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Table Of Contents
Behavioral Change for Buffer Recarving
Understanding Buffer Carving Mechanism
Behavioral Change to Carve-Level 1
Behavioral Change to Command "no bfrp carve-level"
Behavioral Change for Buffer Recarving
Part Number: OL-18534-01
First Published: December 8, 2008Table 1 Feature History
Contents
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Behavioral Change to Carve-Level 1
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Purpose of This Document
This document explains the basics of the buffer recarving functionality, buffer carves that are triggered upon changing MTUs, the behavioral change to carve-level 1 functionality, the addition of the new command bfrp carve-level to enable or disable packet buffer recarve optimization, and behavioral change to the command "no bfrp carve-level".
Overview
Each line card (LC) on a Cisco 12000 series Internet router has two types of memory:
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This document discusses mainly the buffer carving algorithm, the addition of a new CLI command to control buffer optimization, a behavioral change to carve-level 1 functionality and a behavioral change to the command no bfrp carve-level. Buffer carving mechanism is related with the packet memory (SDRAM), which is divided into two banks: ToFab and FrFab (towards the Fabric and from the Fabric). The ToFab memory is used for packets that come in one of the interfaces on the LC and make their way to the fabric, whereas the FrFab memory is used for packets that are going out of an interface on the LC from the fabric.
The Tofab and Frfab queues are the most important concepts to understand in order to efficiently troubleshoot ignored packets in the Cisco 12000 Series Internet Router.
Note
Buffer Carving Algorithm
Cisco IOS software implements a buffer-carving algorithm that divides SDRAM into various buffer sizes. The Cisco 12000 series Internet router connects to several different media types, with different MTUs. A mechanism was needed to make the most efficient use of packet memory buffers. The buffer carving algorithm was devised after studying the makeup of Internet traffic and considering the various media types that the Cisco 12000 series Internet router supports.
Understanding Buffer Carving Mechanism
The Route Processor (RP) runs the buffer carve code to determine the buffer specifications to be sent down to line cards. Line cards apply these specifications on their packet buffers. Route Processor buffer carve code can be invoked under the following conditions:
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Controlling Buffer Recarving
Each line card on Cisco 12000 series Internet router has two sets of packet memories. One of the packet memories is on the ingress side (tofab) and the other is on the egress side(frfab).
The Route Processor calculates buffer specifications and sends it down to the line cards on their power-on. The line cards blindly carve their packet memories based on these specifications. Changes in the buffer specification can happen when the MTU distribution changes on the router, triggering recarving of buffers on line cards (LC). Currently by default, buffer recarving occurs in any of the following cases:
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Network Traffic Disruption
Recarves on line card result in disruption of the traffic flow; the worst-case recarve will result in all line card Tofab and Fr Fab packet memories getting recarved, resulting in a router-wide traffic drop. Recarving in case 1 is acceptable to ensure the biggest packet flowing through the router does not get dropped. Similarly, case 4 is acceptable because it ensures that the biggest packets on the ingress side do not get dropped. However, case 2 and case 3 can be disabled using the bfrp carve-level command.
bfrp carve-level
To ensure control over the frequency of buffer recarving, use the bfrp carve-level command. Whenever buffer recarving occurs, packets sent to that line card are dropped. Use the command bfrp carve-level 0 if you want to disable buffer recarving completely after recarving once to default buffer specifications. Use the command bfrp carve-level 1 if you want to perform buffer recarving only if the change in the MTU is changing the maximum slot MTU configured on any slot. Use the no bfrp carve-level command to restore the default carving mechanism.
bfrp carve-level [0 | 1]
no bfrp carve-level
Syntax Description
0
Applies default buffer distribution and disables buffer recarves completely.
1
Allows recarving only if the change in the MTU is changing the maximum slot MTU configured on any slot.
Command Modes
Global configuration
Defaults
By default, buffer recarving is triggered if any of the cases discussed in the section "Controlling Buffer Recarving" are met.
Command History
Examples
The following example shows how to recarve the default buffer distribution and disable further recarving for any MTU change. The maximum MTU configured on router is 6000:
Router> enableRouter# config tRouter(config)# bfrp carve-level 0The following example shows that recarving is invoked only if the MTU change is changing the maximum slot MTU configured on any slot:
Router(config)# bfrp carve-level 1The following example shows how to restore the default buffer recarving behavior:
Router(config)# no bfrp carve-levelUsage Guidelines
Use the command bfrp carve-level 0 to accommodate all default buffer sizes on the router with a maximum buffer size of 9248 and then prevent further recarves on any MTU change.
Use the command bfrp carve-level 1 to selectively disable buffer recarve, by allowing recarves only for maximum slot MTU changes.
Use the command no bfrp carve-level to restore to default carving mechanism.
Note
Behavioral Change to Carve-Level 1
This section explains the behavioral change to carve-level 1 functionality. It contains the following topics:
Old Behavior
The ToFab and FrFab of all line cards match the queues in the global buffer distribution (dsn).
