Table Of Contents
PPP/MLP MRRU Negotiation Configuration
Prerequisites for PPP/MLP MRRU Negotiation Configuration
Restrictions for PPP/MLP MRRU Negotiation Configuration
Information About PPP/MLP MRRU Negotiation Configuration
Advertising of a Specific MRRU Value
How to Configure PPP/MLP MRRU Negotiation Configuration
Configuring PPP/MLP MRRU Negotiation Configuration on Virtual Templates
Configuring PPP/MLP MRRU Negotiation Configuration on Multilink Groups
Configuring PPP/MLP MRRU Negotiation Configuration on Dialer Interfaces
Configuration Examples for PPP/MLP MRRU Negotiation Configuration
PPP/MLP MRRU Negotiation Configuration on Virtual Templates: Example
PPP/MLP MRRU Negotiation Configuration on Multilink Groups: Example
PPP/MLP MRRU Negotiation Configuration on Dialer Interfaces: Example
PPP/MLP MRRU Negotiation Configuration
First Published: March 1, 2004Last Updated: February 28, 2006The PPP/MLP MRRU Negotiation Configuration feature allows a router to send and receive frames over Multilink PPP (MLP) bundles that are larger than the default Maximum Receive Reconstructed Unit (MRRU) limit of 1524 bytes.
History for the PPP/MLP MRRU Negotiation Configuration Feature
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Contents
•
Prerequisites for PPP/MLP MRRU Negotiation Configuration
•
Restrictions for PPP/MLP MRRU Negotiation Configuration
•
Information About PPP/MLP MRRU Negotiation Configuration
•
How to Configure PPP/MLP MRRU Negotiation Configuration
•
Configuration Examples for PPP/MLP MRRU Negotiation Configuration
Prerequisites for PPP/MLP MRRU Negotiation Configuration
Before performing the tasks to configure PPP/MLP MRRU Negotiation Configuration, you need to understand how to configure PPP and MLP. It will also be useful to be familiar with concepts presented in RFC 1990. See the "Related Documents" and "RFCs" sections for pointers to this information.
Restrictions for PPP/MLP MRRU Negotiation Configuration
This feature and its new interface configuration command are valid only on interfaces that support MLP.
Note
Be careful when configuring MLP MRRU negotiation in a virtual private dialup network (VPDN) environment with an L2TP network server (LNS) that is not running Cisco IOS Release 12.3(7)T. The software performs strict matching on the MRRU values in earlier versions of the Cisco IOS software.
Information About PPP/MLP MRRU Negotiation Configuration
To configure PPP/MLP MRRU Negotiation Configuration, you need to understand the following concepts:
•
Advertising of a Specific MRRU Value
MRRU Negotiation on MLP
Before Cisco IOS Release 12.3(7)T, configuring the MRRU option negotiated on a multilink bundle with the MLP was not possible. Cisco IOS software always advertised an MRRU default value of 1524 bytes, which meant that the maximum transmission unit (MTU) of the peer's bundle interface was restricted to a value of 1524 bytes or fewer if the data transfer was to be successful. Users who wanted to benefit from MLP features had to accept limits on the MTU byte size setting.
The PPP/MLP MRRU Negotiation Configuration feature allows configuration control over MRRU negotiation. A new interface configuration command introduced with this feature, ppp multilink mrru, allows configuring the specific MRRU value that the router will advertise, and optionally establishing a lower boundary on the MRRU value of the peer.
MLP is a method for spreading traffic across multiple physical WAN links while providing packet fragmentation and reassembly, proper sequencing, multivendor interoperability, and load balancing on inbound and outbound traffic. MLP was developed to use the multiple bearer channels in ISDN, but is equally applicable to any situation in which multiple PPP links connect two systems, including asynchronous links.
When MLP is used, several physical interfaces can constitute one logical connection to the peer. To represent the logical connection, software provides a logical interface, often called the bundle interface. This interface will have the IP address, for instance, and the MTU setting of the interface that IP uses when it is deciding whether to fragment an IP datagram that needs to be forwarded. The physical interfaces simply forward individual MLP fragments or frames that are given to them by the bundle interface.
The result of having to decide whether to fragment a packet is that, whereas with simple PPP the interface MTU must not exceed the peer's MRU, with MLP the MTU size of the bundle interface must not exceed the MRRU setting of the peer. The MRRU settings on both sides need not be equal, but the "must not exceed" rule just specified must be followed; otherwise; a system might send several fragments that, when reconstructed as a frame, will be too large for the peer's receive buffer.
Advertising of a Specific MRRU Value
Where a PPP link is destined to join an existing MLP bundle, the MRRU value advertised on that link will be the MRRU of the existing bundle, in configurations where the software can determine which bundle the link is destined to join at the time the Link Control Protocol (LCP) is negotiated.
In Cisco IOS Release 12.0(28)S, this is the case for multilink groups only.
In Cisco IOS Release 12.3(7)T and later releases, this is the case for both multilink groups and dialer profiles that have already been bound to the physical interface at the time LCP is negotiated.
In all other cases, the MRRU value advertised on a link is by an order of preference, as follows:
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The value configured on the link interface with the ppp multilink mrru interface command, or the value inherited from the configuration of the ppp multilink mrru command on the parent interface. If both values are present, the link interface value has precedence.
•
The value of the bundle interface MTU, if the bundle interface is known at the time LCP is negotiated. In Cisco IOS Release 12.0(28)S, this is the case for multilink groups only.
•
The default MRRU value of 1524 bytes.
Peer MRRU Negotiation
By default, any peer MRRU value that falls above the lower boundary of what is considered to be acceptable will be acknowledged.
In addition, the ppp multilink mrru interface command will allow specifying a minimum required peer MRRU value. If a lower value has been configured on a link interface or is inherited from a parent interface, software will send a negative acknowledgment along with the required minimum MRRU value to any peer with MRRU values that are below the established threshold.
When the bundle interface comes up, its MTU will be reduced if it exceeds the peer's MRRU for the duration of the existence of the bundle.
When a link joins a bundle, it must have the same values configured for the local and remote MRRU as the bundle does. If not, the link will be dropped and an error message will be displayed.
How to Configure PPP/MLP MRRU Negotiation Configuration
The following sections describe how to configure the PPP/MLP MRRU Negotiation Configuration feature.
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Configuring PPP/MLP MRRU Negotiation Configuration on Virtual Templates (required for virtual templates)
•
Configuring PPP/MLP MRRU Negotiation Configuration on Multilink Groups (required for multilink groups)
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Configuring PPP/MLP MRRU Negotiation Configuration on Dialer Interfaces (required for dialer interfaces)
Configuring PPP/MLP MRRU Negotiation Configuration on Virtual Templates
In this task, you configure MRRU negotiation on the serial interface. The bundle interface will be a virtual access interface cloned from the virtual template.
SUMMARY STEPS
1.
enable
2.
configure terminal
3.
multilink virtual-template number
4.
interface virtual-template number
5.
ip address ip-address mask
6.
mtu bytes
7.
exit
8.
interface serial slot/port
9.
ppp multilink
10.
ppp multilink mrru [local | remote] mrru-value
11.
mtu bytes
12.
exit
DETAILED STEPS
Troubleshooting Tips
Use the debug ppp negotiation command to verify and troubleshoot MRRU negotiation on virtual templates. Use the show interface command to verify MRRU negotiation on the interfaces.
Configuring PPP/MLP MRRU Negotiation Configuration on Multilink Groups
In this task, you configure MRRU negotiation on the multilink interface. The bundle interface is static, that is, always available.
SUMMARY STEPS
1.
enable
2.
configure terminal
3.
interface multilink number
4.
ip address ip-address mask
5.
ppp multilink mrru [local | remote] mrru-value
6.
mtu bytes
7.
exit
8.
interface serial slot/port
9.
ppp multilink
10.
ppp multilink group group-number
11.
mtu bytes
12.
exit
DETAILED STEPS
Troubleshooting Tips
Use the debug ppp negotiation command to verify and troubleshoot MRRU negotiation on multilink groups. Use the show interface command to verify MRRU negotiation on the interfaces.
Configuring PPP/MLP MRRU Negotiation Configuration on Dialer Interfaces
In this task, you configure MRRU negotiation on the dialer interface. The bundle interface will be a virtual access interface cloned from the dialer interface.
SUMMARY STEPS
1.
enable
2.
configure terminal
3.
interface dialer number
4.
ip address ip-address mask
5.
encapsulation ppp
6.
dialer configuration-commands
7.
ppp multilink
8.
ppp multilink mrru [local | remote] mrru-value
9.
mtu bytes
10.
exit
DETAILED STEPS
Command or Action PurposeStep 1
enable
Example:Router> enable
Enables privileged EXEC mode.
•
Enter your password if prompted.
Step 2
configure terminal
Example:Router# configure terminal
Enters global configuration mode.
Step 3
interface dialer number
Example:Router(config)# interface dialer 1
Defines a dialer rotary group and enters interface configuration mode.
Step 4
ip address ip-address mask
Example:Router(config-if)# ip address 10.13.1.1 255.255.255.0
Sets the IP address for the interface.
Step 5
encapsulation ppp
Example:Router(config-if)# encapsulation ppp
Sets the PPP encapsulation method.
Step 6
dialer configuration-commands
Example:Router(config-if)# dialer string 5550101
Configures dialer interface characteristics.
•
The dialer commands you use depend upon your network configuration. Choose from dialer interface configuration commands such as dialer remote-name, dialer idle-timeout, dialer string, dialer pool, and so on. See the "Configuration Examples for PPP/MLP MRRU Negotiation Configuration" and "Related Documents" sections for dialer interface configuration examples.
Step 7
ppp multilink
Example:Router(config-if)# ppp multilink
Enables MLP on the interface.
Step 8
ppp multilink mrru [local | remote] mrru-value
Example:Router(config-if)# ppp multilink mrru local 1600
Configures the MRRU value negotiated on a multilink bundle when MLP is used.
•
local—(Optional) Configures the local MRRU value. The default values for the local MRRU are the value of the multilink group interface MTU for multilink group members, and 1524 bytes for all other interfaces.
•
remote—(Optional) Configures the minimum value that the software will accept from the peer when it advertises its MRRU. By default, the software accepts any peer MRRU value of 128 or higher. You can specify a higher minimum acceptable MRRU value in a range from 128 to 16384 bytes.
Step 9
mtu bytes
Example:Router(config-if)# mtu 1600
(Optional) Adjusts the maximum packet size or MTU size.
•
The default MTU for serial interfaces is 1500.
Step 10
exit
Example:Router(config-if)# exit
Exits interface configuration mode and returns to global configuration mode.
Troubleshooting Tips
Use the debug ppp negotiation command to verify and troubleshoot MRRU negotiation on multilink groups. Use the show interface command to verify MRRU negotiation on dialer interfaces.
Configuration Examples for PPP/MLP MRRU Negotiation Configuration
This section contains the following examples:
•
PPP/MLP MRRU Negotiation Configuration on Virtual Templates: Example
•
PPP/MLP MRRU Negotiation Configuration on Multilink Groups: Example
•
PPP/MLP MRRU Negotiation Configuration on Dialer Interfaces: Example
PPP/MLP MRRU Negotiation Configuration on Virtual Templates: Example
The following example shows how to configure MRRU negotiation on a virtual template with synchronous serial interfaces. The example also applies to asynchronous serial interfaces.
multilink virtual-template 1!interface virtual-template 1ip address 10.13.1.1 255.255.255.0mtu 1600!interface serial 0/0ppp multilinkppp multilink mrru local 1600mtu 1600!interface serial 0/1ppp multilinkppp multilink mrru local 1600mtu 1600PPP/MLP MRRU Negotiation Configuration on Multilink Groups: Example
The following example shows how to configure MRRU negotiation on multilink groups:
interface multilink 10ip address 10.13.1.1 255.255.255.0ppp multilink mrru local 1600mtu 1600!interface serial 0/0ppp multilinkmultilink-group 10mtu 1600!interface serial 0/1ppp multilinkmultilink-group 10mtu 1600PPP/MLP MRRU Negotiation Configuration on Dialer Interfaces: Example
The following example shows how to configure MRRU negotiation on dialer interfaces:
interface dialer 1ip address 10.13.1.1 255.255.255.0encapsulation pppdialer remote-name 2610-2dialer idle-timeout 30 inbounddialer string 5550101dialer pool 1dialer-group 1no cdp enableppp multilinkppp multilink mrru local 1600mtu 1600Additional References
The following sections provide references related to the PPP/MLP MRRU Negotiation Configuration feature.
Related Documents
Related Topic Document TitleConfiguring media-independent PPP and Multilink PPP
"Part 9: PPP Configuration," in the Cisco IOS Dial Technologies Configuration Guide
PPP and MLP commands
Cisco IOS Dial Technologies Command Reference, Release 12.3 T
Standards
MIBs
MIBs MIBs LinkNone
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
Technical Assistance
Command Reference
This section documents the following modified command.
ppp multilink mrru
To configure the maximum receive reconstructed unit (MRRU) option negotiated on a multilink bundle when Multilink PPP (MLP) is configured, use the ppp multilink mrru command in interface configuration mode. To remove or change the configured MRRU, use the no form of this command.
ppp multilink mrru [local | remote] mrru-bytes
no ppp multilink mrru [local | remote] mrru-bytes
Syntax Description
Defaults
The default values for the local MRRU are the value of the multilink group interface MTU for multilink group members and 1524 bytes for all other interfaces.
The software accepts any peer MRRU value of 128 or higher.Command Modes
Interface configuration
Command History
Usage Guidelines
This command allows the MRRU value to be configured on MLP interfaces and member links. This command is useful for interfaces running an application such as IP Security (IPSec), where the addition of the IPSec header causes the packet to exceed the 1500-byte MTU of a typical IP packet.
When using a large-bundle interface MTU size, you must ensure that the individual frames-per-fragment size passed to the link interfaces is not greater than the link interface MTU setting or the peer MRRU setting. This size limit can be achieved in one of the following two ways:
•
Configure the link interface MTU setting appropriately.
•
Configure fragmentation such that the link MTU settings will never be violated.
When MLP is configured, several physical interfaces can constitute one logical connection to the peer. To represent the logical connection, software provides a logical interface, often called the bundle interface. This interface will have the IP address, for instance, and the MTU setting of the interface that IP uses when it is deciding whether to fragment an IP datagram that needs to be forwarded. The physical interfaces simply forward individual MLP fragments or frames that are given to them by the bundle interface.
The result of having to decide whether to fragment a packet is that, whereas with simple PPP the interface MTU must not exceed the peer's MRRU, with MLP the MTU size of the bundle interface must not exceed the MRRU setting of the peer. The MRRU settings on both sides need not be equal, but the "must not exceed" rule just specified must be followed; otherwise a system might send several fragments that, when reconstructed as a frame, will be too large for the peer's receive buffer.
Once you configure the MRRU on the bundle interface, you enable the router to receive large reconstructed MLP frames. You may want to configure the bundle MTU so that the router can send large MLP frames, although it is not strictly necessary. The maximum recommended value for the bundle MTU is the value of the peer's MTU. The software will automatically reduce the bundle interface MTU if necessary to avoid violating the peer's MRRU.
When the bundle interface MTU is tuned to a higher number, then depending upon the fragmentation configuration, the link interface may be given larger frames to send. There are two possible solutions to this problem, as follows:
•
Ensure that fragmentation is performed such that fragments are sized less than the link interface MTU (refer to the command pages for the ppp multilink fragment disable and ppp multilink fragment delay commands for more information about packet fragments).
•
Configure the MTUs of the link interfaces such that they can send the larger frames.
Note
Be careful when configuring MLP MRRU negotiation in a virtual private dialup network (VPDN) environment when an L2TP network server (LNS) is not running Cisco IOS Release 12.3(7)T. The software performs strict matching on the MRRU values in older versions of Cisco IOS software.
Examples
The following example shows how to configure MRRU negotiation on a virtual template with synchronous serial interfaces. The example also applies to asynchronous serial interfaces.
multilink virtual-template 1!interface virtual-template 1ip address 10.13.1.1 255.255.255.0mtu 1600!interface serial 0/0ppp multilinkppp multilink mrru local 1600mtu 1600!interface serial 0/1ppp multilinkppp multilink mrru local 1600mtu 1600The following example shows how to configure MRRU negotiation on multilink groups:
interface multilink 10ip address 10.13.1.1 255.255.255.0ppp multilink mrru local 1600mtu 1600!interface serial 0/0ppp multilinkmultilink-group 10mtu 1600!interface serial 0/1ppp multilinkmultilink-group 10mtu 1600The following example shows how to configure MRRU negotiation on dialer interfaces:
interface dialer 1ip address 10.13.1.1 255.255.255.0encapsulation pppdialer remote-name 2610-2dialer idle-timeout 30 inbounddialer string 5550101dialer pool 1dialer-group 1no cdp enableppp multilinkppp multilink mrru local 1600Related Commands
Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.
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