36 – VXLAN EVPN Multi-Fabrics with External Routing Block (part 2)


I recommend you to read part 1 if you missed it 🙂

thank you, yves

VXLAN EVPN Multi-Fabric with External Active/Active Gateways

The first use case is simple. Each VXLAN fabric behaves like a traditional Layer 2 network with a centralized routing block. External devices (such as routers and firewalls) provide default gateway functions, as shown in Figure 1.

Figure 8: External Routing Block IP Gateway for VXLAN/EVPN Extended VLAN

Figure 1: External Routing Block IP Gateway for VXLAN/EVPN Extended VLAN

In the Layer 2–based VXLAN EVPN fabric deployment, the external routing block is used to perform routing functions between Layer 2 segments. The same routing block can be connected to the WAN advertising the public networks from each data center to the outside and to propagate external routes to each fabric.

The routing block consists of a “router-on-a-stick” design (from the fabric’s point of view) built with a pair of traditional routers, Layer 3 switches, or firewalls that serve as the IP gateway. These IP gateways are attached to a pair of vPC border nodes that initiate and terminate the VXLAN EVPN tunnels.

Connectivity between the IP gateways and the border nodes is achieved through a Layer 2 trunk carrying all the VLANs that require routing services.

To improve performance with active default gateways in each data center, reducing the hairpinning of east-west traffic for server-to-server communication between sites, and depending on the IP gateway platform of choice, the routing block can be duplicated with the same virtual IP and MAC addresses for all relevant SVIs on both sides. Hence, to use active-active gateways on both fabrics, you must filter communications between gateways that belong to the same First-Hop Routing Protocol (FHRP) group. With OTV as the DCI solution, the FHRP filter will be applied to the OTV control plane.

Figure 2 shows this scenario.

Note: Although VLANs are locally significant per edge device (or even per port), and the Layer 2 virtual network identifier (VNI) is locally significant in each VXLAN EVPN fabric, the following examples assume that the same Layer 2 VNIs (L2VNIs) are reused on both fabrics. The same VLAN ID was also reused on each leaf node and on the border switches. This approach was used to simplify the diagrams and packet walk. In a real production network, the network manager can use different network identifiers for the Layer 2 and 3 VNIs deployed in the individual fabrics.

Figure 9- VXLAN EVPN Layer 2 Fabric with External Routing Block

Figure 9: VXLAN EVPN Layer 2 Fabric with External Routing Block

Figure 2: VXLAN EVPN Layer 2 Fabric with External Routing Block

Figure 2 shows the following:

Note: The basic assumption is that H1 and H2 have already populated their ARP tables with the default gateway information, for example because they previously sent ARP requests targeted to the gateway. As a consequence, the gateway also has H1 and H2 information in its local ARP table.

  1. Host H1 connected to leaf L11 in fabric 1 needs to send a data packet to host H2 connected in vPC mode to a pair of leaf nodes, L12 and L13, in the same fabric 1. Because H1 and H2 are part of different IP subnets, H1 sends the traffic to the MAC address of its default gateway, which is deployed on an external Layer 3 device connected to the same fabric 1. The communication between H1 and the default gateway uses the VXLAN fabric as a pure Layer 2 overlay service.
  2. Traffic from H1 that belongs to VLAN 100 is VXLAN encapsulated by leaf L11 with L2VNI 10100 (locally mapped to VLAN 100) and sent to the egress anycast VTEP address defined in border nodes 1 and 2. This address represents the next hop to reach the MAC address of the default gateway. Layer 3 equal-cost multipath (ECMP) is used in the fabric to load-balance the traffic destined for the egress anycast VTEP between the two border nodes. In the example in Figure 9, border node BL1 is selected as the destination.
  3. BL1 decapsulates the VXLAN frame and bridges the original frames destined for the local default gateway onto VLAN 100 (locally mapped to L2VNI 10100).
  4. The default gateway receives the frame destined for H2, performs a Layer 3 lookup, and subsequently forwards the packet to the Layer 2 segment on which H2 resides.
  5. The Layer 2 flow reaches one of the vPC-connected border nodes (BL2 in this example). BL2 uses the received IEEE 802.1q tag (VLAN 200) to identify the locally mapped L2VNI, 10200, to be used for VXLAN to encapsulates the frame. BL2 then forwards the data packet to the anycast VTEP address defined on the vPC pair of leaf nodes, L12 and L13, on which the destination H2 is connected.
  6. One of the receiving leaf nodes is designated to decapsulate the VXLAN frame and send the original data packet to H2.
  7. Routed communications between endpoints located at the remote site are kept local within fabric 2. This behavior is possible only because FHRP filtering is enabled on the OTV edge devices. In this example, H4 sends traffic destined for H6 using its local default gateway active in data center DC2. East-west routed traffic can consequently be localized within each fabric, eliminating unnecessary interfabric traffic hairpinning.


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