Subnet-Based Vlan Guidelines - Avaya 8800 Planning And Engineering

Layer 3 network design
RSMLT L2 Edge provides:
• Greater scalability—VRRP scales to 255 instances, while RSMLT scales to the maximum
number of VLANs.
• Simpler configuration—Simply enable RSMLT on a VLAN; VRRP requires virtual IP
configuration, along with other parameters.
For connections in pure Layer 3 configurations (using a static or dynamic routing protocol), a
Layer 3 RSMLT configuration is recommended over VRRP. In these instances, an RSMLT
configuration provides faster failover than one with VRRP because the connection is a Layer
3 connection, not just a Layer 2 connection for default gateway redundancy.
Both VRRP and RSMLT can provide default gateway resiliency for end stations. The
configurations of these features are different, but both provide the same end result and are
transparent to the end station.
For more information about RSMLT, see

Subnet-based VLAN guidelines

You can use subnet-based VLANs to classify end-users in a VLAN based on the end-user
source IP addresses. For each packet, the switch performs a lookup, and, based on the source
IP address and mask, determines to which VLAN the traffic belongs. To provide security,
subnet-based VLANs can be used to allow only users on the appropriate IP subnet to access to
the network.
You cannot classify non-IP traffic using a subnet-based VLAN.
You can enable routing in each subnet-based VLAN by assigning an IP address to the subnet-
based VLAN. If no IP address is configured, the subnet-based VLAN is in Layer 2 switch mode
only.
You can enable VRRP for subnet-based VLANs. The traffic routed by the VRRP Master
interface is forwarded by hardware. Therefore, no throughput impact is expected when you
use VRRP on subnet-based VLANs.
You can use subnet-based VLANs to achieve multinetting functionality; however, multiple
subnet-based VLANs on a port can only classify traffic based on the sender IP source address.
Thus, you cannot multinet by using multiple subnet-based VLANs between routers (Layer 3
devices). Multinetting is supported, however, on all end-user-facing ports.
You cannot classify Dynamic Host Configuration Protocol (DHCP) traffic into subnet-based
VLANs because DHCP requests do not carry a specific source IP address; instead, they use
an an all broadcast address. To support DHCP to classify subnet-based VLAN members,
create an overlay port-based VLAN to collect the bootp/DHCP traffic and forward it to the
appropriate DHCP server. After the DHCP response is forwarded to the DHCP client and it
126 Planning and Engineering — Network Design
Routed SMLT on page 89.
November 2010