BGP EVPN TYPE-1 & TYPE-4 in action

 

🧠 Quick Recap Before Type-4

When an ESI (Ethernet Segment Identifier) is configured on dual-homed leafs:

• Each leaf advertises:
  ✅ Type-1 Per-ESI → ESI discovery & mass withdrawal
  ✅ Type-1 Per-EVI → VLAN (EVI) participation, per-VLAN control

• Remote VTEPs use these to know who shares the ESI.

Now, to decide which leaf forwards BUM traffic (Broadcast, Unknown, Multicast) for each VLAN,
they use Type-4 routes.

---

⚙️ What is a Type-4 Route?

Route Type	Name	Purpose
Type-4	Ethernet Segment Route (ES Route)	Used for DF election between PEs sharing the same ESI

---

🔹 Content of Type-4 Route

Each PE advertising a shared ESI sends:

EVPN Type-4 Route
-----------------
- ESI: 0000:0000:0000:0000:000010
- Originating Router’s IP (VTEP IP)
- Route Distinguisher
- Next Hop: VTEP IP
- Extended Communities:
    - ES-Import RT (derived from ESI)
    - DF Algorithm (RFC7432 or RFC8584)

---

🧭 Putting It Together (Flow of Events)

Let’s take this topology again:

        +----------- Spine (RR) -----------+
        |                                 |
     +--+--+                           +--+--+
     |Leaf1|                           |Leaf2|
     +--+--+                           +--+--+
        |                                 |
        +--------- Shared Link -----------+
                  (ESI = 10)
                      |
                   +------+
                   |HostA |
                   +------+
         VLAN 10, VLAN 20

---

Step-by-Step DF Election Process

Step	Action	EVPN Route	Description
1	Leaf1, Leaf2 detect they share ESI=10	Type-1 Per-ESI	Discover multihomed peers
2	Both advertise VLANs on that ESI	Type-1 Per-EVI (Ethernet Tag = VLAN ID)	Indicates which VLANs are active
3	Both advertise ES route	Type-4 (ES Route)	Used for DF election
4	Each PE learns others’ Type-4s	—	Now both know the list of PEs in the ESI group
5	Run DF election algorithm	(default = Modulus algorithm)	Deterministic: based on VLAN ID & VTEP IP
6	Result → per-VLAN DF assignment	—	One leaf becomes DF for VLAN 10, the other for VLAN 20
7	DF starts forwarding BUM; non-DF blocks BUM	—	Unicast flows are unaffected

---

🧮 DF Election Logic Example (Default Modulus)

DF = (VLAN-ID mod Number-of-PEs)

With 2 PEs:

VLAN	Calculation	DF
VLAN 10	10 mod 2 = 0	Leaf1
VLAN 20	20 mod 2 = 0	Leaf1
VLAN 11	11 mod 2 = 1	Leaf2
VLAN 21	21 mod 2 = 1	Leaf2

➡️ So DF role alternates per VLAN.

⚙️ Modern implementations use HRW (Highest Random Weight) algorithm (RFC 8584), which is more stable.

---

🧩 Relationship Between Route Types

Function	Route Type	Advertised per	Key Use
ESI Discovery	Type-1 (Per-ESI)	Once per ESI	Identify multihomed peers
VLAN Participation	Type-1 (Per-EVI)	Once per VLAN	Input to DF election
DF Election	Type-4	Once per ESI	Compute DF/non-DF per VLAN
MAC/IP Reachability	Type-2	Per host	Learned over active PE(s)
Mass Withdrawal	Type-1 Withdrawal	On ESI failure	Flush stale MACs

---

🛰️ Example Control Plane View (Simplified)

Leaf1 → Spine:
  Type-1 (ESI=10, ETAG=0)
  Type-1 (ESI=10, ETAG=10)
  Type-1 (ESI=10, ETAG=20)
  Type-4 (ESI=10, Origin=10.10.10.1)

Leaf2 → Spine:
  Type-1 (ESI=10, ETAG=0)
  Type-1 (ESI=10, ETAG=10)
  Type-1 (ESI=10, ETAG=20)
  Type-4 (ESI=10, Origin=10.10.10.2)

When both Type-4 routes are received, each leaf runs DF election and decides:

VLAN	DF	Non-DF
VLAN 10	Leaf1	Leaf2
VLAN 20	Leaf2	Leaf1

---

🧠 Summary

Route	Scope	Key Function
Type-1 Per-ESI	1 per ESI	ESI discovery & mass withdrawal
Type-1 Per-EVI	1 per VLAN	DF election input (per-VLAN participation)
Type-4	1 per ESI	DF election control (which PE is DF)