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This feature module describes enhancements to session scalability and stability. It includes information on the benefits of the enhancements, supported platforms, related documents, and new commands.
This document includes the following sections:
The session scalability enhancements described in this document are included in Cisco IOS Release 12.1(1) DC1. They can be used in addition to the L2TP scalability enhancements in Cisco IOS Release 12.0(7) DC to achieve high numbers of PPP sessions and L2TP tunnels.
Increased Input and Output Hold-Queue Limits
Prior to this release, the default input hold-queue limit was 75 packets, and the default output hold-queue limit was 40 packets. To accommodate more control messages, these limits were increased to values determined by the amount of DRAM on the node route processor (NRP), as shown in Table 1.
| NRP DRAM | Input Hold-Queue Limit | Output Hold-Queue Limit |
|---|---|---|
64 MB | 500 packets | 80 packets |
128 MB | 1000 packets | 80 packets |
Limiting the Number of Simultaneous Link Control Protocol Session Initiations
A new command enables you to limit the number of simultaneous link control protocol (LCP) session initiations. You can reduce the client session recovery time after a dropout by preventing a chain reaction of LCP session initiation timeouts.
Limiting the Load Metric
A new command enables you to limit the load metric based on the length of the PPP manager process input queue.
These session scalability enhancements are supported on the node route processor (NRP) of the Cisco 6400 UAC.
Standards
None
MIBs
None
RFCs
No new or modified RFCs are supported by these feature enhancements.
Cisco Express Forwarding
To support more than 1000 sessions, you must enable Cisco Express Forwarding (CEF) with the ip cef global configuration command. For more information on CEF, see the "Cisco Express Forwarding " chapter of the Cisco IOS Switching Services Configuration Guide .
Recommended Memory
Cisco recommends at least 128 MB of DRAM on the Cisco 6400 NRP while using these feature enhancements.
Recommended NSP Image
Cisco recommends that you simultaneously run Cisco IOS Release 12.1(1) DB on the NSP while using these enhancements.
See the following sections for configuration tasks for the session scalability enhancements. Each task is optional.
If the show interfaces EXEC command reveals an excessive number of discarded packets because of input hold-queue overflows, increase the input hold-queue limit by completing the following steps, beginning in global configuration mode.
| Command | Purpose | |
|---|---|---|
Step 1 | Router(config)# interface atm slot/subslot/port | Select the ATM interface. |
Step 2 | Router(config-if)# hold-queue length in | Specify the maximum number of packets in the input hold-queue. |
To display the current input hold-queue setting and the number of packets discarded because of input hold-queue overflows, use the show interfaces EXEC command.
If the show interfaces EXEC command reveals an excessive number of discarded packets because of output hold-queue overflows, increase the output hold-queue limit by completing the following steps, beginning in global configuration mode.
| Command | Purpose | |
|---|---|---|
Step 1 | Router(config)# interface atm slot/subslot/port | Select the ATM interface. |
Step 2 | Router(config-if)# hold-queue length out | Specify the maximum number of packets in the output hold-queue. |
To display the current output hold-queue setting and the number of packets discarded because of output hold-queue overflows, use the show interfaces EXEC command.
By default, the system does not limit the number of simultaneously active LCP sessions. Allowing a large number of LCP sessions to start in parallel causes many sessions to timeout and retry, and can result in a chain reaction of LCP session negotiations and excessive session recovery times. The chain reaction can be controlled by limiting the number of simultaneous LCP session initiations. This allows sessions to be established prior to additional initiations.
To limit the number of simultaneous LCP session initiations, use the following command in global configuration mode.
| Command | Purpose |
|---|---|
Router(config)# lcp max-session-starts number | Specifies the maximum number of simultaneous LCP sessions to be negotiated. Value must be between 100 and 3000. |
 |
Note The nominal value depends on many factors. Cisco recommends that you start with the lowest value of 100. Try several numbers and select the one that results in the shortest session recovery time after a link dropout. |
To check the configured limit of LCP session initiations, use the show running-config EXEC command.
To limit the load metric, use the following command in global configuration mode.
| Command | Purpose |
|---|---|
Router(config)# lcp max-load-metric number | Specifies the maximum load metric based on the length of the PPP manager process input queue. |
|
Note The nominal value depends on many factors. Cisco recommends that you start with 100. Try several numbers and select the one that results in the shortest session recovery time after a link dropout. |
To check the configured limit of LCP session initiations, use the show running-config EXEC command.
For general L2TP configuration examples, see the Layer 2 Tunnel Protocol feature module.
