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VOSS: Routing loop caused by incorrect OSPF route overriding the original ISIS route

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TitleVOSS: Routing loop caused by incorrect OSPF route overriding the original ISIS route
Symptoms
Though a particular network was learnt via ISIS, the VSP switches replaces it with a OSPF route advertised by the ASBR in the network. This caused a routing loop as the router advertising the ISIS route only has the direct connectivity to that network.
Environment
  • VOSS
  • All supported software versions.
User-added image
VSP 2 and 3 are in a vIST cluster.
VSP 1 has ISIS adjacencies with VSP2 and VSP 3.
VSP 2 and VSP 3 has separate OSPF adjacency with Third party router. 
10.1.1.0/24 is learnt by VSP 2 and VSP 3 via ISIS. Both the VSP 2&3 redistribute the same route into OSPF. 

When the links are brought up, the VSP 2 and VSP 3 initially has the 10.1.1.0/24 as an ISIS route in the routing table. However after full convergence with OSPF neighbors, the 10.1.1.0/24 is learnt as a OSPF route replacing the ISIS route. Both of the routers shows the other router as the next hop causing a routing loop.
Cause
The issue is caused due to a flaw in routing logic as the original ISIS route should be available in the routing table to reach the destination network. The default route preference for SPBM_L1(ISIS) routes is 5 but it was changed  manually by mistake to 130 which is higher than the OSPF's preference.. If the routes are learnt through multiple protocols, the route with the least preference would be installed in the routing table and so, the OSPF route sent by the each routers were replacing the original ISIS route sent by the VSP 1 device.
 
ip route preference protocol spbm-level1 130
#show ip route preference
*******************************************************************************
                Command Execution Time: Mon Oct 28 15:12:38 2019 SGT
*******************************************************************************

====================================================================================================
                       IP Route Preference - GlobalRouter
====================================================================================================
PROTOCOL        DEFAULT    CONFIG
----------------------------------------------------------------------------------------------------
LOCAL           0          0
STATIC          5          5
SPBM_L1         7          130
OSPF_INTRA      20         20
OSPF_INTER      25         25
EBGP            45         45
RIP             100        100
OSPF_E1         120        120
OSPF_E2         125        125
IBGP            175        175
Resolution
The below command was used to reset the route preference of ISIS routes to default value of 7. This command immediately changed the routing table and ISIS route replaced the OSPF route solving the routing issue.
 
(config)#ip route preference protocol spbm-level1 7

 
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