MPLS Traffic Engineering—
Overload Avoidance Support for IS-IS
First Published: June 23, 2003
Last Updated: February 28, 2006
The MPLS Traffic Engineering—Overload Avoidance Support for IS-IS feature enables users to ensure
that specific routers will be included in Constrained Shortest Path First calculations, even when the
routers indicate through IS-IS that they are overloaded.
History for the MPLS Traffic Engineering—Overload Avoidance Support for IS-IS Feature
Release
Modification
12.0(22)S
This feature was introduced.
12.2(28)SB
This feature was integrated into Cisco IOS Release 12.2(28)SB.
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Contents
•
Feature Overview, page 2
•
Prerequisites, page 2
•
Command Reference, page 2
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© 2002, 2006 Cisco Systems, Inc. All rights reserved.
MPLS Traffic Engineering— Overload Avoidance Support for IS-IS
Feature Overview
Feature Overview
Occasionally you may want a router in an IS-IS network to not carry traffic. For example:
•
A test router in a lab that is connected to a production network.
•
A router configured as an link-state packet (LSP) flooding server (on an nonbroadcast multiaccess
(NBMA) network, for example) in combination with the mesh-group feature.
•
A router that is aggregating virtual circuits (VC), being used only for network management.
•
A router that is experiencing a memory shortage, making its routing table inaccurate or incomplete.
This new feature is implemented through a single command, mpls traffic-eng path-selection overload
allow. For the reader’s convenience, we have also included information about an older command,
set-overload-bit (which had already been fully documented in the Cisco IOS IP Configuration Guide
and Command Reference), because the new command’s function is so closely tied to that older command
To isolate any such router, you turn on the set-overload-bit command (described in detail on page 5).
Doing so will allow other routers to ignore this router in their SPF calculations. No paths through this
router will be seen by other routers in the IS-IS area. (Nevertheless, IP and Connectionless Network
Service (CLNS) prefixes directly connected to this router still will be reachable).
Now, you can selectively override the overload bit with respect to Multiprotocol Label Switching
(MPLS) LSPs by using a new command: mpls traffic-eng path-selection overload allow. This
command enables the operator to include an overloaded node in CSPF.
Prerequisites
Your network must support the following Cisco IOS features in order for this command to have any
effect:
•
MPLS
•
IS-IS
Command Reference
This section documents modified commands only.
•
mpls traffic-eng path-selection overload allow
•
set-overload-bit
Cisco IOS Release: Multiple releases (see the Feature History table)
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MPLS Traffic Engineering— Overload Avoidance Support for IS-IS
mpls traffic-eng path-selection overload allow
mpls traffic-eng path-selection overload allow
To ensure that Label Switched Paths (LSPs) will not be torn down because of routers whose Intermediate
System-to-Intermediate System (IS-IS) overload bit is turned on, use the mpls traffic-eng
path-selection overload allow command in global configuration mode on the LSP’s headend router. To
disable this override, use the no form of the command.
mpls traffic-eng path-selection overload allow {head [middle] [tail] | middle [tail] | tail}
no mpls traffic-eng path-selection overload allow {head [middle] [tail] | middle [tail] | tail}
Syntax Description
head
Allows LSPs to originate at the head-end router even when, on that same
router, the overload bit has been turned on.
middle
Tells the head-end router to allow LSPs to transit midpoint routers that have
their overload bit turned on.
tail
Tells the head-end router to allow LSPs to terminate at tail routers that have
their overload bit turned on.
Command Default
No default behavior or values.
Command Modes
Global configuration
Command History
Release
Modification
12.0(22)S
This command was introduced.
12.2(28)SB
This command was integrated into Cisco IOS Release 12.2(28)SB.
Usage Guidelines
The command is always configured on the head node of the LSP you want to affect.
You must specify which nodes along the LSP are to ignore the overload bit—head only, middle only, tail
only, or some combination of head, middle, and tail.
When you want a combination of locations, you must specify them in this order: head first, then middle,
then tail. No other order will work.
Examples
The following example allows ingress and egress LSPs to be set up on routers whose overload bit is
turned on:
Router(config)# mpls traffic-eng path-selection overload allow head tail
The following example removes the override of the IS-IS overload bit on this node:
Router(config)# no mpls traffic-eng path-selection overload allow head tail
Cisco IOS Release: Multiple releases (see the Feature History table)
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MPLS Traffic Engineering— Overload Avoidance Support for IS-IS
mpls traffic-eng path-selection overload allow
Related Commands
Command
Description
set-overload-bit
Configures a router to signal other routers not to use it as an intermediate hop
in their SPF calculation.
Cisco IOS Release: Multiple releases (see the Feature History table)
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MPLS Traffic Engineering— Overload Avoidance Support for IS-IS
set-overload-bit
set-overload-bit
To configure the router to signal other routers not to use it as an intermediate hop in their shortest path
first (SPF) calculations, use the set-overload-bit command in router configuration mode. To remove the
designation, use the no form of this command.
set-overload-bit
no set-overload-bit
Syntax Description
This command has no arguments or keywords.
Command Default
The overload bit is not set.
Command Modes
Router configuration
Command History
Release
Modification
11.2
This command was introduced.
12.2(28)SB
This command was integrated into Cisco IOS Release 12.2(28)SB.
Usage Guidelines
This command forces the router to set the overload bit (also known as the hippity bit) in its
nonpseudonode link-state packets (LSPs). Normally, the setting of the overload bit is allowed only when
a router runs into problems. For example, when a router is experiencing a memory shortage, it might be
that the link-state database is not complete, resulting in an incomplete or inaccurate routing table. By
setting the overload bit in its LSPs, other routers can ignore the unreliable router in their SPF
calculations until the router has recovered from its problems.
Examples
The following example configures the set-overload bit:
router isis
set-overload-bit
Cisco IOS Release: Multiple releases (see the Feature History table)
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MPLS Traffic Engineering— Overload Avoidance Support for IS-IS
set-overload-bit
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© 2002, 2006 Cisco Systems, Inc. All rights reserved.
Cisco IOS Release: Multiple releases (see the Feature History table)
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