ES discovery and DF election procedures

The ethernet-segment ESI-1 is configured as per the previous section, with all the required parameters. When ethernet-segment no shutdown is executed, PE1 and PE2 advertise an ES route for ESI-1. They both include the route-target auto-derived from the MAC portion of the configured ESI. If the route-target address family is configured in the network, this allows the RR to keep the dissemination of the ES routes under control.

In addition to the ES route, PE1 and PE2 advertise AD per-ESI routes and AD per-EVI routes.

• AD per-ESI routes announce the Ethernet-Segment capabilities, including the mode (single-active or all-active) as well as the ESI label for split-horizon.

• AD per-EVI routes are advertised so that PE3 knows what services (EVIs) are associated with the ESI. These routes are used by PE3 for its aliasing and backup procedures.

When ES routes exchange between PE1 and PE2 is complete, both run the DF election for all the services in the ethernet-segment.

PE1 and PE2 elect a Designated Forwarder (DF) per <ESI, service>. The default DF election mechanism in 7750 SR, 7450 ESS, and 7950 XRS SR OS is service-carving (as per RFC 7432). The following applies when enabled on a specified PE:

• An ordered list of PE IPs where ESI-1 resides is built. The IPs are gotten from the Origin IP fields of all the ES routes received for ESI-1, as well as the local system address. The lowest IP is considered ordinal '0' in the list.

• The local IP can only be considered a ‟candidate” after successful ethernet-segment no shutdown for a specified service.

  Note: The remote PE IPs must be present in the local PE's RTM so that they can participate in the DF election.

• A PE only considers a specified remote IP address as candidate for the DF election algorithm for a specified service if, as well as the ES route, the corresponding AD routes per-ESI and per-EVI for that PE have been received and properly activated.

• All the remote PEs receiving the AD per-ES routes (for example, PE3), interpret that ESI-1 is all-active if all the PEs send their AD per-ES routes with the single-active bit = 0. Otherwise, if at least one PE sends an AD route per-ESI with the single-active flag set or the local ESI configuration is single-active, the ESI behaves as single-active.

• An es-activation-timer can be configured at the redundancy>bgp-evpn-multi-homing>es-activation-timer level or at the service>system>bgp-evpn>eth-seg>es-activation-timer level. This timer, which is 3 seconds by default, delays the transition from non-DF to DF for a specified service, after the DF election has run.

  • This use of the es-activation-timer is different from zero and minimizes the risks of loops and packet duplication because of ‟transient” multiple DFs.

  • The same es-activation-timer should be configured in all the PEs that are part of the same ESI. It is up to the user to configure either a long timer to minimize the risks of loops/duplication or even es-activation-timer=0 to speed up the convergence for non-DF to DF transitions. When the user configures a specific value, the value configured at ES level supersedes the configured global value.

• The DF election is triggered by the following events:

  • config>service>system>bgp-evpn>eth-seg# no shutdown triggers the DF election for all the services in the ESI.

  • Reception of a new update/withdrawal of an ES route (containing an ESI configured locally) triggers the DF election for all the services in the ESI.

  • Reception of a new update/withdrawal of an AD per-ES route (containing an ESI configured locally) triggers the DF election for all the services associated with the list of route-targets received along with the route.

  • Reception of a new update of an AD per-ES route with a change in the ESI-label extended community (single-active bit or MPLS label) triggers the DF election for all the services associated with the list of route-targets received along with the route.

  • Reception of a new update/withdrawal of an AD route per-EVI (containing an ESI configured locally) triggers the DF election for that service.

• When the PE boots up, the boot-timer allows the necessary time for the control plane protocols to come up before bringing up the Ethernet-Segment and running the DF algorithm. The boot-timer is configured at system level - config>redundancy>bgp-evpn-multi-homing# boot-timer - and should use a value long enough to allow the IOMs and BGP sessions to come up before exchanging ES routes and running the DF election for each EVI/ISID.

  • The system does not advertise ES routes until the boot timer expires. This guarantees that the peer ES PEs do not run the DF election either until the PE is ready to become the DF if it needs to.

  • The following show command displays the configured boot-timer as well as the remaining timer if the system is still in boot-stage.

