HP 6125XLG - Page 7

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• Compute a TRILL distribution tree for each VLAN according to the LSDB.
• Use the TRILL distribution tree to guide the forwarding of multi-destination frames, which include
multicast, broadcast, and unknown unicast frames in the VLAN.
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 decapsulated 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 Multicast frame forwarding flow
TRILL selects distribution trees for forwarding multicast, broadcast, and unknown unicast 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, t
wo 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 at RB 4 select the link. For more
information about pseudonode IDs, see Layer 3—IP Routing Configuration Guide.
VLAN 10VLAN 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