tag
B-VLAN tag identifies the VLAN ID and priority of the frame in the SPBM network. The TPID
in the tag is fixed at 0x8100.
I-Tag
Backbone service instance tag contains the following subfields:
• TPID—A value fixed at 0x88E7 to identify the frame as an 802.1ah encapsulated frame.
• I-PCP—Transmission priority of the frame on the BEB.
• I-DEI—Drop priority of the frame on the BEB.
• I-SID—Backbone service instance identifier.
• C-DA—Customer destination MAC address.
• C-SA—Customer source MAC address.
S-Tag This field contains the outer customer VLAN ID and priority.
C-Tag This field contains the inner customer VLAN ID and priority.
Working mechanisms
To forward Layer 2 traffic between customer sites across an SPBM network, you must configure Ethernet
service instances on BEBs' customer network ports, and map Ethernet service instances to SPB VSIs.
SPBM uses ISIS-SPB in the control plane to calculate shortest path trees. It uses IEEE 802.1ah PBB
encapsulation in the data plane to encapsulate and forward traffic.
SPBM uses the following generic process to calculate SPTs and forward traffic:
1. BEBs and BCBs send ISIS-SPB P2P hellos to establish and maintain adjacencies.
2. Adjacent nodes send LSPs to advertise their respective topology data. Eventually, the LSDBs of all
nodes are synchronized.
3. Each node selects the forwarding path: