Both LSR and LER roles behave in the same way with this type of configuration. Before installing CBF information in the forwarding path, the system performs a consistency check on the CBF information of the ECMP set of tunnel next hops that resolve an LDP prefix FEC.
If no LSP, in the full ECMP set, has been assigned with a class forwarding policy configuration, the set is considered as inconsistent from a CBF perspective. The system, then, programs in the forwarding path, the whole ECMP set without any CBF information, and regular ECMP spraying occurs over the full set.
If the ECMP set is assigned to more than one class forwarding policy, the set is inconsistent from a CBF perspective. Then, the system programs, in the forwarding path, the whole ECMP set without any CBF information, and regular ECMP spraying occurs over the full set.
A full ECMP set is consistent from a CBF perspective when the ECMP:
is assigned to a single class forwarding policy
contains either an LSP assigned to the default set (implicit or explicit)
contains an LSP assigned to a non-default set that has explicit FC mappings
If there is no default set in a consistent ECMP set, the system automatically selects one set as the default one. The selected set is one set with the lowest ID among those referenced by the LSPs of the ECMP set.
If the ECMP set is consistent from a CBF perspective, the system programs in the forwarding path all the LSPs which have CBF configuration, and packets classified to a specific FC are forwarded by using the LSPs of the corresponding forwarding set.
If there are more than one LSPs in a forwarding set, the system performs a modulo operation on these LSPs only to select one. As a result, ECMP spraying occurs for multiple packets of this forwarding class. Also, the system programs, in the forwarding path, the remaining LSPs of the ECMP set, without any CBF information. These LSPs are not used for class-based forwarding.
If there is no operational LSP in a specific forwarding set, the system forwards packets which have been classified to the corresponding forwarding class onto the default set. Additionally, if there is no operational LSP in the default set, the system reverts to regular ECMP spraying over the full ECMP set.
If the user changes (by adding, modifying or deleting) the CBF configuration associated with an LSP that was previously selected as part of an ECMP set, then the FEC resolution is automatically updated, and a CBF consistency check is performed. Moreover, the user changes can update the forwarding configuration.
The LSR role applies to incoming labeled LDP traffic whose FEC is resolved to IGP IPv4 shortcuts.
The LER role applies to the following:
IPv4 and IPv6 prefixes in GRT (with an IPv4 BGP NH)
VPN-v4 and VPN-v6 routes
However, LER does not apply to any service which uses either explicit binding to an SDP (static or T-LDP signaled services), or auto-binding to SDP (BGP-AD VPLS, BGP-VPLS, BGP-VPWS, Dynamic MS-PW).
For BGP-LU, ECMP+CBF is supported only in the absence of the VPRN label. Therefore, ECMP+CBF is not supported when a VPRN label runs on top of BGP-LU (itself running over LDPoRSVP).
The CBF capability is available with any system profile. The number of sets is limited to four with system profile None or A, and to six with system profile B. This capability does not apply to CPM generated packets, including OAM packets, which are looked-up in RTM, and which are forwarded over tunnel next hops. These packets are forwarded by using either regular ECMP, or by selecting one nexthop from the set.