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5. Ethernet Segment

5. Ethernet Segment​

As indicated in [RFC7209], each Ethernet segment needs a unique identifier in an EVPN. This section defines how such identifiers are assigned and how they are encoded for use in EVPN signaling. Later sections of this document describe the protocol mechanisms that utilize the identifiers.

When a customer site is connected to one or more PEs via a set of Ethernet links, then this set of Ethernet links constitutes an "Ethernet segment". For a multihomed site, each Ethernet segment (ES) is identified by a unique non-zero identifier called an Ethernet Segment Identifier (ESI). An ESI is encoded as a 10-octet integer in line format with the most significant octet sent first. The following two ESI values are reserved:

  • ESI 0 denotes a single-homed site.

  • ESI {0xFF} (repeated 10 times) is known as MAX-ESI and is reserved.

In general, an Ethernet segment SHOULD have a non-reserved ESI that is unique network wide (i.e., across all EVPN instances on all the PEs). If the CE(s) constituting an Ethernet segment is (are) managed by the network operator, then ESI uniqueness should be guaranteed; however, if the CE(s) is (are) not managed, then the operator MUST configure a network-wide unique ESI for that Ethernet segment. This is required to enable auto-discovery of Ethernet segments and Designated Forwarder (DF) election.

In a network with managed and non-managed CEs, the ESI has the following format:

               +---+---+---+---+---+---+---+---+---+---+
               | T |          ESI Value                |
               +---+---+---+---+---+---+---+---+---+---+

Where:

T (ESI Type) is a 1-octet field (most significant octet) that specifies the format of the remaining 9 octets (ESI Value). The following six ESI types can be used:

  • Type 0 (T=0x00) - This type indicates an arbitrary 9-octet ESI value, which is managed and configured by the operator.

  • Type 1 (T=0x01) - When IEEE 802.1AX LACP is used between the PEs and CEs, this ESI type indicates an auto-generated ESI value determined from LACP by concatenating the following parameters:

  • CE LACP System MAC address (6 octets). The CE LACP System MAC address MUST be encoded in the high-order 6 octets of the ESI Value field.

  • CE LACP Port Key (2 octets). The CE LACP port key MUST be encoded in the 2 octets next to the System MAC address.

  • The remaining octet will be set to 0x00.

As far as the CE is concerned, it would treat the multiple PEs that it is connected to as the same switch. This allows the CE to aggregate links that are attached to different PEs in the same bundle.

This mechanism could be used only if it produces ESIs that satisfy the uniqueness requirement specified above.

  • Type 2 (T=0x02) - This type is used in the case of indirectly connected hosts via a bridged LAN between the CEs and the PEs. The ESI Value is auto-generated and determined based on the Layer 2 bridge protocol as follows: If the Multiple Spanning Tree Protocol (MSTP) is used in the bridged LAN, then the value of the ESI is derived by listening to Bridge PDUs (BPDUs) on the Ethernet segment. To achieve this, the PE is not required to run MSTP. However, the PE must learn the Root Bridge MAC address and Bridge Priority of the root of the Internal Spanning Tree (IST) by listening to the BPDUs. The ESI Value is constructed as follows:

  • Root Bridge MAC address (6 octets). The Root Bridge MAC address MUST be encoded in the high-order 6 octets of the ESI Value field.

  • Root Bridge Priority (2 octets). The CE Root Bridge Priority MUST be encoded in the 2 octets next to the Root Bridge MAC address.

  • The remaining octet will be set to 0x00.

This mechanism could be used only if it produces ESIs that satisfy the uniqueness requirement specified above.

  • Type 3 (T=0x03) - This type indicates a MAC-based ESI Value that can be auto-generated or configured by the operator. The ESI Value is constructed as follows:

  • System MAC address (6 octets). The PE MAC address MUST be encoded in the high-order 6 octets of the ESI Value field.

  • Local Discriminator value (3 octets). The Local Discriminator value MUST be encoded in the low-order 3 octets of the ESI Value.

This mechanism could be used only if it produces ESIs that satisfy the uniqueness requirement specified above.

  • Type 4 (T=0x04) - This type indicates a router-ID ESI Value that can be auto-generated or configured by the operator. The ESI Value is constructed as follows:

  • Router ID (4 octets). The system router ID MUST be encoded in the high-order 4 octets of the ESI Value field.

  • Local Discriminator value (4 octets). The Local Discriminator value MUST be encoded in the 4 octets next to the IP address.

  • The low-order octet of the ESI Value will be set to 0x00.

This mechanism could be used only if it produces ESIs that satisfy the uniqueness requirement specified above.

  • Type 5 (T=0x05) - This type indicates an Autonomous System (AS)-based ESI Value that can be auto-generated or configured by the operator. The ESI Value is constructed as follows:

  • AS number (4 octets). This is an AS number owned by the system and MUST be encoded in the high-order 4 octets of the ESI Value field. If a 2-octet AS number is used, the high-order extra 2 octets will be 0x0000.

  • Local Discriminator value (4 octets). The Local Discriminator value MUST be encoded in the 4 octets next to the AS number.

  • The low-order octet of the ESI Value will be set to 0x00.

This mechanism could be used only if it produces ESIs that satisfy the uniqueness requirement specified above.

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