5
Figure 4 Multicast ECMP
TRILL distribution trees support Equal Cost Multiple Path (ECMP), also known as multicast
ECMP. When multicast ECMP is enabled, TRILL assigns equal-cost links to different TRILL
distributions trees to improve load sharing performance.
When N equal-cost links exist in the network, each TRILL distribution tree selects an equal-cost
link for forwarding packets through
J mod N
in root bridge priority order. J is the priority
sequence number of a TRILL distribution tree and starts from 0.
As shown in
{
The link directly connecting RB 1 to RB 2 is assigned to the TRILL distribution tree rooted at
RB 1.
{
The link RB 1-RB 3-RB 2 is assigned to the TRILL distribution tree rooted at RB 4.
TRILL distribution trees support fast root switching. When an RB detects that the root of a
distribution tree is unreachable, the RB deletes the LSP of the root from its LSDB. This triggers
recalculation of all distribution trees in the TRILL network. Multidestination traffic is switched to
new distribution trees.
Layer 3 forwarding mechanisms
IMPORTANT:
•
TRILL Layer 3 forwarding is available in Release 1138P01 and later versions.
•
TRILL Layer 3 forwarding is supported only on FE and FX cards.
•
TRILL can perform Layer 3 forwarding only for IPv4 unicast traffic.
As shown in
, distribution layer device RB 1 is the gateway of a data center network. RB 1
forwards Layer 3 traffic between VLANs in the data center network and Layer 3 traffic between the
data center and external networks.
