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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.
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
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.
In the scenario of a switch stack, the SDM template on the master switch overrides a newly added switch’s SDM template.
Configuring and verifying the SDM template of a switch is fairly simple.
show sdm prefer or
show sdm prefer [vlan | routing | default]
SW1#show sdm prefer The current template is "desktop default" template. The selected template optimizes the resources in the switch to support this level of features for 8 routed interfaces and 1024 VLANs. number of unicast mac addresses: 6K number of IPv4 IGMP groups + multicast routes: 1K number of IPv4 unicast routes: 8K number of directly-connected IPv4 hosts: 6K number of indirect IPv4 routes: 2K number of IPv4 policy based routing aces: 0 number of IPv4/MAC qos aces: 512 number of IPv4/MAC security aces: 1K
Change the SDM Template
config t sdm prefer [vlan | routing | default] reload
What are the results of resource exhaustion? 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.