Per flow hashing uses information in a packet as an input to the hash function ensuring that any specific flow maps to the same egress LAG port/ECMP path. Note that because the hash uses information in the packet, traffic for the same SAP/interface may be sprayed across different ports of a LAG or different ECMP paths. If this is not wanted, other hashing methods described in this section can be used to change that behavior. Depending on the type of traffic that needs to be distributed into an ECMP and, or LAG, different variables are used as input to the hashing algorithm that determines the next hop selection. The following describes default per flow hashing behavior for those different types of traffic:
VPLS known unicast traffic is hashed based on the IP source and destination addresses for IP traffic, or the MAC source and destination addresses for non-IP traffic. The MAC SA/DA are hashed and then, if the Ethertype is IPv4 or IPv6, the hash is replaced with one based on the IP source address/destination address.
VPLS multicast, broadcast and unknown unicast traffic.
Traffic transmitted on SAPs is not sprayed on a per-frame basis, but instead, the service ID selects ECMP and LAG paths statically.
Traffic transmitted on SDPs is hashed on a per packet basis in the same way as VPLS unicast traffic. However, per packet hashing is applicable only to the distribution of traffic over LAG ports, as the ECMP path is still chosen statically based on the service ID.
Data is hashed twice to get the ECMP path. If LAG and ECMP are performed on the same frame, the data is hashed again to get the LAG port (three hashes for LAG). However, if only LAG is performed, then hashing is only performed twice to get the LAG port.
Multicast traffic transmitted on SAPs with IGMP snooping enabled is load-balanced based on the internal multicast ID, which is unique for every (s,g) record. This way, multicast traffic pertaining to different streams is distributed across different LAG member ports.
The hashing procedure that used to be applied for all VPLS BUM traffic would result in PBB BUM traffic being sent out on BVPLS SAP to follow only a single link when MMRP was not used. Therefore, traffic flooded out on egress BVPLS SAPs is now load spread using the algorithm described above for VPLS known unicast.
Unicast IP traffic routed by a router is hashed using the IP SA/DA in the packet.
MPLS packet hashing at an LSR is based on the whole label stack, along with the incoming port and system IP address. Note that the EXP/TTL information in each label is not included in the hash algorithm. This method is referred to as Label-Only Hash option and is enabled by default, or can be re-instated in CLI by entering the lbl-only option. A few options to further hash on the headers in the payload of the MPLS packet are also provided. For more details, see Changing default per flow hashing inputs.
VLL traffic from a service access point is not sprayed on a per-packet basis, but as for VPLS flooded traffic, the service ID selects one of the ECMP/LAG paths. The exception to this is when shared-queuing is configured on an Epipe SAP, or Ipipe SAP, or when H-POL is configured on an Epipe SAP. In those cases, traffic spraying is the same as for VPLS known unicast traffic. Packets of the above VLL services received on a spoke SDP are sprayed the same as for VPLS known unicast traffic.
Note that Cpipe VLL packets are always sprayed based on the service-id in both directions.
Multicast IP traffic is hashed based on an internal multicast ID, which is unique for every record similar to VPLS multicast traffic with IGMP snooping enabled.
If the ECMP index results in the selection of a LAG as the next hop, then the hash result is hashed again and the result of the second hash is input to the modulo like operation to determine the LAG port selection.
When the ECMP set includes an IP interface configured on a spoke SDP (IES/VPRN spoke interface), or a Routed VPLS spoke SDP interface, the unicast IP packets—which is sprayed over this interface—is not further sprayed over multiple RSVP LSPs/LDP FEC (part of the same SDP), or GRE SDP ECMP paths. In this case, a single RSVP LSP, LDP FEC next-hop or GRE SDP ECMP path is selected based on a modulo operation of the service ID. In case the ECMP path selected is a LAG, the second round of the hash, hashes traffic based on the system, port or interface load-balancing settings.
In addition to the above described per-flow hashing inputs, the system supports multiple options to modify default hash inputs.