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14.1.2. All-Active Redundancy Mode

14.1.2. All-Active Redundancy Mode​

For a given ES, if the remote PE has imported the set of Ethernet A-D per ES routes from one or more PEs and none of them have the "Single-Active" flag in the ESI Label extended community set, then the remote PE MUST deduce that the ES is operating in All-Active redundancy mode. A remote PE that receives a MAC/IP Advertisement route with a non-reserved ESI SHOULD consider the advertised MAC address to be reachable via all PEs that have advertised reachability to that MAC address's EVI/ES via the combination of an Ethernet A-D per EVI route for that EVI/ES (and Ethernet tag, if applicable) AND an Ethernet A-D per ES route for that ES. The remote PE MUST use received MAC/IP Advertisement routes and Ethernet A-D per EVI/per ES routes to construct the set of next hops for the advertised MAC address.

Each next hop comprises an MPLS label stack that is to be used by the egress PE to forward the packet. This label stack is determined as follows:

  • If the next hop is constructed as a result of a MAC route, then this label stack MUST be used. However, if the MAC route doesn't exist for that PE, then the next hop and the MPLS label stack are constructed as a result of the Ethernet A-D routes. Note that the following description applies to determining the label stack for a particular next hop to reach a given PE, from which the remote PE has received and imported Ethernet A-D routes that have the same ESI and Ethernet tag as the ones present in the MAC advertisement. The Ethernet A-D routes mentioned in the following description refer to the ones imported from this given PE.

  • If a set of Ethernet A-D per ES routes for that ES AND an Ethernet A-D route per EVI exist, only then must the label from that latter route be used.

The following example explains the above.

Consider a CE (CE1) that is dual-homed to two PEs (PE1 and PE2) on a LAG interface (ES1), and is sending packets with source MAC address MAC1 on VLAN1 (mapped to EVI1). A remote PE, say PE3, is able to learn that MAC1 is reachable via PE1 and PE2. Both PE1 and PE2 may advertise MAC1 in BGP if they receive packets with MAC1 from CE1. If this is not the case, and if MAC1 is advertised only by PE1, PE3 still considers MAC1 as reachable via both PE1 and PE2, as both PE1 and PE2 advertise a set of Ethernet A-D per ES routes for ES1 as well as an Ethernet A-D per EVI route for <EVI1, ES1>.

The MPLS label stack to send the packets to PE1 is the MPLS LSP stack to get to PE1 (at the top of the stack) followed by the EVPN label advertised by PE1 for CE1's MAC.

The MPLS label stack to send packets to PE2 is the MPLS LSP stack to get to PE2 (at the top of the stack) followed by the MPLS label in the Ethernet A-D route advertised by PE2 for <ES1, VLAN1>, if PE2 has not advertised MAC1 in BGP.

We will refer to these label stacks as MPLS next hops.

The remote PE (PE3) can now load balance the traffic it receives from its CEs, destined for CE1, between PE1 and PE2. PE3 may use N-tuple flow information to hash traffic into one of the MPLS next hops for load balancing of IP traffic. Alternatively, PE3 may rely on the source MAC addresses for load balancing.

Note that once PE3 decides to send a particular packet to PE1 or PE2, it can pick one out of multiple possible paths to reach the particular remote PE using regular MPLS procedures. For instance, if the tunneling technology is based on RSVP-TE LSPs and PE3 decides to send a particular packet to PE1, then PE3 can choose from multiple RSVP-TE LSPs that have PE1 as their destination.

When PE1 or PE2 receives the packet destined for CE1 from PE3, if the packet is a known unicast, it is forwarded to CE1. If it is a BUM packet, then only one of PE1 or PE2 must forward the packet to the CE. Whether PE1 or PE2 forwards this packet to the CE is determined based on which of the two is the DF.

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