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Services Are Not Switched to the Backup Static Route When the Next Hop of the Primary Static Route Is Down

Publication Date:  2012-07-27 Views:  33 Downloads:  0
Issue Description
The version of the NE40E is NE40E&80E V300R002C06B325. The NE40E is configured with two static routes for the 219.147.255.192/28 network segment. The purpose is to switch services to 219.147.146.118 when 219.147.146.114 is Down.
ip route-static 219.147.255.192 255.255.255.240 219.147.146.114
ip route-static 219.147.255.192 255.255.255.240 219.147.146.118 preference 150 
 
Alarm Information
Null
Handling Process
View the routing table. The result shows that the next hop of 219.147.255.192 is still 219.147.146.114, with the outbound interface changing to a Null0 interface.
<HL-HR-XSN-C-1.IDC>dis ip rou 219.147.255.192
Route Flags: R - relay, D - download to fib
---------------------------------------
Routing Table : Public
Summary Count : 1

Destination/Mask Proto Pre Cost Flags NextHop Interface
219.147.255.192/28 Static 60 0 RD 219.147.146.114 NULL0
Continue to view the routing table of 219.147.146.114. The next hop of this static route is found to be 0.0.0.0.
<HL-HR-XSN-C-1.IDC>dis ip rou 219.147.146.114
Route Flags: R - relay, D - download to fib
--------------------------------------------------
Routing Table : Public
Summary Count : 1

Destination/Mask Proto Pre Cost Flags NextHop Interface

219.147.146.0/24 Static 254 0 D 0.0.0.0 NULL0
In the configuration, a static route of 219.147.146.0/24 is found to point to the Null0 interface. This static route is used for BGP to advertise routes in network mode.
ip route-static 219.147.146.0 255.255.255.0 NULL0 preference 254
Configure outbound interfaces for the two 219.147.255.192/28 static routes. Then the services resume to normal.
ip route-static 219.147.255.192 255.255.255.240 219.147.146.114 GigabitEthernet2/0/0
ip route-static 219.147.255.192 255.255.255.240 219.147.146.118 GigabitEthernet2/0/1 preference 150 
 
Root Cause
Generally, when packets are forwarded along a static route for which no outbound interface is designated, route iteration is required. According to the iterative algorithm, routes are found based on the longest prefix match principle, which can be seen from the following three static routes of the NE40E.
ip route-static 219.147.146.0 255.255.255.0 NULL0 preference 254
ip route-static 219.147.255.192 255.255.255.240 219.147.146.114
ip route-static 219.147.255.192 255.255.255.240 219.147.146.118 preference 150
Because no outbound interface is configured, when interface 219.147.146.114 is Down, the router searches for the iterative route of route 219.147.146.114. Then a route in the 219.147.146.0/24 segment is found as the iterative route, incorrectly. If an outbound interface is configured for the static route, no next-hop iteration will be conducted. The router directly searches the destination segment 219.147.255.192/28 for a route for service forwarding. 
 
Suggestions
During route configuration, try to avoid configuring static routes in the network segment where the blackhole route belongs. If the static routes have to be in the same network segment as the blackhole route for local service forwarding and route advertisement by BGP, add outbound interfaces to the static routes. 

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