9.2.2. Route Resolution

9.2.2. Route Resolution

If the Ethernet Segment Identifier field in a received MAC/IP Advertisement route is set to the reserved ESI value of 0 or MAX-ESI, then if the receiving PE decides to install forwarding state for the associated MAC address, it MUST be based on the MAC/IP Advertisement route alone.

If the Ethernet Segment Identifier field in a received MAC/IP Advertisement route is set to a non-reserved ESI, and the receiving PE is locally attached to the same ESI, then the PE does not alter its forwarding state based on the received route. This ensures that local routes are preferred to remote routes.

If the Ethernet Segment Identifier field in a received MAC/IP Advertisement route is set to a non-reserved ESI, then if the receiving PE decides to install forwarding state for the associated MAC address, it MUST be when both the MAC/IP Advertisement route AND the associated set of Ethernet A-D per ES routes have been received. The dependency of MAC route installation on Ethernet A-D per ES routes is to ensure that MAC routes don't get accidentally installed during a mass withdraw period.

To illustrate this with an example, consider two PEs (PE1 and PE2) connected to a multihomed Ethernet segment ES1. All-Active redundancy mode is assumed. A given MAC address M1 is learned by PE1 but not PE2. On PE3, the following states may arise:

T1 When the MAC/IP Advertisement route from PE1 and the set of Ethernet A-D per ES routes and Ethernet A-D per EVI routes from PE1 and PE2 are received, PE3 can forward traffic destined to M1 to both PE1 and PE2.

T2 If after T1 PE1 withdraws its set of Ethernet A-D per ES routes, then PE3 forwards traffic destined to M1 to PE2 only.

T2' If after T1 PE2 withdraws its set of Ethernet A-D per ES routes, then PE3 forwards traffic destined to M1 to PE1 only.

T2'' If after T1 PE1 withdraws its MAC/IP Advertisement route, then PE3 treats traffic to M1 as unknown unicast.

T3 PE2 also advertises a MAC route for M1, and then PE1 withdraws its MAC route for M1. PE3 continues forwarding traffic destined to M1 to both PE1 and PE2. In other words, despite M1 withdrawal by PE1, PE3 forwards the traffic destined to M1 to both PE1 and PE2. This is because a flow from the CE, resulting in M1 traffic getting hashed to PE1, can get terminated, resulting in M1 being aged out in PE1; however, M1 can be reachable by both PE1 and PE2.