Brocade Multi-Service IronWare Multicast Configuration Guide (Supporting R05.6.00) - Manual (Page 34)

22 Multi-Service IronWare Multicast Configuration Guide
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IP multicast boundaries
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Hash based load Distribution
• Depends on a hash function to distribute the multicast cache entries.
• hash function based S, G, next-hop addresses.
• Splits the cache entries by choosing a different RPF neighbor and in turn splits the traffic.
• Load balancing is based on the distribution of the keys S, G, next-hop.
• Least disruptive as the hashing redistributes only those cache entries that are affected during
link flaps.
• Some paths may not be utilized for the distribution of the multicast entries. For example, for
the ECMP paths from R3 to R6, R7 and R8, only paths R3 to R7 and R3 to R8 are being
utilized.
Deleting path
When an ECMP path goes down, all the multicast entries using that path get redistributed among
the other available paths.
Adding path
No redistribution (default behavior without rebalance option) of the cache entries when a new path
is added to the ECMP set.
• Here Optimal utilization of the paths is traded off in favor of not disturbing the existing flow.
• It also requires a full branch setup towards the source or RP of the multicast distribution tree
sometimes.
• When a path flaps i.e. goes down and comes back up, the multicast entries which had been
using this path would not be using this path anymore and it becomes worse if a subset of paths
go down and come back up one by one, resulting in only the path(s) that didn’t flap to carry all
entries.
Dynamic rebalancing
• Option to rebalance the traffic immediately on a new next-hop or path addition.
• Both CONFIG and EXEC level option.
• Helps in both new next-hop and path addition and path flap cases.
• The existing flows will be disturbed with least disruption by using the hashing method.
Limitations and prerequisites
The following limitations and prerequisites apply to the configuration of ECMP path load balancing.
• The hash method is a load splitting method and hence traffic load balancing is not supported.
• S based and S,G based hashing is not supported.
• The hash method is a load splitting method and not a load balancing method and hence the
load balancing effect due to load splitting the multicast entries is only a best effort and the
splitting is actually based on the number of S, G flows and the number of next-hops and the
actual distribution of the S,G and the next-hop addresses.