Disable Default Route Advertisement Before a Cutover
If a device sets the overload (OL) bit to 1 in IS-IS LSPs, the routes passing through the device are switched to a backup path. If the device is enabled to advertise its default route, the default route is not switched to the backup path.
Scenario
In Figure 1-9, IS-IS neighbor relationships are established between Device A and Device B, between Device A and Device C, between Device B and Device D, and between Device C and Device D. The primary path passes through Device A, Device C, and Device D, and the backup path passes through Device A, Device B, and Device D. The problem may occur in either of the following scenarios:
1. during a cutover, the set-overload command is run in the IS-IS view on Device C to switch services immediately to the backup path;
2. the set-overload on-startup command is run in the IS-IS view on Device C to trigger route switchover in case of a master/slave MPU switchover or device restart.
Configuration Requirements
Default route advertisement must be disabled in the IS-IS view before the cutover and then re-enabled after the cutover.
<HUAWEI> system-view
[HUAWEI] isis 1
[HUAWEI-isis-1] undo default-route-advertise
Misconfiguration Risks
Risk description:
If the set-overload [ on-startup ] and default-route-advertise commands are run in the IS-IS view on Device C, the default route is not switched to the backup path during the cutover. If no specific routes are available to Device C, services are interrupted.
After the timer (600s by default) that starts upon a device restart or master/slave MPU switchover expires, the default route is not switched to the backup path.
Identification method:
Run the display isis process-id lsdb local verbose command in any view to check LSP information. If the OL bit is 1 and default route advertisement is enabled, the problem has occurred. In the following example, the OL bit is 1, and default route advertisement is enabled.
<HUAWEI> display isis 1 lsdb local verbose
ATTENTION: System is overloaded
Manual overload set YES OverLoad on Startup NO
System Memory Low NO Memory Allocate Failure NO
Level-2 Link State Database
LSPID Seq Num Checksum Holdtime Length ATT/P/OL
-------------------------------------------------------------------------------
abcd.0001.0031.00-00* 0x0000008e 0x6726 1183 76 0/0/1
SOURCE abcd.0001.0031.00
NLPID IPV4
AREA ADDR 10
INTF ADDR 10.10.1.1
INTF ADDR 30.1.1.2
+NBR ID aaaa.0001.0030.00 COST: 10
+IP-Extended 10.10.1.1 255.255.255.255 COST: 0
+IP-Extended 30.1.1.0 255.255.255.0 COST: 10
abcd.0001.0031.00-01* 0x00000001 0x289a 1179 34 0/0/0
SOURCE abcd.0001.0031.00
+IP-Extended 0.0.0.0 0.0.0.0 COST: 0
*(In TLV)-Leaking Route, *(By LSPID)-Self LSP, +-Self LSP(Extended),
ATT-Attached, P-Partition, OL-Overload
Recovery measures:
Before the cutover, run the undo default-route-advertise command in the IS-IS view. After the cutover, run the default-route-advertise command again.
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