    A:PE1# show redundancy bgp-evpn-multi-homing 
    ===============================================================================
    Redundancy BGP EVPN Multi-homing Information
    ===============================================================================
    Boot-Timer              : 10 secs                 
    Boot-Timer Remaining    : 0 secs                  
    ES Activation Timer     : 3 secs                  
    ===============================================================================
    

• When service-carving mode auto is configured (default mode), the DF election algorithm runs the function [V(evi) mod N(peers) = i(ordinal)] to identify the DF for a specified service and ESI, as described in the following example.

  As shown in Figure: ES discovery and DF election, PE1 and PE2 are configured with ESI-1. Given that V(10) mod N(2) = 0, PE1 is elected DF for VPLS-10 (because its IP address is lower than PE2's and it is the first PE in the candidate list).

  Note: The algorithm takes the configured evi in the service as opposed to the service-id itself. The evi for a service must match in all the PEs that are part of the ESI. This guarantees that the election algorithm is consistent across all the PEs of the ESI. The evi must be always configured in a service with SAPs/SDP bindings that are created in an ES.

• A manual service-carving option is allowed so that the user can manually configure for which evi identifiers the PE is primary: service-carving mode manual / manual evi <start-evi> to <end-evi>

  • The system is the PE forwarding/multicasting traffic for the evi identifiers included in the configuration. The PE is secondary (non-DF) for the non-specified evi identifiers.

  • If a range is configured but the service-carving is not mode manual, then the range has no effect.

  • Only two PEs are supported when service-carving mode manual is configured. If a third PE is configured with service-carving mode manual for an ESI, the two non-primary PEs remain non-DF regardless of the primary status.

  • For example, as shown in Figure: ES discovery and DF election: if PE1 is configured with service-carving manual evi 1 to 100 and PE2 with service-carving manual evi 101 to 200, then PE1 is the primary PE for service VPLS 10 and PE2 the secondary PE.

• When service-carving is disabled, the lowest originator IP wins the election for a specified service and ESI:

  config>service>system>bgp-evpn>eth-seg>service-carving> mode off

  The following show command displays the ethernet-segment configuration and DF status for all the EVIs and ISIDs (if PBB-EVPN is enabled) configured in the ethernet-segment.

  *A:PE1# show service system bgp-evpn ethernet-segment name "ESI-1" all 
  ===============================================================================
  Service Ethernet Segment
  ===============================================================================
  Name                    : ESI-1
  Admin State             : Up                 Oper State         : Up
  ESI                     : 01:00:00:00:00:71:00:00:00:01
  Multi-homing            : allActive          Oper Multi-homing  : allActive
  Source BMAC LSB         : 71-71              
  ES BMac Tbl Size        : 8                  ES BMac Entries    : 1
  Lag Id                  : 1                  
  ES Activation Timer     : 0 secs             
  Exp/Imp Route-Target    : target:00:00:00:00:71:00
  
  Svc Carving             : auto               
  ES SHG Label            : 262142             
  ===============================================================================
  ===============================================================================
  EVI Information 
  ===============================================================================
  EVI                 SvcId               Actv Timer Rem      DF
  -------------------------------------------------------------------------------
  1                   1                   0                   no
  -------------------------------------------------------------------------------
  Number of entries: 1
  ===============================================================================
  -------------------------------------------------------------------------------
  DF Candidate list
  -------------------------------------------------------------------------------
  EVI                                     DF Address
  -------------------------------------------------------------------------------
  1                                       192.0.2.69
  1                                       192.0.2.72
  -------------------------------------------------------------------------------
  Number of entries: 2
  -------------------------------------------------------------------------------
  -------------------------------------------------------------------------------
  ===============================================================================
  ISID Information 
  ===============================================================================
  ISID                SvcId               Actv Timer Rem      DF
  -------------------------------------------------------------------------------
  20001               20001               0                   no
  -------------------------------------------------------------------------------
  Number of entries: 1
  ===============================================================================
                                   
  -------------------------------------------------------------------------------
  DF Candidate list
  -------------------------------------------------------------------------------
  ISID                                    DF Address
  -------------------------------------------------------------------------------
  20001                                   192.0.2.69
  20001                                   192.0.2.72
  -------------------------------------------------------------------------------
  Number of entries: 2
  -------------------------------------------------------------------------------
  -------------------------------------------------------------------------------
  ===============================================================================
  BMAC Information 
  ===============================================================================
  SvcId                                   BMacAddress
  -------------------------------------------------------------------------------
  20000                                   00:00:00:00:71:71
  -------------------------------------------------------------------------------
  Number of entries: 1
  ===============================================================================
  

