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8.5. Designated Forwarder Election

8.5. Designated Forwarder Election​

Consider a CE that is a host or a router that is multihomed directly to more than one PE in an EVPN instance on a given Ethernet segment. One or more Ethernet tags may be configured on the Ethernet segment. In this scenario, only one of the PEs, referred to as the Designated Forwarder (DF), is responsible for certain actions:

  • Sending multicast and broadcast traffic, on a given Ethernet tag on a particular Ethernet segment, to the CE.

  • Flooding unknown unicast traffic (i.e., traffic for which a PE does not know the destination MAC address), on a given Ethernet tag on a particular Ethernet segment to the CE, if the environment requires flooding of unknown unicast traffic.

Note that this behavior, which allows selecting a DF at the granularity of <ES, VLAN> or <ES, VLAN bundle> for multicast, broadcast, and unknown unicast traffic, is the default behavior in this specification.

Note that a CE always sends packets belonging to a specific flow using a single link towards a PE. For instance, if the CE is a host, then, as mentioned earlier, the host treats the multiple links that it uses to reach the PEs as a Link Aggregation Group (LAG). The CE employs a local hashing function to map traffic flows onto links in the LAG.

If a bridged network is multihomed to more than one PE in an EVPN network via switches, then the support of All-Active redundancy mode requires the bridged network to be connected to two or more PEs using a LAG.

If a bridged network does not connect to the PEs using a LAG, then only one of the links between the bridged network and the PEs must be the active link for a given <ES, VLAN> or <ES, VLAN bundle>. In this case, the set of Ethernet A-D per ES routes advertised by each PE MUST have the "Single-Active" bit in the flags of the ESI Label extended community set to 1.

The default procedure for DF election at the granularity of <ES, VLAN> for VLAN-based service or <ES, VLAN bundle> for VLAN-(aware) bundle service is referred to as "service carving". With service carving, it is possible to elect multiple DFs per Ethernet segment (one per VLAN or VLAN bundle) in order to perform load balancing of multi-destination traffic destined to a given segment. The load balancing procedures carve up the VLAN space per ES among the PE nodes evenly, in such a way that every PE is the DF for a disjoint set of VLANs or VLAN bundles for that ES. The procedure for service carving is as follows:

  1. When a PE discovers the ESI of the attached Ethernet segment, it advertises an Ethernet Segment route with the associated ES-Import extended community attribute.

  2. The PE then starts a timer (default value = 3 seconds) to allow the reception of Ethernet Segment routes from other PE nodes connected to the same Ethernet segment. This timer value should be the same across all PEs connected to the same Ethernet segment.

  3. When the timer expires, each PE builds an ordered list of the IP addresses of all the PE nodes connected to the Ethernet segment (including itself), in increasing numeric value. Each IP address in this list is extracted from the "Originating Router's IP address" field of the advertised Ethernet Segment route. Every PE is then given an ordinal indicating its position in the ordered list, starting with 0 as the ordinal for the PE with the numerically lowest IP address. The ordinals are used to determine which PE node will be the DF for a given EVPN instance on the Ethernet segment, using the following rule:

Assuming a redundancy group of N PE nodes, for VLAN-based service, the PE with ordinal i is the DF for an <ES, VLAN V> when (V mod N) = i. In the case of VLAN-(aware) bundle service, then the numerically lowest VLAN value in that bundle on that ES MUST be used in the modulo function.

It should be noted that using the "Originating Router's IP address" field in the Ethernet Segment route to get the PE IP address needed for the ordered list allows for a CE to be multihomed across different ASes if such a need ever arises.

  1. The PE that is elected as a DF for a given <ES, VLAN> or <ES, VLAN bundle> will unblock multi-destination traffic for that VLAN or VLAN bundle on the corresponding ES. Note that the DF PE unblocks multi-destination traffic in the egress direction towards the segment. All non-DF PEs continue to drop multi-destination traffic in the egress direction towards that <ES, VLAN> or <ES, VLAN bundle>.

In the case of link or port failure, the affected PE withdraws its Ethernet Segment route. This will re-trigger the service carving procedures on all the PEs in the redundancy group. For PE node failure, or upon PE commissioning or decommissioning, the PEs re-trigger the service carving. In the case of Single-Active multihoming, when a service moves from one PE in the redundancy group to another PE as a result of re-carving, the PE, which ends up being the elected DF for the service, SHOULD trigger a MAC address flush notification towards the associated Ethernet segment. This can be done, for example, using the IEEE 802.1ak Multiple VLAN Registration Protocol (MVRP) 'new' declaration.

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