How Trill Works; Trill Forwarding Mechanisms - H3C S12500 Series Configuration Manual

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
When a unicast frame enters the TRILL network, the ingress RB encapsulates the original Ethernet
1.
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
S1
VLAN 10
Inner D-MAC = S2
Inner S-MAC = S1
Inner VLAN = 10
Payload
Ethernet frame
The outer Ethernet header enables traditional Ethernet switches to forward TRILL frames. RBs can be
connected through traditional Ethernet switches.
Figure 2, a unicast frame is forwarded as follows:
Ingress RB
RB 1
VLAN 200
Outer D-MAC = RB 2
Outer S-MAC = RB 1
Outer VLAN = 200
Egress RB = RB 3
Ingress RB = RB 1
Inner D-MAC = S2
Inner S-MAC = S1
Inner VLAN = 10
Payload
TRILL frame
Unicast frame
Transit RB
RB 2
VLAN 300
Outer D-MAC = RB 3
Outer S-MAC = RB 2
Outer VLAN = 300
Egress RB = RB 3
Ingress RB = RB 1
Inner D-MAC = S2
Inner S-MAC = S1
Inner VLAN = 10
Payload
TRILL frame
3
Egress RB
RB 3
VLAN 10
Inner D-MAC = S2
Inner S-MAC = S1
Inner VLAN = 10
Payload
Ethernet frame
S2