4
As shown in
, when a multicast frame from VLAN 10 enters the TRILL network, RB 1,
which is an ingress RB, encapsulates the multicast frame into a TRILL frame. In the frame, the
egress RB is RB 2, the root bridge of the TRILL distribution tree for VLAN 10. When the frame
arrives at the root bridge, it is distributed throughout the TRILL distribution tree. Then, the TRILL
frame is de-encapsulated by RB 3 and sent to the destination station S2. Because the network
segment where RB 4 resides does not have a receiver of this frame, RB 4 drops the frame.
Figure 3 Layer 2 multicast frame forwarding flow
TRILL selects distribution trees for forwarding multidestination frames based on the VLANs to
which the frames belong. Because the topologies of TRILL distribution trees are different, traffic
can be load shared. However, equal-cost links are not used for load sharing.
When N equal-cost links exist in the network, each TRILL distribution tree selects the link with
the largest pseudonode ID for forwarding packets. As shown in
, two equal-cost links
exist between RB 1 and RB 2. Assume the link directly connecting RB 1 to RB 2 has the largest
pseudonode ID. Both the TRILL distribution tree rooted at RB 1 and the TRILL distribution tree
rooted at RB 4 select the link. For more information about pseudonode IDs, see
Layer 3—IP
Routing Configuration Guide
.
VLAN 10
VLAN 10
S1
S2
VLAN 200
TRILL network
RB 1
Ethernet frame
Payload
Inner VLAN = 10
Inner S-MAC = S1
Inner D-MAC = Multi
TRILL frame
Ingress RB = RB 1
Outer VLAN = 200
Egress RB = RB 2
Outer S-MAC = RB 1
Outer D-MAC = All RBs
Payload
Inner VLAN = 10
Inner S-MAC = S1
Inner D-MAC = Multi
TRILL frame
Ingress RB = RB 1
Outer VLAN = 200
Egress RB = RB 2
Outer S-MAC = RB 2
Outer D-MAC = All RBs
Payload
Inner VLAN = 10
Inner S-MAC = S1
Inner D-MAC = Multi
RB 4
RB 3
Ethernet frame
Payload
Inner VLAN = 10
Inner S-MAC = S1
Inner D-MAC = Multi
RB 2
Multicast frame
Distribution tree
of VLAN 10
Root bridge of
distribution tree
Leaf of
distribution tree
