H3C Technologies H3C S12500-X Series Switches - Manual (Page 9)

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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
Different types of frames are forwarded using different forwarding mechanisms. The following sections
describe these mechanisms.
Unicast frame forwarding mechanism
As shown in Figure 2 , a unicast frame is forwarded as follows:
1. When a unicast frame enters the TRILL network, the ingress RB encapsulates the original Ethernet
frame with a TRILL header (like an IP header) and an outer Ethernet header (like the Ethernet header
of a regular Ethernet frame).
2. RBs forward the frame hop by hop according to the egress RB nickname in the TRILL header to the
egress RB 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.
3. 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 Unicast frame forwarding flow
The outer Ethernet header enables traditional Ethernet switches to forward TRILL frames. RBs can be
connected through traditional Ethernet switches.
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