A remote PE (PE3 in Figure: Backup PE) imports the AD routes per ESI, where the single-active flag is set. PE3 interprets that the Ethernet-Segment is single-active if at least one PE sends an AD route per-ESI with the single-active flag set. MACs for a specified service and ESI are learned from a single PE, that is, the DF for that <ESI, EVI>.
The remote PE installs a single EVPN-MPLS destination (TEP, label) for a received MAC address and
a backup next-hop to the PE for which the AD routes
per-ESI and per-EVI are received. For instance, in
the following command, 00:ca:ca:ba:ca:06 is
associated with the remote ethernet-segment
eES 01:74:13:00:74:13:00:00:74:13. That
eES is resolved to PE (192.0.2.73), which is the DF
on the ES.
*A:PE3# show service id 1 fdb detail
===============================================================================
Forwarding Database, Service 1
===============================================================================
ServId MAC Source-Identifier Type Last Change
Age
-------------------------------------------------------------------------------
1 00:ca:ca:ba:ca:02 sap:1/1/1:2 L/0 06/12/15 00:33:39
1 00:ca:ca:ba:ca:06 eES: Evpn 06/12/15 00:33:39
01:74:13:00:74:13:00:00:74:13
1 00:ca:fe:ca:fe:69 eMpls: EvpnS 06/11/15 21:53:47
192.0.2.69:262118
1 00:ca:fe:ca:fe:70 eMpls: EvpnS 06/11/15 19:59:57
192.0.2.70:262140
1 00:ca:fe:ca:fe:72 eMpls: EvpnS 06/11/15 19:59:57
192.0.2.72:262141
-------------------------------------------------------------------------------
No. of MAC Entries: 5
-------------------------------------------------------------------------------
Legend: L=Learned O=Oam P=Protected-MAC C=Conditional S=Static
===============================================================================
*A:PE3# show service id 1 evpn-mpls
===============================================================================
BGP EVPN-MPLS Dest
===============================================================================
TEP Address Egr Label Num. MACs Mcast Last Change
Transport
-------------------------------------------------------------------------------
192.0.2.69 262118 1 Yes 06/11/2015 19:59:03
ldp
192.0.2.70 262139 0 Yes 06/11/2015 19:59:03
ldp
192.0.2.70 262140 1 No 06/11/2015 19:59:03
ldp
192.0.2.72 262140 0 Yes 06/11/2015 19:59:03
ldp
192.0.2.72 262141 1 No 06/11/2015 19:59:03
ldp
192.0.2.73 262139 0 Yes 06/11/2015 19:59:03
ldp
192.0.2.254 262142 0 Yes 06/11/2015 19:59:03
bgp
-------------------------------------------------------------------------------
Number of entries : 7
-------------------------------------------------------------------------------
===============================================================================
===============================================================================
BGP EVPN-MPLS Ethernet Segment Dest
===============================================================================
Eth SegId TEP Address Egr Label Last Change
Transport
-------------------------------------------------------------------------------
01:74:13:00:74:13:00:00:74:13 192.0.2.73 262140 06/11/2015 19:59:03
ldp
-------------------------------------------------------------------------------
Number of entries : 1
-------------------------------------------------------------------------------
===============================================================================
If PE3 sees only two single-active PEs in the same ESI, the second PE is the backup PE. Upon receiving an AD per-ES/per-EVI route withdrawal for the ESI from the primary PE, the PE3 starts sending the unicast traffic to the backup PE immediately.
If PE3 receives AD routes for the same ESI and EVI from more than two PEs, the PE does not install any backup route in the data path. Upon receiving an AD per-ES/per-EVI route withdrawal for the ESI, it flushes the MACs associated with the ESI.