EVPN Type 1 (Ethernet Auto-Discovery) Explained

Type 1 advertisements are used for two distinct functions – Fast Convergence and Aliasing. EVPN Fast Convergence allows PE devices to change the next-hop adjacencies for all MAC addresses associated with a particular Ethernet Segment. EVPN aliasing allows traffic to be balanced across multiple egress points.

Type 1 routes are only advertised if Ethernet Segment Identifier is set to non-zero value, meaning that Type 1 routes are only originate for multi-homed sites.

Please refer to the following cheatsheet if you are not familiar with EVPN Terminology.

In the following example, Type 1 routes will be advertised by both PE1 and PE5 for CE5 Ethernet Segment. PE5 will not advertise Type 1 for CE4’s Segment, as CE4 is single-homed.

EVPN MPLS Type 1 For Topology Diagram (RFC 7432 Explained)
EVPN MPLS Type 1 Topology Diagram

The format of Type 1 Route as described in RFC 7432 is shown below:

EVPN MPLS Type 1 Route Format as defined in RFC 7432
EVPN MPLS Type 1 Route Format

 

PE5’s configuration for CE5 site in Juniper format:

ge-0/0/2 {
    flexible-vlan-tagging;
    encapsulation flexible-ethernet-services;
    esi {
        00:00:00:00:00:00:10:00:00:05;
        all-active;
    }
    unit 100 {
        encapsulation vlan-bridge;
        vlan-id 100;
    }
}

The following capture shows Type 1 Ethernet Auto Discover (A-D) route used for Fast Convergence and was taken on PE5. Please note that Ethernet TAG is set to MAX-ET (FFFF:FFF) and MPLS label is the NLRI is set to 0. Extended Community attribute carrying the actual MPLS label is included in the route. Refer to section 8.2.1 of RFC 7432 for more information.

Path Attribute - MP_REACH_NLRI
...
 Network layer reachability information (27 bytes)
   EVPN NLRI: Ethernet AD Route
      AFI: Ethernet AD Route (1)
          Length: 25
          Route Distinguisher: 0001780002050000 (120.0.2.5:0)
          ESI: 00 00 00 00 00 10 00 00 05
             ESI Type: ESI 9 bytes value (0)
             ESI 9 bytes value: 00 00 00 00 00 10 00 00 05
          Ethernet Tag ID: 4294967295 (FFFF:FFFF)
          MPLS Label Stack: 0 (bottom)

 Path Attribute - EXTENDED_COMMUNITIES
...
  Carried extended communities: (2 communities)
    Community Transitive Two-Octet AS Route Target: 100:100
      Community type high: Transitive Two-Octet AS (0x00)
      Subtype as2: Route Target (0x02)
      Two octets AS specific: 100
      Four octets AN specific: 100
    Community Transitive EVPN ESI MPLS Label: All active redundancy
                                              Label: 315232
      Community type high: Transitive EVPN (0x06)
      Subtype evpn: ESI MPLS Label (0x01)
      .... ...0 = Single active bit: All-Active redundancy
      Two octets Value specific: 0x0000
      MPLS Label: 315232

The following Type 1 route is used for Aliasing.
RFC allows per <ESI, Ethernet Tag ID> or per <ESI> advertisements for a given MAC-VRF. MPLS label provided in this advertisement is used for known unicast load-balancing as described in Section 14 of RFC 7432.

The following Wireshark capture of Type 1 route was taken on PE5’s MPLS network-facing interface. Notice the Ethernet Tag value – router is using per <ESI> granularity by setting Ethernet TAG ID to 0. Refer to section 8.4.1 of RFC 7432 for more information.

