page_title: IEEE 802.1aq
section_title: Example
hierarchical_section_title: IEEE 802.1aq / Details / Example
caption_attribution_description: English: Figure 2 - two ECMP paths between nodes 7 and 5.
context_page_description: Shortest Path Bridging, specified in the IEEE 802.1aq standard, is a computer networking technology intended to simplify the creation and configuration of networks, while enabling multipath routing.
  It is the replacement for the older spanning tree protocols: IEEE 802.1D, IEEE 802.1w, IEEE 802.1s. These blocked any redundant paths that could result in a layer 2 loop, whereas SPB allows all paths to be active with multiple equal cost paths, provides much larger layer 2 topologies, supports faster convergence times, and improves the efficiency by allowing traffic to load share across all paths of a mesh network. It is designed to virtually eliminate human error during configuration and preserves the plug-and-play nature that established Ethernet as the de facto protocol at layer 2.
  The technology provides logical Ethernet networks on native Ethernet infrastructures using a link state protocol to advertise both topology and logical network membership. Packets are encapsulated at the edge either in media access control-in-media access control 802.1ah or tagged 802.1Q/802.1ad frames and transported only to other members of the logical network.
context_section_description: We will work through SPBM behavior on a small example, with emphasis on the shortest path trees for unicast and multicast.
  The network shown in Figure 1 consists of 8 participating nodes numbered 0 through 7. These would be switches or routers running the IEEE 802.1aq protocol. Each of the 8 participating nodes has a number of adjacencies numbered 1..5. These would likely correspond to interface indexes, or possibly port numbers. Since 802.1aq does not support parallel interfaces each interface corresponds to an adjacency. The port / interface index numbers are of course local and are shown because the output of the computations produce an interface index (in the case of unicast) or a set of interface indexes (in the case of multicast) which are part of the forwarding information base (FIB) together with a destination MAC address and backbone VID.
  The network has a fully meshed inner core of four nodes (0..3) and then four outer nodes (4,5,6 and 7), each dual-homed onto a pair of inner core nodes.
  Normally when nodes come from the factory they have a MAC address assigned which becomes a node identifier but for the purpose of this example we will assume that the nodes have MAC addresses of the form 00:00:00:00:N:00 where N is the node id (0..7) from Figure 1. Therefore, node 2 has a MAC address of 00:00:00:00:02:00. Node 2 is connected to node 7 (00:00:00:00:07:00) via node 2's interface/5.
  The IS-IS protocol runs on all the links shown since they are between participating nodes. The IS-IS hello protocol has a few additions for 802.1aq including information about backbone VIDs to be used by the protocol. We will assume that the operator has chosen to use backbone VIDs 101 and 102 for this instance of 802.1aq on this network.
  The node will use their MAC addresses as the IS-IS SysId and join a single IS-IS level and exchange link state packets (LSPs in IS-IS terminology). The LSPs will contain node information and link information such that every node will learn the full topology of the network. Since we have not specified any link weights in this example, the IS-IS protocol will pick a default link metric for all links, therefore all routing will be minimum hop count.
  After topology discovery the next step is distributed calculation of the unicast routes for both ECMP VIDs and population of the unicast forwarding tables (FIBs).
  Consider the route from Node 7 to Node 5: there are a number of equal cost paths. 802.1aq specifies how to choose two of them: the first is referred to as the Low PATH ID path. This is the path which has the minimum node id on it. In this case the Low PATH ID path is the 7->0->1->5 path (as shown in red in Figure 2). Therefore, each node on that path will create a forwarding entry toward the MAC address of node five using the first ECMP VID 101. Conversely, 802.1aq specifies a second ECMP tie breaking algorithm called High PATH ID. This is the path with the maximum node identifier on it and in the example is the 7->2->3->5 path (shown in blue in Figure 2).
  Node 7 will therefore have a FIB that among other things indicates:
  MAC 00:00:00:05:00 / vid 101 the next hop is interface/1.
  MAC 00:00:00:05:00 / vid 102 the next hop is interface/2.
  Node 5 will have exactly the inverse in its FIB:
  MAC 00:00:00:07:00 / vid 101 the next hop is interface/1.
  MAC 00:00:00:07:00 / vid 102 the next hop is interface/2.
  The intermediate nodes will also produce consistent results so for example node 1 will have the following entries.
  MAC 00:00:00:07:00 / vid 101 the next hop is interface/5.
  MAC 00:00:00:07:00 / vid 102 the next hop is interface/4.
  MAC 00:00:00:05:00 / vid 101 the next hop is interface/2.
  MAC 00:00:00:05:00 / vid 102 the next hop is interface/2.
  And Node 2 will have entries as follows:
  MAC 00:00:00:05:00 / vid 101 the next hop is interface/2.
  MAC 00:00:00:05:00 / vid 102 the next hop is interface/3.
  MAC 00:00:00:07:00 / vid 101 the next hop is interface/5.
  MAC 00:00:00:07:00 / vid 102 the next hop is interface/5.
  If we had an attached non participating device at Node 7 talking to a non participating device
caption: Figure 2 - two ECMP paths between nodes 7 and 5
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