Design Reference

RSMLT provides superior router redundancy in core networks (for example, IP subnet B) in which
OSPF is used. 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.
RSMLT router failure and recovery
This section describes the failure and recovery of router R1 in
SMLT and RSMLT in Layer 2 and 3
environments on page 55.
R3 and R4 both use R1 as their next-hop to reach IP subnet A. Even though R4 sends packets to
R2, these packets are routed directly to subnet A at R2. R3 sends packets towards R1; these
packets are also sent directly to subnet A. After R1 fails, with the help of SMLT, all packets are
directed to R2. R2 provides routing for R2 and R1.
After OSPF converges, R3 and R4 change their next-hop to R2 to reach IP subnet A. You can
configure the hold-up timer (that is, the amount of time R2 routes for R1 in the event of failure) to a
time period greater than the routing protocol convergence or to indefinite (that is, the pair always
routes for each other). Avaya recommends that you configure the hold up and hold down timer to
1.5 times the convergence time of the network.
In an application where you use RSMLT at the edge instead of VRRP, Avaya recommends that you
configure the hold-up timer value to indefinite.
After R1 restarts after a failure, it first becomes active as a VLAN bridge. Using the bridge
forwarding table, packets destined to R1 are switched to R2 for as long as the hold-down timer
value. These packets are routed at R2 for R1. Like VRRP, to converge routing tables, the hold-down
timer value needs to be greater than the one required by the routing protocol.
After the hold-down time expires and the routing tables have converged, R1 starts routing packets
for itself and also for R2. Therefore, it does not matter which one of the two routers is used as the
next-hop from R3 and R4 to reach IP subnet A.
If you configure single-homed IP subnets on R1 or R2, Avaya recommends that you add another
routed VLAN to the virtual interswitch trunks (vIST). As a traversal VLAN or subnet, this additional
routed VLAN needs lower routing protocol metrics to avoid unnecessary Internet Control Message
Protocol (ICMP) redirect generation messages. This recommendation also applies to VRRP
implementations.
RSMLT guidelines
Use the following guidelines when you create RSMLT configurations:
• RSMLT is based on SMLT so all SMLT configuration rules apply. Enable RSMLT on the SMLT
aggregation switches on an individual VLAN basis. The VLAN must be a member of SMLT
links and the vIST trunk.
• The VLAN must be routable (IP address configured). On all four routers in a square or full-
mesh topology, configure an Interior Routing Protocol, such as OSPF, although the protocol is
independent from RSMLT.
• Routing protocols and static routes can be used with RSMLT.
• RSMLT pair switches provide backup for each other. As long as one of the two routers in an
vIST pair is active, traffic forwarding is available for both next-hops.
For design examples using RSMLT, see the following sections.
Layer 3 switch clustering and RSMLT
56 Network Design Reference for Avaya VSP 4000 Series June 2015
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