In a BGP EVPN VXLAN design, even if BGP EVPN peers exchange routes successfully, the remote VTEP must be able to reach the BGP next-hop (typically the source VTEP’s loopback) via the underlay network to program the VXLAN tunnel. An unreachable next-hop in the underlay IGP (OSPF in this case) is a common and fundamental issue that prevents the installation of MAC/IP routes into the forwarding table, making it the most logical first step in troubleshooting.

An SR-TE Policy with an explicitly defined path (using a list of SIDs or adjacency SIDs) is the correct tool to steer traffic along a specific, deterministic path that is independent of the IGP’s calculated shortest path. This allows the engineer to manually define a route that circumvents the congested link, providing precise traffic engineering control.

Enabling NetStream aggregation is the most effective way to reduce the volume of exported data while maintaining visibility. Aggregation combines multiple raw flows that share common characteristics (like source/destination IP pairs) into a single aggregated record before export. This significantly reduces the number of records sent to the collector, alleviating the overload, while still providing meaningful traffic pattern information for analysis.

The Site of Origin (SOO) extended community is used to identify routes originating from a particular VPN site. When a PE receives a route from a CE, it tags it with an SOO. If the PE later receives the same route from another PE (via MP-BGP), it checks the SOO. If the SOO matches one configured for a local CE site, the PE discards the route. This prevents a routing loop where a route could be advertised from one CE to the PE, across the MPLS core to another PE, and then back down to another CE at the same physical site, which is crucial during failover scenarios.

BGP-LS (BGP Link-State) is a key protocol in SDN architectures. Its core function is for network devices (acting as BGP speakers) to collect the detailed link-state information from their IGP (OSPF or IS-IS) databases, aggregate it, and then advertise this comprehensive network topology and attribute information to a central controller. This gives the controller a complete, real-time view of the network’s state, which it uses for path computation, optimization, and service provisioning.

To run IS-IS in an IPv6-only underlay while also supporting Segment Routing (for MPLS data plane, i.e., SR-MPLS), you need IS-IS Multi-Topology (MT). MT allows IS-IS to maintain separate topologies (and SPF calculations) for different address families. You would typically enable MT for IPv6 unicast (MT-ID 2) to build the IPv6 underlay, while also enabling the SR extensions (which are topology-agnostic) to distribute Segment Routing information like Segment IDs (SIDs).

The core architectural difference is in how the path is defined and signaled. LDP is a separate protocol that passively assigns and distributes labels based on the existing IGP shortest path. The path is implicit (follow IGP). In contrast, Segment Routing encodes the path as an ordered list of instructions (segments) in the packet header. The source (or an SR-TE controller) determines this path. The IGP or BGP distributes the segment identifiers (SIDs), but the path itself is carried in the packet, making the network stateless with respect to flows.

Scalability is the primary reason. In a full-mesh BGP design, every VTEP must establish a BGP session with every other VTEP. This results in n*(n-1)/2 sessions, which becomes unmanageable as the data center grows (e.g., hundreds of VTEPs). Deploying Route Reflectors allows each VTEP to establish sessions only with one or a few RRs (O(n) sessions). The RRs then reflect EVPN routes to all other VTEPs, maintaining the necessary control plane information exchange while drastically improving scalability and manageability.

Virtual links are a temporary design workaround, not a best practice for stable production networks. Their major drawback is that they make the backbone area (Area 0) dependent on the stability and topology of a non-backbone transit area. If the transit area experiences instability or re-convergence, it directly impacts the virtual link and, by extension, the integrity of the backbone. This creates a fragile design that is harder to troubleshoot and contradicts the principle of maintaining a simple, robust backbone.

In a BGP EVPN VXLAN design, even if BGP EVPN peers exchange routes successfully, the remote VTEP must be able to reach the BGP next-hop (typically the source VTEP’s loopback) via the underlay network to program the VXLAN tunnel. An unreachable next-hop in the underlay IGP (OSPF in this case) is a common and fundamental issue that prevents the installation of MAC/IP routes into the forwarding table, making it the most logical first step in troubleshooting.0