3
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 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
The outer Ethernet header enables traditional Ethernet switches to forward TRILL frames and
connect RBs.
•
Multidestination frame forwarding mechanism.
In a TRILL network, RBs perform the following operations:
{
Compute a TRILL distribution tree for each VLAN based on the LSDB.
{
Guide the forwarding of multidestination frames in each VLAN by using its TRILL distribution
tree.
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
