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Redistribution between OSPF and ISIS leads to routing black hole

Publication Date:  2013-12-26 Views:  150 Downloads:  0
Issue Description
The topology is shown as below.




R1, R2 and R3 are connected via Frame Relay; all of them are in Area 0 of OSPF. On R2, redistribute the network segment of 10.1.20.0 into OSPF. R1 and R3 are ISIS neighbors of R4, repectively. On R1 and R3, OSPF and ISIS redistribute routes into each other, and after the convergence of routes, R4 is not able to reach 10.1.20.0.
Alarm Information
NA
Handling Process
1. Configure the below commands on ISIS process of R1 and R3 to adjust the priority of OSPF ASE route in ISIS to 160. Now on R1 and R3, the ISIS route 10.1.24.0/24 learned from R4 is with priority of 160, while the priority of OSPF ASE route learnt from R2 is 150, the router will choose OSPF ASE route and add it into the IP routing table.
 
acl number 2000
rule 10 permit source 10.1.20.0 0.0.0.255

route-policy pre permit node 10
if-match acl 2000
apply preference 160
route-policy pre permit node 20

isis 1
preference route-policy pre
#

<R1>dis ip ro 10.1.20.0
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Table : Public
Summary Count : 1
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface

      10.1.20.0/24  O_ASE   150  1           D   10.1.123.2      Serial0/0/0

<R3>dis ip ro 10.1.20.0
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Table : Public
Summary Count : 1
Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface

      10.1.20.0/24  O_ASE   150  1           D   10.1.123.2      Serial0/0/0

2. Perform ping test and tracert test on R4 to validate that he packets can be forwarded normally.

<R4>ping 10.1.20.2
  PING 10.1.20.2: 56  data bytes, press CTRL_C to break
    Reply from 10.1.20.2: bytes=56 Sequence=1 ttl=254 time=90 ms
    Reply from 10.1.20.2: bytes=56 Sequence=2 ttl=254 time=80 ms
    Reply from 10.1.20.2: bytes=56 Sequence=3 ttl=254 time=80 ms
    Reply from 10.1.20.2: bytes=56 Sequence=4 ttl=254 time=50 ms
    Reply from 10.1.20.2: bytes=56 Sequence=5 ttl=254 time=60 ms

  --- 10.1.20.2 ping statistics ---
    5 packet(s) transmitted
    5 packet(s) received
    0.00% packet loss
    round-trip min/avg/max = 50/72/90 ms

<R4>tracert 10.1.20.2

traceroute to  10.1.20.2(10.1.20.2), max hops: 30 ,packet length: 40,press CTRL_C to break

1 10.1.34.3 20 ms  40 ms  40 ms

2 10.1.123.1 60 ms  60 ms 10.1.123.2 60 ms
Root Cause
When OSPF and ISIS redistribute routes into each other, because of the consequence of configuration and the convergence of routes, below phenomenon is appeared. In general cases, R1 and R3 learn the route 10.1.20.0/24 from R2, which is ASE route with priority of 150. When performing the redistributions on R1 and R3, first, R3 redistributes this route into ISIS area of R4, and then R4 refreshes its routing table and advertise this route to its neighbor R1. Now there are two routes of 10.1.20.0/24, one is ISIS route, and the other is OSPF ASE route. Because the priority of ISIS (15) is higher than OSPF ASE route (150), so the router will choose the ISIS route. After that, R1 will redistribute this ISIS route into OSPF, when R1 will generate an external route, which the destination network is 10.1.20.0/24 and next hop is R1 itself, and will advertise this route to OSPF neighbor R3. Therefore R3 will receive two ASE routes to 10.1.20.0/24 but with different next hops, and this route will be in IP routing table of R3. This way there is a loop, R4->R3->R1->R4.
Suggestions
1. When redistributing routes between different routing protocols, be careful about the loops. Take actions to routes that are redistributed like filtering routes to avoid loops.

2. Route redistribution between different routing protocols will makes the changes of priority. Be careful on the changes and take actions to control routes.

END