Ensure that Routes Imported an IGP Take Precedence over IGP Routes
An IGP on multiple ASBRs is configured to import routes from a different protocol with a lower route priority than the IGP. However, only one of the ASBRs can succeed in importing the routes as IGP routes, and the other ASBRs learn the IGP routes from this ASBR. As a result, upstream traffic bypasses these ASBRs.
Scenario
In Figure 1-8, Router A and Router B are ASBRs and are configured to import BGP routes to OSPF as OSPF routes. To guide forwarding of upstream traffic, Router A advertises the OSPF routes to Router C, and Router B advertises the OSPF routes to Router D. Router B also learns the OSPF routes from Router A, and then the BGP routes on Router B become inactive because the priority of BGP routes is lower than that of OSPF routes. As a result, Router B does not advertise the BGP routes to Router D. In this case, upstream traffic of Router D bypasses Router B and flows to Router A through Router C.
Configuration Requirements
The priority value of IGP routes must be larger than that of imported routes. The larger the value, the lower the priority.
Misconfiguration Risks
Risk description:
Upstream traffic flows through the same ASBR according to IGP routes and then is forwarded using the original routes if the following conditions are met:
An IGP on multiple ASBRs is configured to import routes from a different protocol.
The priority of the imported routes is lower than that of IGP routes.
Identification method:
Check whether an IGP is configured to import routes from a different protocol.
# Run the display current-configuration configuration ospf command in any view to check whether OSPF is configured to import routes from a different protocol. In the following example, BGP routes are imported to OSPF.
<HUAWEI> display current-configuration configuration ospf
#
ospf 1 router-id 1.1.1.6
import-route bgp
area 0.0.0.0
network 1.1.19.6 0.0.0.0
#
return
# Run the display current-configuration configuration ospfv3 command in any view to check whether OSPFv3 is configured to import routes from a different protocol. In the following example, static routes are imported to OSPFv3.
<HUAWEI> display current-configuration configuration ospfv3
#
ospfv3 1
router-id 1.1.1.1
import-route static
#
return
# Run the display current-configuration configuration isis command in any view to check whether IS-IS is configured to import routes from a different protocol. In the following example, static routes are imported to IS-IS.
<HUAWEI> display current-configuration configuration isis
#
isis 1
cost-style wide
network-entity 10.000a.0011.0006.00
import-route static
#
return
Check the priority of the imported routes.
# Display OSPF configurations. In the following example, the priority of OSPF intra-area, inter-area, ASE, and NSSA routes is 200.
<HUAWEI> system-view
[HUAWEI] ospf 1
[HUAWEI-ospf-1] display this
#
ospf 1
preference 200
preference ase 200
#
return
# Display OSPFv3 configurations. In the following example, the priority of OSPFv3 intra-area, inter-area, ASE, and NSSA routes is 100.
<HUAWEI> system-view
[HUAWEI] ospfv3 1
[HUAWEI-ospfv3-1] display this
#
ospfv3 1
router-id 1.1.19.6
preference 100
preference ase 100
#
return
# Display IS-IS configurations. In the following example, the priority of IS-IS intra-area and inter-area routes is 200.
<HUAWEI> system-view
[HUAWEI] isis 1
[HUAWEI-isis-1] display this
#
isis 1
cost-style wide
network-entity 00.0005.0000.0019.0006.00
preference 200
#
return
# Display BGP configurations. In the following example, the priorities of EBGP, IBGP, and local routes are 200, 180, and 150, respectively.
<HUAWEI> system-view
[HUAWEI] bgp
[HUAWEI-bgp] display this
#
bgp 100
#
ipv4-family unicast
undo synchronization
preference 200 180 150
#
# Display RIP configurations. In the following example, the priority of RIP routes is 200.
<HUAWEI> display current-configuration configuration rip
#
rip 1
preference 200
#
return
# Display RIPng configurations. In the following example, the priority of RIPng routes is 200.
<HUAWEI> display current-configuration configuration ripng
#
ripng 1
preference 200
#
return
# Display static route configurations. In the following example, the priority of static routes is 200.
<HUAWEI> display current-configuration configuration
#
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ip route-static 1.1.1.1 32 NULL 0 preference 200
#
If no route priority is configured, Huawei devices use the following default priorities:
Recovery measures:
Reduce the priority value of IGP routes to a value smaller than that of imported routes using the preference command. The smaller the value, the higher the priority. Prevent the priority change from affecting normal services.
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