Lists changes to SHG label encoding due to RFC 7432, the impact on EVPN PE routers, and Cisco's recommendations for mitigating interoperability risks.
To adhere to RFC 7432 recommendations, the encoding or decoding of MPLS label is modified for extended community. Earlier, the lower 20 bits of extended community were used to encode the split-horizon group (SHG) label. The SHG label encoding now uses a higher 20 bits of extended community.
According to this change, routers running old and new software release versions in the same ethernet segment decodeextended communities differently. This change causes inconsistent SHG labels on peering EVPN PE routers. Almost always, the router drops BUM packets with incorrect SHG labels. However, remote PE may accept such packets and forward them to CE in certain conditions, potentially causing a loop. One such instance is when the label incorrectly reads NULL.
To overcome this problem, Cisco recommends you to:
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Minimize the time both PEs are running different software release versions.
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Before upgrading to a new release, isolate the upgraded node and shut down the corresponding AC bundle.
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After upgrading both the PEs to the same release, you can bring both into service.
Similar recommendations apply to peering PEs with different vendors with SHG label assignment that does not adhere to RFC 7432.