H3C Technologies H3C S6800 Series Switches - Manual (Page 11)

4
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 tasks:
{
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.
As shown in Figure 3 , 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 Figure 4 , two equal-cost links exist
between RB 1 and RB 3. Assume the link directly connecting RB 1 to RB 3 has the largest
pseudonode ID. Both the TRILL distribution tree rooted at RB 3 and the TRILL distribution tree rooted
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