H3C Technologies H3C S6800 Series Switches - Manual (Page 10)

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How TRILL works
TRILL establishes and maintains adjacencies between RBs by periodically advertising Hello frames,
distributes LSPs among RB neighbors, and generates an LSDB for all RBs in the network. Based on the
LSDB, each RB uses the SPF algorithm to calculate forwarding entries destined to other RBs.
TRILL forwarding mechanisms
TRILL uses different mechanisms to forward Layer 2 and Layer 3 traffic. The following sections describe
these mechanisms.
Layer 2 forwarding mechanisms
TRILL forwards the traffic within a VLAN by using the following mechanisms:
• Unicast frame forwarding mechanism.
As shown in
Figure 2 , a unicast frame is forwarded as follows:
a. When a unicast frame enters the TRILL network, the ingress RB encapsulates the original
Ethernet frame with the following headers:
− A TRILL header (similar to an IP header).
− An outer Ethernet header (similar to the Ethernet header of a regular Ethernet frame).
b. RBs forward the frame hop by hop according to the egress RB nickname in the TRILL header in
the same way routers forward IP packets. Each hop replaces the outer Ethernet header with an
appropriate outer Ethernet header, and decrements the hop count in the TRILL header.
c. Upon receiving the TRILL frame, the egress RB de-encapsulates it to obtain the original Ethernet
frame, and sends the frame to the target device.
Figure 2 Layer 2 unicast frame forwarding flow
VLAN 10 VLAN 200 VLAN 300 VLAN 10
Ingress RB
RB 1
S1 S2
Ethernet frame TRILL frame TRILL frame Ethernet frame
Egress RB
RB 3
Transit RB
RB 2
Ingress RB = RB 1
Outer VLAN = 200
Egress RB = RB 3
Outer S-MAC = RB 1
Outer D-MAC = RB 2
Payload
Inner VLAN = 10
Inner S-MAC = S1
Inner D-MAC = S2
Ingress RB = RB 1
Outer VLAN = 300
Egress RB = RB 3
Outer S-MAC = RB 2
Outer D-MAC = RB 3
Payload
Inner VLAN = 10
Inner S-MAC = S1
Inner D-MAC = S2
Payload
Inner VLAN = 10
Inner S-MAC = S1
Inner D-MAC = S2
Payload
Inner VLAN = 10
Inner S-MAC = S1
Inner D-MAC = S2
Unicast frame