SMLT and RSMLT in Layer 3 environments shows a typical redundant network example with user aggregation, core, and server access layers. To minimize the creation of many IP subnets, one VLAN (VLAN 1, IP subnet A) spans all wiring closets.
SMLT provides the loop-free topology and forwards all links for VLAN 1, IP subnet A.
The aggregation layer switches are configured with routing enabled and provide active-active default gateway functionality through RSMLT.
After you enable RSMLT on a VLAN (on both aggregation devices), the cluster devices simply inform each other (over vIST messaging) of their physical IP and MAC on that VLAN. Thereafter, the two cluster devices take mutual ownership of their IP addresses on that VLAN. This action means each cluster device routes IP traffic that is directed to the physical MAC of the IP or the physical MAC of the peer IP on that VLAN, and when one of them is down the other cluster device:
Replies to ARP requests for both the IP and the peer IP on that VLAN
Replies to pings to the IP and the peer IP on that VLAN
In this case, routers R1 and R2 forward traffic for IP subnet A. RSMLT provides both router failover and link failover. For example, if the Split MultiLink Trunk link between R2 and R4 is broken, the traffic fails over to R1 as well.
For IP subnet A, VRRP with a backup master can provide the same functionality as RSMLT, as long as no additional router is connected to IP subnet A.
RSMLT provides superior router redundancy in core networks (IP subnet B), where OSPF is used for the routing protocol. Routers R1 and R2 provide router backup for each other, not only for the edge IP subnet A, but also for the core IP subnet B. Similarly routers R3 and R4 provide router redundancy for IP subnet C and also for core IP subnet B.