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This chapter describes the switching database manager (SDM) features built into your switch router. This chapter includes the following topics:
The Gigabit Ethernet interfaces use the forwarding engine and the ternary content addressable memory (TCAM) to implement the Layer 2 and Layer 3 search engine functionality. The Layer 2 and Layer 3 switching information is maintained in TCAM. The switching database manager (SDM) is the Layer 3 software subsystem that manages the Layer 3 switching information maintained in TCAM.
The switching database manager (SDM) resides on the central processor and its primary function is to maintain the Layer 3 switching database. SDM maintains the address entries contained in TCAM in an appropriate order. SDM manages TCAM space by partitioning protocol-specific switching information into multiple regions.
The forwarding engine on the Gigabit Ethernet interfaces performs packet processing on multiple channels simultaneously and extracts the packet header information. It uses this packet header information to issue exact-match or longest-match address searches in the switching information stored in TCAM.
TCAM identifies the location index of the Layer 2 or Layer 3 address and conveys it to the forwarding engine. The forwarding engine uses this location index to further derive information associated with each Layer 2 and Layer 3 address.
The key benefits of SDM in Layer 3 switching are its ability to organize the switching information in TCAM into protocol specific-regions and its ability to configure the size of these protocol regions. SDM enables exact-match and longest-match address searches, which result in high-speed forwarding.
SDM partitions TCAM space into multiple regions. Each region is protocol specific. SDM interacts with the individual protocol control layer to store Layer 3 switching information. SDM consists of the following types of regions:
Table 10-1 lists default partitioning for each protocol region in TCAM.
| Protocol Region | Lookup Type | Key Size | Default Size | No. of TCAM Entries |
|---|---|---|---|---|
ipx-bvi-network | Exact-match | 32 bits | 32 | 32 |
ip-adjacency | Exact-match | 32 bits | 2048 | 2048 |
ipx-node | Exact-match | 64 bits | 2048 | 4096 |
ip-prefix | Longest-match | 32 bits | 8192 | 8192 |
ipx-network | Exact-match | 32 bits | 6144 | 6144 |
ip-mcast | Longest-match | 64 bits | 3072 | 6144 |
l2-switching | Exact-match | 64 bits | 1024 | 2048 |
This section describes the commands necessary to configure the switching database manager (SDM). It includes commands necessary to configure the autolearn feature in SDM and commands required to configure the size of the SDM regions. The commands described in this section are unique to Layer 3 switching software.
SDM groups entries based on their mask lengths into "buckets." The size of each bucket in the protocol region varies. The autolearn feature in SDM automatically saves the mask length distribution (bucket size distribution) for the longest-match region in the switching database. SDM uses this information to set up the partitions, which are effective during the next system reboot.
Layer 3 switching enables the autolearn feature by default. The no form of the sdm autolearn command disables the SDM autolearn feature. You can reenable autolearn by issuing the sdm autolearn command from global configuration mode.
2948G-L3# configure terminal 2948G-L3 (config)# sdm autolearn 2948G-L3 (config)# Ctrl-Z
TCAM space consists of 32K entries, each entry being 32 bits wide. Since SDM is responsible for managing TCAM space, SDM partitions the entire TCAM space for each protocol region based on user configuration. A change in the partition configuration takes effect the next time you reboot the system.
The protocol region size in SDM is represented by the number of 32-bit or 64-bit entries. The combined size of all the application regions should be calculated in terms of 32-bit TCAM entries and should not exceed 32K,which is the total TCAM size.
To configure SDM size for each protocol region, use the following steps beginning in global configuration mode.
| Step | Command | Purpose | ||
|---|---|---|---|---|
| | |||
| | Returns to privileged EXEC mode. |
The following output is an example of configuring 2K entries for the ip-prefix region.
2948G-L3 # configure terminal 2948G-L3 (config)# sdm size ip-prefix k-entries 2 2948G-L3 (config)# Ctrl-Z
The combined size entered for all the protocol regions should not exceed 32K, which is the total TCAM size. To display the supported size of SDM, use the show sdm size command from global configuration mode.
2948G-L3 # show sdm size Switching Database Region Sizes : IPX Direct Network :256 32-bit entries IP Adjacency :2048 32-bit entries IPX Node :1024 64-bit entries IP Prefix :2048 32-bit entries IPX Network :2048 32-bit entries IP Multicast :1024 64-bit entries MAC Addr :2048 64-bit entries
Posted: Fri Apr 14 09:51:08 PDT 2000
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