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Understanding prefix scale of QFX5000 switches in "lpm-profile" mode

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Article ID: KB34576 KB Last Updated: 11 Jun 2019Version: 1.0
Summary:
This KB will answer to following questions about prefix scale of QFX5000 switches in lpm-profile" mode
- What is the maximum scale of QFX5000 series?
- Why more prefixes can be programmed than max number in the output of "show pfe route summary hw"?
- How can I check utilization of forwarding table memory in PFE?

 

Solution:

In "lpm-profile" mode,
- It uses a combination of TCAMs and SRAMs in PFE to implement longest prefix match
- Its algorithm groups entries with common prefixes together
- TCAM stores the root prefix and SRAM stores the list of prefixes associated with the root prefix
- The number of entries in this secondary list determine the total number of prefixes that can be supported.
- Depending on the pattern of incoming prefixes, the scale number could vary
=> There is no fixed max scale number for prefixes
 
You can see max numbers per prefix types in the output of "show pfe route summary hw" command
and case by case, you may see the number of prefixes in "Used" column exceeds the number of prefixes in "Max" column.
- Numbers in MAX column are provided by PFE chip vendor and those are minimum guaranteed numbers
- Because the reason explained above, more than the MAX number of prefixes can be programmed like example below.
 
root@QFX5K> show pfe route summary hw
Profile active: lpm-profile
Type            Max       Used      Free      % free
----------------------------------------------------
IPv4 Host       8192      822       5558      67.85
IPv4 LPM        131072    136855    0         0.00
IPv4 Mcast      4096      0         2779      67.85
IPv6 Host       4096      906       2779      67.85
IPv6 LPM(< 64)  16384     17781     0         0.00
IPv6 LPM(> 64)  16384     11        0         0.00
IPv6 Mcast      2048      0         1390      67.87
 

There is no other way to monitor the utilization of TCAM and SRAM. Only way to know how much prefixes can be programmed  is trying to programm
the exact same pattern of the prefixes you want to use until you see table full error.

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