Avaya 8800 Planning And Engineering, Network Design page 89

For IP subnet A, VRRP Backup-Master can provide the same functions as RSMLT, as long as an
additional router is not connected to IP subnet A.
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
Layer 3 environments
R3 and R4 both use both 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 its packets towards R1; these
packets are also sent directly to subnet A. When 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. The network
administrator can set the hold-up timer (that is, for 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 set the hold up and hold down timer
to 1.5 times the convergence time of the network.
In an application where RSMLT is used at the edge instead of VRRP, Avaya recommends that you
set the hold-up timer value to indefinite.
When R1 reboots after a failure, it first becomes active as a VLAN bridge. Using the bridging
forwarding table, packets destined to R1 are switched to R2 for as long as the hold-down timer is
configured. 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.
When 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 single-homed IP subnets are configured on R1 or R2, Avaya recommends that you add another
routed VLAN to the ISTs. As a traversal VLAN/subnet, this additional routed VLAN needs lower
routing protocol metrics to avoid unnecessary ICMP redirect generation messages. This
recommendation also applies to VRRP implementations.
RSMLT guidelines
Because RSMLT is based on SMLT, all SMLT configuration rules apply. In addition, RSMLT is
enabled on the SMLT aggregation switches on a per-VLAN basis. The VLAN must be a member of
SMLT links and the IST trunk.
The VLAN also must be routable (IP address configured). On all four routers in a square or full-
mesh topology, an Interior Routing Protocol, such as OSPF, must be configured, although the
protocol is independent from RSMLT.
You can use any routing protocol, including static routes, with RSMLT.
June 2016
on page 88.
Planning and Engineering — Network Design
Comments on this document? infodev@avaya.com
Network redundancy
Figure 25: SMLT and RSMLT in
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