BGP EVPN VXLAN Intra VNI Communication - MAC Learning Process

BGP EVPN VXLAN Intra VNI Communication - MAC Learning Process

BGP EVPN VXLAN Control Plane

In this section, We will learn the control plane and Data Plane of BGP VXLAN EVPN in two parts.

  1. BGP VXLAN EVPN – Intra VNI Communication ( MAC / MAC-IP Learning Process )
  2. BGP VXLAN EVPN – Inter VNI Communication

We will also see , difference between usage of MAC and MAC/IP reachability Information.

BGP VXLAN EVPN (Intra VNI) Control plane Process

For Intra VNI Communication to happen, MAC Advertisement from Leaf to Leaf happens, first we will see , how Mac Adress learning happens and then are advertised to other Peers in VXLAN fabric.

MAC Address Learning Process:

When Host is connected to Leaf switch, Local VTEP learns the MAC address from Ingress frame and install this information in to MAC address table as well as VNI Specific layer 2 Routing Information base ( L2RIB) also called as MAC VRF and then gets advertised between leaf switches by using BGP EVPN Route-type 2 ( MAC Advertisement Route ) update.

Below figure explains BGP EVPN Control plane operational MAC advertisement.

Phase 1 : MAC Address table Update

As soon as Host VM BEEF power On and Comes UP, it will send the GARP packet  and Leaf 101 will receive this packet on E1/2 Interface and thus stores this MAC address information in to MAC address table.

Below command shows the AMC address table of leaf 101

Phase 2 L2RIB Update:

Now once MAC address of Host BEEF is stored in MAC table , this MAC will also get installed in to VNI1000 Specific L2RIB by Layer 2 Forwarder ( L2FWDER ) . This VNI specific L2RIB is also called as MAC VRF.

Below figure shows L2RIB of L2VNI 10000 on leaf 101.

The Process below shows  How L2FWDER component notice new Mac address information on Eth1/2 ( Interface Index 0x1a000200) . The received frame has 802.1Q tag of VLAN id 10 and it is due to Vlan 10 , L2FWDER knows that MAC address info belongs to VNI1000.

 Phase 3: BGP MAC Route Export on Local VTEP

Now in this phase, Leaf 101, will export the MAC address of host BEEF from L2RIB to BGP Loc-RIB , from where , it gets installed in to Adj-RIB-Out via Output Policy engine. The BGP Process of Leaf-101 attaches the path attributes based on BGP peer type (iBGP/eBGP/RR-Client) and sends BGP EVPN Route Type 2 update to Spine -11. Spine -11 forwards this update to rest of its RR-Clients ( in this case leaf-102 ).

The BGP path attributes “MP_REACH_NLRI”, which is carried with the BGP updates, contains the NLRI information , where address information includes MAC address along with RD for all MAC in VNI 10000.

We have already discussed how RD and RT are calculated. There is also a MPLS label stack 1, in NLRI , which includes the L2VPN identifier. MPLS label stack = 1: VNI = 1:10000.

The Update message also includes two BGP extended community attributes. First attributes is Route target , which is used for route export/import by VTEP switches. Second attribute is Encapsulation type , which defines encapsulation used in Data Plane ( TYPE 8 = VXLAN ).

Below figure explains the BGP EVPN attributes that are advertised along with MAC of host BEEF.

In above figure , following are the BGP attributes attached while it is advertised.

  • Route Distinguisher
  • [2] - BGP EVPN Route-Type 2, MAC/IP Advertisement Route
  • [0] - Ethernet Segment Identifier (ESI), all zeroed out = single homed site
  • [0] - Ethernet Tag Id, EVPN routes must use value 0
  • [48] - Length of MAC address
  • [1000.0010.beef] - MAC address
  • [0] - Length of IP address
  • [] - Carried IP address
  • /216 - Length of the MAC VRF NLRI in bits: RD (8 octets) + MAC address (6 octets) + L2VNI Id (3 octets) + ESI (10 octets) = 27 octets = 216 bits.

Below figure explains some more in depth information, like Next hop address and MPLS Label stack ( L2VNI ID) , which are only visible in HEX portion of the capture

  • Next HOP: HEX c0 a8 64 65 = BIN
  • MPLS Label Stack 1: HEX 00 27 10 = 10000( L2VNI ID )

Phase 4: BGP L2VPN MAC route import on Remote VTEP:

Once VTEP switch Leaf-102 receives the MAC Route advertisement and install it in to Adj-Rib-In database , routes gets imported in to Loc-RIB based on EVPN policy ( Import RT : 65000:10000). During this Process RD of the received routes will get changed to Local RD if necessary.

In this example subnet in Leaf-101 has VLAN-Id 10 while in Leaf-102 VLAN-Id for the same subnet is 20. This means that the original RD for L2VNI10000 generated by Leaf-101 will change by Leaf-102 to when it moves route from Adj-RIB-In into Loc-RIB as L2VNI specific route.

Below example , shows the BGP table of leaf-102 , the Upper RIB entry is original NLRI , while lower RIB shows Modified RD = Local RD for same NLRI.

Now once the Route is installed in to Loc-RIB , with its local RD , the Route is sent from local-RIB to L2RIB.


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