In addition to Layer 2 virtualization, the SPBM model is extended to also support Routed SPBM, otherwise called SPBM IP Shortcuts.
Unlike Layer 2 VSN, with SPBM IP shortcuts, no I-SID configuration is required. Instead, SPBM nodes propagate Layer 3 reachability as “leaf” information in the IS-IS LSPs using Extended IP reachability TLVs (TLV 135), which contain routing information such as neighbors and locally configured subnets. SPBM nodes receiving the reachability information can use this information to populate the routes to the announcing nodes. All TLVs announced in the IS-IS LSPs are grafted onto the shortest path tree (SPT) as leaf nodes.
The following figure shows a network running SPBM IP shortcuts.
In this example, BEB A receives a packet with a destination IP address in the subnet of VLAN 14 and knows to forward the packet to BEB D based on the IP route propagation within IS-IS. After a route lookup, BEB A knows that BEB D is the destination for the subnet and constructs a new B-MAC header with destination B-MAC: D. BCBs B and C need only perform normal Ethernet switching to forward the packet to BEB D. A route lookup is only required once, at the source BEB, to identify BEB D as the node that is closest to the destination subnet.
In contrast to IP routing or Multiprotocol Label Switching (MPLS), SPBM IP shortcuts provide a simpler method of forwarding IP packets in an Ethernet network using the preestablished Ethernet FIBs on the BEBs. SPBM allows a network to make the best use of routing and forwarding techniques, where only the BEBs perform an IP route lookup and all other nodes perform standard Ethernet switching based on the existing SPT. This allows for end to end IP-over-Ethernet forwarding without the need for ARP, flooding, or reverse learning.
In the previous example, the SPBM nodes in the core that are not enabled with IP shortcuts can be involved in the forwarding of IP traffic. Since SPBM nodes only forward on the MAC addresses that comprise the B-MAC header, and since unknown TLVs in IS-IS are relayed to the next hop but ignored locally, SPBM nodes need not be aware of IP subnets to forward IP traffic.
With IP shortcuts, there is only one IP routing hop, as the SPBM backbone acts as a virtualized switching backplane.
The following figure shows a sample campus network implementing SPBM IP shortcuts.
To enable IP shortcuts on the BEBs, you can configure a circuitless IP address (loopback address) and specify this address as the IS-IS source address. This source address is automatically advertised into IS-IS using TLV 135.
In addition, to advertise routes from the BEBs into the SPBM network, you must enable route redistribution of direct, static, OSPF, RIP, or BGP routes into IS-IS. To advertise IPv6 routes from the BEBs into the SPBM network, you must enable route redistribution of IPv6 direct, IPv6 static, and OSPFv3 routes into IS-IS.