Implementing Cisco IP Switched Networks, 300-115. SDM template of parent  Configure and verify switch administration.

SDM stands for Switching Database Manager. The SDM manages layer 2 and layer 3 switching information. It is maintained on the Ternary Content Addressable Memory, or TCAM. The TCAM itself is used for forwarding lookups. These lookups are done in hardware by the ACL engine. It's supposed to be fast compared to doing it in software. The ACL engine determines how the switch will handle each packet by either permitting or denying. Although the TCAM can provide a performance increase, there are limited amount of entries which can lead to resource contention and exhaustion.

Architecture

Most switches have one TCAM and it is shared between Layer 2 and Layer 3 for forwarding entries, RACLs, VACLs, and QoS ACLs.

The table structure is broken out into the following tables:

- Layer 2 Learning - Port learning policies

- Layer 2 Forwarding - Learned unicast and multicast addresses

- Layer 3 Routing - Used for unicast and multicast route lookups

- ACL and QoS - Identifying traffic according to security and QoS ACLs

SDM Templates

There are three predefined templates:

- Routing Template - Maximizes resources for unicast routing and is used when a switch is used as a router or route aggregator.

- VLAN Template - Unicast routing is disabled and allows for the maximum number of supported MAC addresses. Implemented when a switch is purely a layer 2 device.

- Default Template - The best of both worlds, a mixture between the routing and VLAN template.

Each template has two versions, the Desktop Template and the Aggregator Template. The Aggregator Template is limited to specific switches. When modifying the SDM template on a switch it is required to reload the switch in order to use the new SDM template.

Configuration

Configuring and verifying the SDM template of a switch is simple.

show sdm prefer or show sdm prefer

SW1#show sdm prefer

The current template is "desktop default" template.

Change the SDM Template

config t

sdm prefer

reload

Resources Exhaust

During layer 2 forwarding and learning, new learned addresses are flooded to all ports within the ingress VLAN.

During layer 3 routing, layer 3 unicast and multicast routes are learned only in software and not in TCAM which results in slower routing processes.

 
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