BGP in IP Fabric:

In an IP fabric, Border Gateway Protocol (BGP) is used to manage the routing between leaf and spine devices. Each device can have the same or different Autonomous System (AS) numbers depending on the network design.

Multipath Multiple-AS:

Option B: If every device in the fabric has a different AS number, then enabling Equal-Cost Multi-Path (ECMP) routing requires the multipath multiple-as statement. This configuration allows BGP to consider multiple paths across different AS numbers as equal cost, enabling efficient load balancing across the network.

Same AS Number Configuration:

Option A: It's possible for all leaf devices to use the same AS number in an IP fabric, which simplifies the configuration. EBGP (External BGP) will still function correctly in this setup because BGP considers the peering relationship rather than strictly enforcing different AS numbers in this specific use case.

Conclusion:

Option B: Correct---This statement is essential for enabling ECMP in a multi-AS environment.

Option A: Correct---Leaf devices can share the same AS number without needing special EBGP configuration.

Understanding the Configuration:

The exhibit shows a BGP configuration on a Border-Leaf device. The BGP group UNDERLAY is used for the underlay network, OVERLAY for EVPN signaling, and PROVIDER for connecting to the provider network.

The OVERLAY group has the accept-remote-nexthop statement, which is designed to accept the next-hop address learned from the remote peer as is, without modifying it.

Problem Identification:

The BGP session to Border-Leaf-2 is not established. A common issue in EVPN-VXLAN environments is related to next-hop reachability, especially when accept-remote-nexthop is configured.

In typical EVPN-VXLAN setups, the next-hop address should be reachable within the overlay network. However, the accept-remote-nexthop can cause issues if the next-hop IP address is not directly reachable or conflicts with the expected behavior in the overlay.

Corrective Action:

delete protocols bgp group OVERLAY accept-remote-nexthop: Removing this command will ensure that the device uses its own IP address as the next-hop in BGP advertisements, which is standard practice in many EVPN-VXLAN setups. This change should help establish the BGP session with Border-Leaf-2. Data Center

Reference: Proper handling of BGP next-hop attributes is critical in establishing and maintaining stable BGP sessions, especially in complex multi-fabric environments like EVPN-VXLAN. Removing accept-remote-nexthop aligns with best practices in many scenarios.

Understanding EVPN-VXLAN Architectures:

EVPN-VXLAN overlays allow for scalable Layer 2 and Layer 3 services in modern data centers.

CRB (Centralized Routing and Bridging): In this architecture, the Layer 3 routing is centralized on spine devices, while the leaf devices focus on Layer 2 switching and VXLAN tunneling. This setup is optimal when the goal is to centralize routing for ease of management and to avoid complex routing at the leaf level.

ERB (Edge Routing and Bridging): This architecture places routing functions on the leaf devices, making it a distributed model where each leaf handles routing for its connected hosts.

Architecture Choice for Spine Routing:

Given the requirement to ensure Layer 3 routing happens on the spine devices, the CRB (Centralized Routing and Bridging) architecture is the correct choice. This configuration offloads routing tasks to the spine, centralizing control and potentially simplifying the overall design.

With CRB, the spine devices perform all routing between VXLAN segments. Leaf switches handle local switching and VXLAN encapsulation, but routing decisions are centralized at the spine level.

This model is particularly advantageous in scenarios where centralized management and routing control are desired, reducing the complexity and configuration burden on the leaf switches.

Data Center

Reference:

The CRB architecture is commonly used in data centers where centralized control and simplified management are key design considerations. It allows the spines to act as the primary routing engines, ensuring that routing is handled in a consistent and scalable manner across the fabric.

Understanding the Exhibit Setup:

The network diagram shows an EVPN-VXLAN setup, a common design for modern data centers enabling Layer 2 and Layer 3 services over an IP fabric.

Leaf1 and Leaf2 are the leaf switches connected to Server1 and Server2, respectively, with each server in a different subnet (172.16.1.0/24 and 172.16.2.0/24).

Spine1 and Spine2 are part of the IP fabric, interconnecting the leaf switches.

EVPN-VXLAN Basics:

EVPN (Ethernet VPN) provides Layer 2 and Layer 3 VPN services using MP-BGP.

VXLAN (Virtual Extensible LAN) encapsulates Layer 2 frames into Layer 3 packets for transmission across an IP network.

VTEP (VXLAN Tunnel Endpoint) interfaces on leaf devices handle VXLAN encapsulation and decapsulation.

Integrated Routing and Bridging (IRB):

IRB interfaces are required on leaf1 and leaf2 (where the endpoints are directly connected) to route between different subnets (in this case, between 172.16.1.0/24 and 172.16.2.0/24).

The IRB interfaces provide the necessary L3 gateway functions for inter-subnet communication.

Traffic Flow Analysis:

Traffic from Server1 (172.16.1.1) destined for Server2 (172.16.2.1) must traverse from leaf1 to leaf2.

