How Trill Works; Trill Forwarding Mechanisms - HP 5920 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

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
a. When a unicast frame enters the TRILL network, the ingress RB encapsulates the original
Ethernet frame with the following headers:
−
−
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
S1
Inner D-MAC = S2
Inner S-MAC = S1
Inner VLAN = 10
Ethernet frame
Figure 2, a unicast frame is forwarded as follows:
A TRILL header (similar to an IP header).
An outer Ethernet header (similar to the Ethernet header of a regular Ethernet frame).
Ingress RB
RB 1
VLAN 10
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
Unicast frame
Transit RB
RB 2
VLAN 200
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
Payload
TRILL frame
3
Egress RB
RB 3
VLAN 300
Inner VLAN = 10
Payload
TRILL frame
S2
VLAN 10
Inner D-MAC = S2
Inner S-MAC = S1
Inner VLAN = 10
Payload
Ethernet frame