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15. MAC Mobility

15. MAC Mobility​

It is possible for a given host or end-station (as defined by its MAC address) to move from one Ethernet segment to another; this is referred to as 'MAC Mobility' or 'MAC move', and it is different from the multihoming situation in which a given MAC address is reachable via multiple PEs for the same Ethernet segment. In a MAC move, there would be two sets of MAC/IP Advertisement routes -- one set with the new Ethernet segment and one set with the previous Ethernet segment -- and the MAC address would appear to be reachable via each of these segments.

In order to allow all of the PEs in the EVPN instance to correctly determine the current location of the MAC address, all advertisements of it being reachable via the previous Ethernet segment MUST be withdrawn by the PEs, for the previous Ethernet segment, that had advertised it.

If local learning is performed using the data plane, these PEs will not be able to detect that the MAC address has moved to another Ethernet segment, and the receipt of MAC/IP Advertisement routes, with the MAC Mobility extended community attribute, from other PEs serves as the trigger for these PEs to withdraw their advertisements. If local learning is performed using the control or management planes, these interactions serve as the trigger for these PEs to withdraw their advertisements.

In a situation where there are multiple moves of a given MAC, possibly between the same two Ethernet segments, there may be multiple withdrawals and re-advertisements. In order to ensure that all PEs in the EVPN instance receive all of these correctly through the intervening BGP infrastructure, introducing a sequence number into the MAC Mobility extended community attribute is necessary.

In order to process mobility events correctly, an implementation MUST handle scenarios in which sequence number wraparound occurs.

Every MAC mobility event for a given MAC address will contain a sequence number that is set using the following rules:

  • A PE advertising a MAC address for the first time advertises it with no MAC Mobility extended community attribute.

  • A PE detecting a locally attached MAC address for which it had previously received a MAC/IP Advertisement route with a different Ethernet segment identifier advertises the MAC address in a MAC/IP Advertisement route tagged with a MAC Mobility extended community attribute with a sequence number one greater than the sequence number in the MAC Mobility extended community attribute of the received MAC/IP Advertisement route. In the case of the first mobility event for a given MAC address, where the received MAC/IP Advertisement route does not carry a MAC Mobility extended community attribute, the value of the sequence number in the received route is assumed to be 0 for the purpose of this processing.

  • A PE detecting a locally attached MAC address for which it had previously received a MAC/IP Advertisement route with the same non-zero Ethernet segment identifier advertises it with:

  1. no MAC Mobility extended community attribute, if the received route did not carry said attribute.

  2. a MAC Mobility extended community attribute with the sequence number equal to the highest of the sequence number(s) in the received MAC/IP Advertisement route(s), if the received route(s) is (are) tagged with a MAC Mobility extended community attribute.

  • A PE detecting a locally attached MAC address for which it had previously received a MAC/IP Advertisement route with the same zero Ethernet segment identifier (single-homed scenarios) advertises it with a MAC Mobility extended community attribute with the sequence number set properly. In the case of single-homed scenarios, there is no need for ESI comparison. ESI comparison is done for multihoming in order to prevent false detection of MAC moves among the PEs attached to the same multihomed site.

A PE receiving a MAC/IP Advertisement route for a MAC address with a different Ethernet segment identifier and a higher sequence number than that which it had previously advertised withdraws its MAC/IP Advertisement route. If two (or more) PEs advertise the same MAC address with the same sequence number but different Ethernet segment identifiers, a PE that receives these routes selects the route advertised by the PE with the lowest IP address as the best route. If the PE is the originator of the MAC route and it receives the same MAC address with the same sequence number that it generated, it will compare its own IP address with the IP address of the remote PE and will select the lowest IP. If its own route is not the best one, it will withdraw the route.

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