BGP 3107 labelled route ECMP is not supported on 7210 SAS platforms. References to BGP 3107 labelled route ECMP are included in this section only for completeness of the feature description.
When the responder node has multiple equal cost next-hops for an LDP FEC or a BGP label IPv4 prefix, it replies in the DSMAP TLV with the downstream information of the outgoing interface that is part of the ECMP next-hop set for the prefix.
However, when a BGP label route is resolved to an LDP FEC (of the BGP next-hop of the BGP label route), ECMP can exist at both the BGP and LDP levels. The following next-hop selection is performed in this case:
For each BGP ECMP next hop of the label route, a single LDP next hop is selected even if multiple LDP ECMP next hops exist. Therefore, the number of ECMP next hops for the BGP IPv4 label route is equal to the number of BGP next-hops.
ECMP for a BGP IPv4 label route is only supported at the provider edge (PE) router (BGP label push operation) and not at ABR and ASBR (BGP label swap operation). Therefore, at an LSR, a BGP IPv4 label route is resolved to a single BGP next hop, which is resolved to a single LDP next hop.
LSP trace will return one downstream mapping TLV for each next-hop of the BGP IPv4 label route. It will also return the exact LDP next-hop that the datapath programmed for each BGP next-hop.
In the following description of LSP ping and LSP trace behavior, generic references are made to specific terms as follows: FEC can represent either an LDP FEC or a BGP IPv4 label router, and a Downstream Mapping TLV can represent either the DSMAP TLV or the DDMAP TLV:
If the user initiates an LSP trace of the FEC without the path-destination option specified, the sender node does not include multi-path information in the Downstream Mapping TLV in the echo request message (multipath type=0). In this case, the responder node replies with a Downstream Mapping TLV for each outgoing interface that is part of the ECMP next-hop set for the FEC. The sender node will select the first Downstream Mapping TLV only for the subsequent echo request message with incrementing TTL.
If the user initiates an LSP ping of the FEC with the path-destination option specified, the sender does not include the Downstream Mapping TLV. However, the user can configure the interface option, part of the same path-destination option, to direct the echo request message at the sender node to be sent from a specific outgoing interface that is part of an ECMP path set for the FEC.
If the user initiates an LSP trace of the FEC with the path-destination option specified but configured to exclude a Downstream Mapping TLV in the MPLS echo request message using the CLI command downstream-map-tlv {none}, the sender node does not include the Downstream Mapping TLV. However, the user can configure the interface option, part of the same path-destination option, to direct the echo request message at the sender node to be sent out a specific outgoing interface that is part of an ECMP path set for the FEC.
If the user initiates an LSP trace of the FEC with the path-destination option specified, the sender node includes the multipath information in the Downstream Mapping TLV in the echo request message (multipath type=8). The path-destination option allows the user to exercise a specific path of a FEC in the presence of ECMP. The user enters a specific address from the 127/8 range, which is then inserted in the multipath type 8 information field of the Downstream Mapping TLV. The CPM code at each LSR in the path of the target FEC runs the same hash routine as the datapath and replies in the Downstream Mapping TLV with the specific outgoing interface the packet would have been forwarded to if it had not expired at this node and if the DEST IP field in the packet’s header was set to the 127/8 address value inserted in the multipath type 8 information.
The ldp-treetrace tool always uses the multipath type=8 value and inserts a range of 127/8 addresses instead of a single address to exercise multiple ECMP paths of an LDP FEC. The behavior is the same as the lsp-trace command with the path-destination option enabled.
The path-destination option can also be used to exercise a specific ECMP path of an LDP FEC tunneled over an RSVP LSP or ECMP path of an LDP FEC stitched to a BGP FEC in the presence of BGP ECMP paths. The user must enable the use of the DDMAP TLV either globally (config>test-oam>mpls-echo-request-downstream-map ddmap) or within the specific ldp-treetrace or LSP trace test (downstream-map-tlv ddmap option).