The following example shows a configuration implementing the enhancements documented in this feature module as well as in the Layer 2 Tunnel Protocol Scalability Enhancements feature module. The input hold queue limit on an ATM interface is set to 1200, and virtual template 1 is used to preclone 2000 virtual access interfaces. VPDN group 1 is set to use 7 retransmission attempts, with the retransmission timeouts beginning at 2 seconds and ending at 4 seconds, and the L2TP tunnel timeout is set to 10 seconds. The local RWS is set to 500 packets. The number of simultaneous LCP session initiations are limited to 100, and the load metric is limited to 100.
! vpdn enable ! vpdn-group 1 accept-dialin protocol l2tp virtual-template 1 terminate from hostname LAC1 local name LNS1 l2tp tunnel receive-window 500 l2tp tunnel nosession-timeout 10 l2tp tunnel retransmit retries 7 l2tp tunnel retransmit timeout min 2 l2tp tunnel retransmit timeout max 4 ! virtual-template 1 pre-clone 2000 ! interface ATM 0/0/0 hold-queue 1200 in ! interface FastEthernet 0/0/0 ip address negotiated no ip directed-broadcast ! interface Virtual-Template 1 ip unnumbered FastEthernet 0/0/0 no ip directed-broadcast no keepalive peer default ip address pool pool-1 ppp authentication chap ! lcp max-session-starts 100 lcp max-load-metric 100 !
This section documents new commands. All other commands used with this feature are documented in the Cisco IOS Release 12.1 command reference publications.
To limit load metric, use the lcp max-load-metric global configuration command. To disable this limit, use the no form of the command.
lcp max-load-metric number
Syntax Description
number Maximum load metric based on the length of the PPP manager process input queue.
Defaults
Unlimited
Command Modes
Global configuration
Command History
12.1(1) DC1 This command was introduced on the Cisco 6400 NRP.
Release
Modification
Usage Guidelines
The nominal limit depends on many factors. Try several numbers and select the one that results in the shortest session recovery time after a link dropout.
Examples
The following example limits the load metric to 100:
lcp max-load-metric 100
To limit the number of simultaneous link control protocol (LCP) session initiations, use the lcp max-session-starts global configuration command. To disable this limit, use the no form of the command.
lcp max-session-starts number
Syntax Description
number Maximum number of simultaneous LCP session initiations.
Defaults
Unlimited number of simultaneous LCP sessions initiations
Command Modes
Global configuration
Command History
12.1(1) DC1 This command was introduced on the Cisco 6400 NRP.
Release
Modification
Usage Guidelines
Range of possible values: 100 to 3000.
The nominal limit depends on many factors. Try several numbers and select the one that results in the shortest session recovery time after a link dropout.
Examples
The following example limits the number of simultaneous LCP session initiations to 100:
lcp max-session-starts 100
control messages--Signaling messages that provide the control of setup, maintenance, and tear-down of L2TP sessions and tunnels.
L2TP--Layer 2 Tunnel Protocol. An Internet Engineering Task Force (IETF) standards track protocol defined in RFC 2661 that provides tunneling of PPP. Based upon the best features of L2F and PPTP, L2TP provides an industry-wide interoperable method of implementing VPDN.
L2TP access concentrator--See LAC.
L2TP network server--See LNS.
L2TP session--Communications transactions between the LAC and LNS that support tunneling of a single PPP connection. There is a one-to-one relationship among the PPP connection, L2TP session, and L2TP call.
LAC--L2TP access concentrator. A node that acts as one side of an L2TP tunnel endpoint and is a peer to the L2TP network server (LNS). The LAC sits between an LNS and a remote system and forwards packets to and from each. Packets sent from the LAC to the LNS require tunneling with the L2TP protocol as defined in this document. The connection from the LAC to the remote system is either local or a PPP link.
LCP--Link control protocol. Protocol that establishes, configures, and tests data-link connections for use by PPP. See also PPP.
link control protocol--See LCP.
LNS--L2TP network server. A node that acts as one side of an L2TP tunnel endpoint and is a peer to the L2TP access concentrator (LAC). The LNS is the logical termination point of a PPP session that is being tunneled from the remote system by the LAC. Analogous to the Layer 2 Forwarding (L2F) home gateway (HGW).
Layer 2 Tunnel Protocol--See L2TP.
Point-to-Point Protocol--See PPP.
PPP--Point-to-Point Protocol. A protocol that encapsulates network layer protocol information over point-to-point links. PPP is defined in RFC 1661.
Virtual Private Dialup Networking--See VPDN.
VPDN--Virtual Private Dialup Networking. A system that permits the physical dialup connection to appear to be connected directly to a home network while actually residing elsewhere on the network. A virtual pipe is connected between the physical dialup connections and the termination point at the home network.
Posted: Fri Aug 4 17:37:15 PDT 2000
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