Based on the result of the DF election or the manual service-carving, the control plane on the non-DF (PE1) instructs the data path to remove the LAG SAP (associated with the ESI) from the default flooding list for BM traffic (unknown unicast traffic may still be sent if the EVI label is a unicast label and the source MAC address is not associated with the ESI). On PE1 and PE2, both LAG SAPs learn the same MAC address (coming from the CE). For instance, in the following show commands, 00:ca:ca:ba:ce:03 is learned on both PE1 and PE2 access LAG (on ESI-1). However, PE1 learns the MAC as 'Learned' whereas PE2 learns it as 'Evpn'. This is because of the CE2 hashing the traffic for that source MAC to PE1. PE2 learns the MAC through EVPN but it associates the MAC to the ESI SAP, because the MAC belongs to the ESI.

*A:PE1# show service id 1 fdb detail 
===============================================================================
Forwarding Database, Service 1
===============================================================================
ServId    MAC               Source-Identifier        Type     Last Change
                                                     Age      
-------------------------------------------------------------------------------
1         00:ca:ca:ba:ce:03 sap:lag-1:1              L/0      06/11/15 00:14:47
1         00:ca:fe:ca:fe:70 eMpls:                   EvpnS    06/11/15 00:09:06
                            192.0.2.70:262140
1         00:ca:fe:ca:fe:72 eMpls:                   EvpnS    06/11/15 00:09:39
                            192.0.2.72:262141
-------------------------------------------------------------------------------
No. of MAC Entries: 3
-------------------------------------------------------------------------------
Legend:  L=Learned O=Oam P=Protected-MAC C=Conditional S=Static
===============================================================================

*A:PE2# show service id 1 fdb detail 
===============================================================================
Forwarding Database, Service 1
===============================================================================
ServId    MAC               Source-Identifier        Type     Last Change
                                                     Age      
-------------------------------------------------------------------------------
1         00:ca:ca:ba:ce:03 sap:lag-1:1              Evpn     06/11/15 00:14:47
1         00:ca:fe:ca:fe:69 eMpls:                   EvpnS    06/11/15 00:09:40
                            192.0.2.69:262141
1         00:ca:fe:ca:fe:70 eMpls:                   EvpnS    06/11/15 00:09:40
                            192.0.2.70:262140
-------------------------------------------------------------------------------
No. of MAC Entries: 3
-------------------------------------------------------------------------------
Legend:  L=Learned O=Oam P=Protected-MAC C=Conditional S=Static
===============================================================================

When PE1 (non-DF) and PE2 (DF) exchange BUM packets for evi 1, all those packets are sent including the ESI label at the bottom of the stack (in both directions). The ESI label advertised by each PE for ESI-1 can be displayed by the following command:

*A:PE1# show service system bgp-evpn ethernet-segment name "ESI-1" 
===============================================================================
Service Ethernet Segment
===============================================================================
Name                    : ESI-1
Admin State             : Up                 Oper State         : Up
ESI                     : 01:00:00:00:00:71:00:00:00:01
Multi-homing            : allActive          Oper Multi-homing  : allActive
Source BMAC LSB         : 71-71              
ES BMac Tbl Size        : 8                  ES BMac Entries    : 1
Lag Id                  : 1                  
ES Activation Timer     : 0 secs             
Exp/Imp Route-Target    : target:00:00:00:00:71:00

Svc Carving             : auto               
ES SHG Label            : 262142             
===============================================================================
*A:PE2# show service system bgp-evpn ethernet-segment name "ESI-1" 

===============================================================================
Service Ethernet Segment
===============================================================================
Name                    : ESI-1
Admin State             : Up                 Oper State         : Up
ESI                     : 01:00:00:00:00:71:00:00:00:01
Multi-homing            : allActive          Oper Multi-homing  : allActive
Source BMAC LSB         : 71-71              
ES BMac Tbl Size        : 8                  ES BMac Entries    : 0
Lag Id                  : 1                  
ES Activation Timer     : 20 secs            
Exp/Imp Route-Target    : target:00:00:00:00:71:00

Svc Carving             : auto               
ES SHG Label            : 262142             
===============================================================================