The traffic will be VXLAN encapsulated on leaf1, sent over the IP fabric, and decapsulated on leaf2.

Since the communication is between different subnets, the IRB interfaces on leaf1 and leaf2 are crucial for routing the traffic correctly.

Correct Statements:

C . An IRB Interface must be configured on leaf1 and leaf2: This is necessary to perform the inter-subnet routing for traffic between Server1 and Server2.

D . Traffic from server1 to server2 will transit the VXLAN tunnel between leaf1 and leaf2: This describes the correct VXLAN operation where the traffic is encapsulated by leaf1 and decapsulated by leaf2.

Data Center

Reference:

In EVPN-VXLAN architectures, the leaf switches often handle both Layer 2 switching and Layer 3 routing via IRB interfaces. This allows for efficient routing within the data center fabric without the need to involve the spine switches for every routing decision.

The described traffic flow aligns with standard EVPN-VXLAN designs, where direct VXLAN tunnels between leaf switches enable seamless and scalable communication across a data center network.

Understanding the Configuration:The configuration shown in the exhibit involves an EVPN (Ethernet VPN) setup using BGP as the routing protocol. The export policy named CHANGE_NH is applied to the BGP group evpn-peer, which includes a rule to change the next hop for routes that match the policy.Issue with Next Hop Not Changing:The policy CHANGE_NH is correctly configured to change the next hop to 203.0.113.10 for the matching routes. However, the next hop remains unchanged when advertising EVPN routes to the peer 203.0.113.2.Reason for the Issue:In Junos OS, when exporting routes for VPNs (including EVPN), the next-hop change defined in a policy will not take effect unless the vpn-apply-export parameter is used in the BGP configuration. This parameter ensures that the export policy is applied specifically to VPN routes.The vpn-apply-export parameter must be included to apply the next-hop change to EVPN routes.Answer: Explanation:D . The vpn-apply-export parameter must be applied to this peer: This is the correct solution because the next hop in EVPN routes won't be altered without this parameter in the BGP configuration. It instructs the BGP process to apply the export policy to the EVPN routes.Data Center

Reference:This behavior is standard in EVPN deployments with Juniper Networks devices, where the export policies applied to VPN routes require explicit invocation using vpn-apply-export to take effect.

Understanding ERB Architecture:

ERB (Edge Routed Bridging) architecture is a network design where the routing occurs at the edge (leaf devices) rather than in the spine devices. In a VXLAN overlay network with EVPN as the control plane, leaf devices typically act as both Layer 2 (L2) and Layer 3 (L3) VXLAN gateways.

Placement of VXLAN Gateways:

Option B: All leaf devices will have L2 VXLAN gateways to handle the bridging of VLAN traffic into VXLAN tunnels.

Option C: All leaf devices will also have L3 VXLAN gateways to route traffic between different VXLAN segments (VNIs) and external networks.

Option E: Spine devices in an ERB architecture generally do not function as VXLAN gateways. They primarily focus on forwarding traffic between leaf nodes and do not handle VXLAN encapsulation/decapsulation.

Conclusion:

Option B: Correct---All leaf devices will have L2 VXLAN gateways.

Option C: Correct---All leaf devices will have L3 VXLAN gateways.

Option E: Correct---Spine devices will not act as VXLAN gateways

DHCP Behavior in Factory-Default Configuration:

Option B: In the factory-default configuration, Juniper switches are designed to send DHCP requests on all operational interfaces. This behavior ensures that the switch can obtain an IP address for management and further configuration from any available DHCP server.

Option D: The DHCP server can provide additional configuration parameters, including the required Junos version. This allows for automated provisioning and ensures that the switch is running the correct software version.

Conclusion:

Option B: Correct---The switch will use any operational interface to request an IP address via DHCP.

Option D: Correct---The DHCP server can specify Junos version requirements, enabling automated software management.

Requirements for AI Workloads:

The scenario requires a network that supports at least 400 Gbps Ethernet and does not require Layer 2 VLANs. This setup is well-suited for a pure Layer 3 network, which can efficiently route traffic between devices without the overhead or complexity of maintaining Layer 2 domains.

Choosing the Right Network Architecture:

Option D: An IP fabric using EBGP (External BGP) is ideal for this scenario. In a typical IP fabric, EBGP is used to handle routing between spine and leaf switches, creating a scalable and efficient network. Since there is no need for Layer 2 VLANs, the pure IP fabric design with EBGP provides a straightforward and effective solution.

Options A, B, and C involve more complex architectures (like VXLAN or EVPN), which are unnecessary when there's no requirement for Layer 2 overlays or VLANs.

Conclusion:

Option D: Correct---An IP fabric with EBGP is the most suitable and straightforward architecture for a network that needs to support high-speed AI workloads without Layer 2 VLANs.