Information Technology Reference
In-Depth Information
There are two options. The first is to configure the router to set the OL-bit on startup for a
predefined amount of time. This feature is configured as follows:
router isis
set-overload-bit on-startup timeout
The only problem is that BGP might converge much faster than the configured time. The
second option is to use the following configuration:
router isis
set-overload-bit on-startup wait-for-bgp
This option allows BGP to signal to IS-IS that it is converged and to remove the OL-bit. If
BGP does not signal IS-IS in 10 minutes, the OL-bit is removed. This prevents IS-IS from
becoming stuck in an overloaded state. This feature was added to Cisco IOS Release 12.0(7)S.
For information on integration into other Cisco IOS trains, reference Cisco DDTs CSCdp01872.
It is recommended that you use the
wait-for-bgp
option.
OSPF Maximum Metric on Startup
The OSPF protocol does not have an OL-bit. In OSPF, the OL-bit functionality provided
by IS-IS needs to be simulated through the manipulation of metrics. An OSPF router can
set all metrics in its router link-state advertisement (LSA) and can network LSA to the
maximum metric. This ensures that the router will not be used as a transit path but will still
be reachable for BGP session formation. The configuration options are as follows:
router ospf 100
max-metric router-lsa on-startup timeout
The OSPF max-metric feature can operate in cooperation with BGP using the following
command:
router ospf 100
max-metric router-lsa on-startup wait-for-bgp
The OSPF router reissues its router LSA and any network LSAs after notification from
BGP that BGP is converged. If BGP does not notify OSPF in 10 minutes, the router LSA
and network LSAs are regenerated with the proper metrics to prevent the router from
becoming stuck in a nontransit state. You can verify the OSPF maximum metric on-startup
feature with the
show ip ospf protocol
command, as shown in Example 3-10.
Example 3-10
Verifying the OSPF Maximum Metric on Startup
Routing Process "ospf 100" with ID 10.1.1.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an area border and autonomous system boundary router
Redistributing External Routes from,
static, includes subnets in redistribution
Originating router-LSAs with maximum metric, Time remaining: 00:01:18
Condition: on startup while BGP is converging, State: active
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