 Path Attribute - MP_REACH_NLRI
 ...
  Network layer reachability information (62 bytes)
   EVPN NLRI: Ethernet AD Route
      AFI: Ethernet AD Route (1)
         Length: 25
         Route Distinguisher: 0001780002050064 (120.0.2.5:100)
         ESI: 00 00 00 00 00 10 00 00 05
             ESI Type: ESI 9 bytes value (0)
             ESI 9 bytes value: 00 00 00 00 00 10 00 00 05
          Ethernet Tag ID: 0
          MPLS Label Stack: 315168 (bottom)

Advertised Type 1 routes as observed on PE1:

root@PE1 > show route advertising-protocol bgp 120.0.1.1 extensive
* 1:120.0.2.1:100::10000005::0/304 AD/EVI (1 entry, 1 announced)
 BGP group IBGP-RR type Internal
 Route Distinguisher: 120.0.2.1:100
 Route Label: 320576
 Nexthop: Self
 Flags: Nexthop Change
 Localpref: 100
 AS path: [100] I
 Communities: target:100:100

* 1:120.0.2.1:0::10000005::FFFF:FFFF/304 AD/ESI (1 entry, 1 announced)
 BGP group IBGP-RR type Internal
 Route Distinguisher: 120.0.2.1:0
 Nexthop: Self
 Flags: Nexthop Change
 Localpref: 100
 AS path: [100] I
 Communities: target:100:100 esi-label:all-active (label 320656)

Advertised Type 1 route as observed on PE5:

root@PE5>show route advertising-protocol bgp 120.0.1.1 extensive
* 1:120.0.2.5:100::10000005::0/304 AD/EVI (1 entry, 1 announced)
 BGP group IBGP-RR type Internal
 Route Distinguisher: 120.0.2.5:100
 Route Label: 315168
 Nexthop: Self
 Flags: Nexthop Change
 Localpref: 100
 AS path: [100] I
 Communities: target:100:100

* 1:120.0.2.5:0::10000005::FFFF:FFFF/304 AD/ESI (1 entry, 1 announced)
 BGP group IBGP-RR type Internal
 Route Distinguisher: 120.0.2.5:0
 Nexthop: Self
 Flags: Nexthop Change
 Localpref: 100
 AS path: [100] I
 Communities: target:100:100 esi-label:all-active (label 315232)

EVPN Instance information captured on PE devices:

PE1

root@PE1> show evpn instance extensive

    ESI: 00:00:00:00:00:00:10:00:00:05
      Status: Resolved by IFL ge-0/0/2.100
      Local interface: ge-0/0/2.100, Status: Up/Forwarding
      Number of remote PEs connected: 1
        Remote PE        MAC label  Aliasing label  Mode
        120.0.2.5        0          315168          all-active
      Designated forwarder: 120.0.2.1
      Backup forwarder: 120.0.2.5
      Last designated forwarder update: Apr 04 15:43:15
      Advertised MAC label: 320576
      Advertised aliasing label: 320576
      Advertised split horizon label: 320656

PE5
    ESI: 00:00:00:00:00:00:10:00:00:05
      Status: Resolved by IFL ge-0/0/2.100
      Local interface: ge-0/0/2.100, Status: Up/Forwarding
      Number of remote PEs connected: 1
        Remote PE        MAC label  Aliasing label  Mode
        120.0.2.1        320576     320576          all-active
      Designated forwarder: 120.0.2.1
      Backup forwarder: 120.0.2.5
      Last designated forwarder update: Apr 04 15:43:23
      Advertised MAC label: 315168
      Advertised aliasing label: 315168
      Advertised split horizon label: 315232

PE2 (remote PE not show on the diagram. No direct connection to CE5)
  Number of ethernet segments: 1
    ESI: 00:00:00:00:00:00:10:00:00:05
      Status: Unresolved
      Number of remote PEs connected: 2
        Remote PE        MAC label  Aliasing label  Mode
        120.0.2.1        320576     320576          all-active
        120.0.2.5        0          315168          all-active

Control Plane Wireshark capture can be downloaded from GitHub. Please note that MPLS label values are assigned dynamically and might be different:

EVPN Control Plane PCAP

For more information on EVPN, please refer to our other articles on this topic:

http://www.bgphelp.com/tag/evpn/

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