Ethernet Segment in EVPN: All You Need to Know
Ethernet Segment in EVPN: All You Need to Know
If a Customer Edge (CE) is multihomed to two or more Provider Edge (PEs), the set of Ethernet links that attach CE to PEs an Ethernet Segment (ES). To the CE, an ES looks like a Link Aggregation Group (LAG).
Each Ethernet Segment has a unique non-zero identifier, known as the Ethernet Segment Identifier (ESI). ESI is a 10-octet integer in line format, sent with the most significant octet first.
The ESI’s value has to be unique and non-reserved across all EVPN instances on a PE. If the CE is managed by the network operator, the uniqueness of the ESI is guaranteed.
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| Fig 1.1- EVPN Ethernet Segment |
However, if the CE is not managed by the network operator, a network-wide unique ESI must be configured for that ES. The uniqueness helps with the auto-discovery of the ES and DF election.
Reserved values of the ESI
- ESI 0, which represents a single-homed CE (i.e. CE is linked to only one PE)
- Maximum ESI value 0xFF (10 times), which is reserved.
The ESI type is a 1-octet field (most significant octet) that represents the type of ESI. It specifies the next 9 octets, i.e. the ESI value.
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| Fig 1.2- Ethernet Segment |
Types of ESI
Type 0 (T=0x00) - indicates an arbitrary ESI value which is configured by the network operator.
Type 1 (T=0x01) - indicates an auto-generated ESI value by LACP when IEEE 802.1AX LACP is used between CEs and PEs.
Type 2 (T=0x02) - is used in the case of indirectly connected hosts (bridged LAN). The ESI value is automatically generated by the L2 bridge protocol.
Type 3 (T=0x03) - is a MAC-based ESI value that is auto-generated or configured by the network operator.
Type 4 (T=0x04) - is a Router-ID based ESI value that is auto-generated or configured by the network operator.
Type 5 (T=0x05) - this is AS based ESI value that is auto generated or configured by the network operator.
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