EVPN Type 4 (Ethernet Segment route) Explained

Ethernet Segment Routes are needed in multi-homing scenario and used for Designated Forwarder Election. Designated Forwarder is responsible for sending broadcast, unknown multicast and multicast (BUM) traffic to the CE on a particular Ethernet Segment.

RFC 7432 allows selecting a DF at the granularity of <ES, VLAN> for VLAN-based service and <ES, VLAN bundle> for VLAN-aware service. This enables load-balancing of BUM traffic at a VLAN or VLAN-bundle level.

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EVPN Type 3 (Inclusive Multicast Ethernet Tag route) Explained

Type 3 routes are required for Broadcast, Unknown Unicast and Multicast (BUM) traffic delivery across EVPN networks. Type 3 advertisements provide information about P-tunnels that should be used to send BUM traffic.

Without Type 3 advertisements, ingress router would not know how to deliver BUM traffic to other PE devices that comprise given EVPN instance.

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EVPN Type 2 (MAC/IP Advertisement route) Explained

Type 2 routes are used to advertise MAC addresses and IP addresses that might be associated with aforementioned MAC addresses.

In order to advertise Type 2 routes, PE needs to learn MAC addresses from the directly attached CEs. This is done via normal data-plane learning mechanisms. RFC 7432 also allows for MAC address learning via control plane interaction between PE and CE, although we have not see this implemented by any vendors.

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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.

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EVPN Terminology Reference (RFC 7432, etc)

  • AC, A-C: Attachment Circuit
  • AD, A-D: Ethernet Auto-Discovery route
  • Ethernet Segment (ES): When a customer site (device or network) is connected to one or more PEs via a set of Ethernet links, then that set of links is referred to as an ‘Ethernet segment’.
  • Ethernet Segment Identifier (ESI): A unique non-zero identifier that identifies an Ethernet segment is called an ‘Ethernet Segment Identifier’. ESI 0 denotes a single-homed site.  ESI {0xFF} (repeated 10 times) is known as MAX-ESI.
  • Ethernet Tag: An Ethernet tag identifies a particular broadcast domain, e.g., a VLAN. An EVPN instance consists of one or more broadcast domains.
  • Ethernet Tag ID: 32-bit field containing either a 12-bit or 24-bit identifier that identifies a particular broadcast domain (e.g., a VLAN) in an EVPN instance.
  • EVI: An EVPN Instance spanning the Provider Edge (PE) devices participating in that EVPN.
  • EVPN: Ethernet Virtual Private Network.
  • IPL: IP address length
  • IRB: Integrated Routing and Bridging interface
  • NVE: Network Virtualization Edge
  • MAC-VRF: A Virtual Routing and Forwarding table for Media Access Control (MAC) addresses on a PE.
  • MAX-ET: Reserved Ethernet Tag ID {0xFFFFFFFF} is known as MAX-ET
  • ML: MAC address length
  • TS: Tenant System
  • VA: Virtual Appliance