How does a bridge know which ports belong to a specific VLAN? By configuration. Hence, VLAN is not an overlay network virtualization technology. Thus, spines are not relieved from any duties when it comes to packet forwarding and knowledge of virtual networks. This is done by every node in the packet’s path as it inexorably moves from the source to the destination. It also records that (red VLAN, MAC A) is reachable via Port A in its MAC forwarding table. It filters out Port A from this list of ports. So, in Figure 4-1, when node L1 receives a packet from A destined to E, and it does not have an entry for (red VLAN, MAC E) in its MAC forwarding table, it floods the packet out all ports which belong to the red VLAN. The other thing the bridge does is record the (VLAN, source MAC address) in the MAC forwarding table as being reachable on the ingress port. This filtering is called self-forwarding check. However, the bridge filters out the ingress port from this list of ports the packet is flooded to. What this means is that whenever a bridge receives a packet whose combination of VLAN and destination MAC address is not in the MAC forwarding table, the bridge sends the packet out every port that is in the same VLAN as the incoming packet. 802.1Q bridges use what is called “ flood-and-learn” to populate the MAC forwarding table. If a packet is received without a VLAN tag, the bridge port has a default VLAN tag associated with it. Similarly, we’ll assume that L1 connects to host A on Port A.Īn 802.1Q bridge forwards a packet based on the VLAN and the destination MAC address of the packet. To simplify the mappings a reader has to maintain, we’ll refer to the MAC address of A as MAC A. In an 802.1Q bridge, these virtual networks will be VLANs. There are two virtual networks, red and blue. Hosts (or endpoints) A through F are attached to the leaves. The figure shows a simple Clos network with two spine switches labeled S1 and S2 and four leaf switches labeled L1 through L4.

A compare-and-contrast table at the end helps solidify these concepts further.įor the sake of this discussion, let us assume the network shown inįigure 4-1. We then use this as the basis to describe how EVPN solves the same problem, albeit differently. We also introduce the terminology commonly used in bridging. IEEE 802.1Q defined the standard that describes how VLAN-aware bridges work, and so traditional bridging is also called 802.1Q bridging or plain. To understand how bridging works with EVPN, we begin with a quick study of how bridging works in a traditional 802.1Q bridge.

Cumulus linux show mac address table