Label Distribution Protocol
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MPLS Guide
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3. At ASBR-5, the process is repeated. For example, in Figure 29, ASBR-5 will go
through steps 1 and 2 to choose between ASBR-1 and ASBR-2 and perform a
second recursive ECMP lookup to choose the path to that ASBR.
When there are several candidate upstream LSRs, the LSR must select one
upstream LSR. The algorithm used for the LSR selection is a local decision. If the
LSR selection is done over a LAN interface, the procedure described in ECMP Hash
Algorithm should be applied to ensure that the same upstream LSR is elected from
the set of candidate receivers on that LAN.
The ECMP hash algorithm ensures that there is a single forwarder over the LAN for
a particular LSP.
5.2.5.3.1 ECMP Hash Algorithm
The ECMP hash algorithm requires the opaque value of the FEC (see Table 39) and
is based on RFC 6388 section 2.4.1.1.
The following hash is performed: H = (CRC32 (Opaque Value)) modulo N, where N
is the number of upstream LSRs. The Opaque Value is the field identified in the FEC
element after Opaque Length. The Opaque Length field indicates the size of the
opaque value used in this calculation.
The candidate upstream LSRs are numbered from lowest to highest IP addresses.
The selected upstream LSR is the LSR that has the value H, as calculated by the
hash algorithm.
5.2.6 Intra-AS Non-segmented mLDP
Non-segmented mLDP intra-AS (inter-area) is supported on Option C only. Figure 30
shows a typical intra-AS topology with a backbone IGP area 0 and access
non-backbone IGP areas 1 and 2. In these topologies, the ABRs usually do a
next-hop-self for BGP label routes, which requires recursive FEC.
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