New Behavior
At carve level 1, fix ensures change in largest MTU configured on slot, recarves both ToFab and FrFab buffers on the same line card and only FrFab buffers on other card, keeping the ToFab buffers of other card unchanged.
However, in some rare cases though global distribution changes due to MTU change, same will not be reflected in the ToFab of other line card due above restriction. So, there could be some extra buffer queues which are not in global buffer distribution.
Buffers are allocated dynamically for incoming packets. If incoming packet matches the criteria for extra queue found in ToFab direction, it will allocate buffer from that queue. So buffers in extra queue(in ToFab direction) do not go waste.
The following example shows the logs of caveat at carve level-1. The command output in the following example shows that an extra buffer queue is added in ToFab for slot 2. The example includes the following hardware and software configurations:
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Step 1
Step 2
Step 3
Router# show controllers queuesGlobal MTU information:Max MTU data size = 4592MTUs considered in SDRAM buffer carving:Index MTU Data Size0 15681 4592Input SDRAM buffer distribution = default :Index Data Size Min Data Size Percent Buffers Fixed?0 80 1 50 yes1 608 81 30 yes2 1616 1488 15 yes3 4592 3671 3 no4 9296 7431 2 noResulting global SDRAM buffer distribution (includes MTUs):Index Data Size Min Data Size Percent Buffers Fixed?0 80 1 50 yes1 608 81 30 yes2 1616 1488 15 yes3 4592 3671 3 no4 9296 7431 2 noContd...FrFab SDRAM buffer carve-data for slot 0:valid element = yes, percent buffers unused = 0Buffer distribution:Index Data Size Percent Buffers0 80 511 608 312 1616 153 4592 3ToFab SDRAM buffer carve-data for slot 0:valid element = yes, percent buffers unused = 0Buffer distribution:Index Data Size Percent Buffers0 80 511 608 312 1616 153 4592 3FrFab SDRAM buffer carve-data for slot 2:valid element = yes, percent buffers unused = 0Buffer distribution:Index Data Size Percent Buffers0 80 511 608 312 1616 153 4592 3ToFab SDRAM buffer carve-data for slot 2:valid element = yes, percent buffers unused = 0Buffer distribution:Index Data Size Percent Buffers0 80 501 608 312 1616 153 1952 1 <<<< Mismatch with global dsn4 4592 3FrFab SDRAM buffer carve-data for slot 3:valid element = yes, percent buffers unused = 0Buffer distribution:Index Data Size Percent Buffers0 80 511 608 312 1616 153 4592 3ToFab SDRAM buffer carve-data for slot 3:valid element = yes, percent buffers unused = 0Buffer distribution:Index Data Size Percent Buffers0 80 531 608 322 1616 16Behavioral Change to Command "no bfrp carve-level"
This section explains the behavioral change to the command "no bfrp carve-level". It contains the following topics:
Old Behavior
Traversing back from carve level 0 or 1 to the default carve using the command "no bfrp carve-level", restores to the default carving mechanism without performing any carving. Only future MTU changes trigger recarve of both Tofab and From Fab according to their default carve mechanism.
New Behavior
Executing the command "no bfrp carve-level", triggers a carving on both ToFab and FromFab direction according to the default carving mechanism. The feature change is that if you traverse back from carve-level 0 or 1 to default carve mechanism, using the command "no bfrp carve-level", recarving takes place. Below is the output showing the new behavior. It triggers router wide recarving, suggested to use on a maintenance window (MW).
Step 1
GSR-12012(config)# do show running-config | i carvebfrp carve-level 0Step 2
GSR-12012(config)# no bfrp carve-levelGSR-12012(config)#05:05:42: %RP-6-CARVE_BUF: Carve Level change may recarve the buffers << indicative messageGSR-12012(config)#endGSR-12012(config)#endGSR-12012#05:08:51: %SYS-5-CONFIG_I: Configured from console by consoleStep 3
GSR-12012# show controllers queues <<< buffers are recarved according to default carve mechanismGlobal MTU information:Max MTU data size = 4592MTUs considered in SDRAM buffer carving:Index MTU Data Size0 4592Input SDRAM buffer distribution = default :Index Data Size Min Data Size Percent Buffers Fixed?0 80 1 50 yes1 608 81 30 yes2 1616 1488 15 yes3 4592 3671 3 no4 9296 7431 2 noContd...Resulting global SDRAM buffer distribution (includes MTUs):Index Data Size Min Data Size Percent Buffers Fixed?0 80 1 50 yes1 608 81 30 yes2 1616 1488 15 yes3 4592 3671 3 no4 9296 7431 2 noFrFab SDRAM buffer carve-data for slot 8:valid element = yes, percent buffers unused = 0Buffer distribution:Index Data Size Percent Buffers0 80 511 608 312 1616 153 4592 3ToFab SDRAM buffer carve-data for slot 8:valid element = yes, percent buffers unused = 0Buffer distribution:Index Data Size Percent Buffers0 80 511 608 312 1616 153 4592 3
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