WALKTHROUGH: OSPF Route Preference

Back to the Route Preference Lab

So you're stuck or don't trust your work. Here is the cheat/check page. Note that this is all done from the perspective of J2300-1. Adapt to other routers as needed.

1. Locate a destination in your routing table which has multiple equal cost paths and was populated by OSPF

admin@J2300-1> show route 

inet.0: 49 destinations, 49 routes (49 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

3.3.3.0/24         *[OSPF/150] 00:01:13, metric 12610, tag 3
                    > to 10.1.111.101 via fe-0/0/1.1001
4.4.4.0/24         *[OSPF/150] 22:23:07, metric 1000, tag 4
                    > to 10.1.111.101 via fe-0/0/1.1001
5.1.1.0/25         *[Static/5] 03:33:46
                      Receive
5.2.1.0/25         *[Static/5] 03:33:17
                      Receive
5.5.5.0/24         *[OSPF/150] 17:03:05, metric 3515, tag 5
                    > to 10.0.12.2 via fe-0/0/1.12
7.7.7.0/24         *[OSPF/150] 17:03:05, metric 1000, tag 7
                      to 10.0.12.2 via fe-0/0/1.12
                    > to 10.0.13.3 via fe-0/0/1.13
10.0.0.1/32        *[Direct/0] 1d 06:04:19
                    > via lo0.0
10.0.0.2/32        *[OSPF/10] 01:03:21, metric 5
                    > to 10.0.12.2 via fe-0/0/1.12
10.0.0.3/32        *[OSPF/10] 01:03:21, metric 5
                    > to 10.0.13.3 via fe-0/0/1.13
                                        
admin@J2300-1> 

7.7.7.0 has more than one potential next hop.

2. For areas 0.0.0.1, 0.0.0.2 and 0.0.0.3, log into one of those areas internal routers that connects to an ABR for which your router serves as the ABR

Login using the router name for the username, and juniper123 for the password.

user@mgmtbox~ $ ssh R2@10.99.10.2
R2@10.99.10.2's password: 
--- JUNOS 10.4R6.5 built 2011-07-23 10:01:44 UTC
R2@LR:R2> 

3. What is the best route from one side of the area to the other side? (i.e. from R1 to R2 in area 0.0.0.1, R6 to R8 in area 0.0.0.2 or R9 to R10 in area 0.0.0.3?

R2@LR:R2> show route 10.1.0.1    

inet.0: 50 destinations, 50 routes (50 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.1.0.1/32        *[OSPF/150] 00:01:25, metric 10020, tag 0
                    > to 10.1.23.103 via fxp4.123

R2@LR:R2> 

Best route points to R3.

4. Perform a traceroute from one side of the area to the other and confirm the path

R2@LR:R2> traceroute 10.1.0.1 no-resolve    
traceroute to 10.1.0.1 (10.1.0.1), 30 hops max, 40 byte packets
 1  10.1.23.103  0.652 ms  0.561 ms  0.534 ms
 2  10.1.34.104  0.846 ms  0.783 ms  0.829 ms
 3  10.1.0.1  1.159 ms  1.089 ms  1.114 ms

R2@LR:R2> 

Trace goes to R3, to R4 to R1.

5. For your area, configure the backbone link that connects the two ABRs together to participate in another area without harming the integrity of your backbone

admin@J2300-1> edit 
Entering configuration mode

[edit]
admin@J2300-1# edit protocols ospf area 1 

[edit protocols ospf area 0.0.0.1]
admin@J2300-1# set interface fe-0/0/1.13 secondary 

[edit protocols ospf area 0.0.0.1]
admin@J2300-1# commit 
commit complete

[edit protocols ospf area 0.0.0.1]
admin@J2300-1# run show ospf neighbor 
Address          Interface              State     ID               Pri  Dead
10.0.12.2        fe-0/0/1.12            Full      10.0.0.2         128    35
  Area 0.0.0.0
10.0.13.3        fe-0/0/1.13            Full      10.0.0.3         128    32
  Area 0.0.0.0
10.1.111.101     fe-0/0/1.1001          Full      10.1.0.1         128    10
  Area 0.0.0.1
10.0.13.3        fe-0/0/1.13            Full      10.0.0.3         128    32
  Area 0.0.0.1
10.101.0.6       fe-0/0/1.101           Full      10.0.0.6         128    34
  Area 101.101.101.101
10.101.0.5       fe-0/0/1.101           Full      10.0.0.5         128    34
  Area 101.101.101.101
10.101.0.4       fe-0/0/1.101           Full      10.0.0.4         128    36
  Area 101.101.101.101
10.101.0.3       fe-0/0/1.101           Full      10.0.0.3         128    35
  Area 101.101.101.101
10.101.0.2       fe-0/0/1.101           Full      10.0.0.2         128    34
  Area 101.101.101.101
10.101.0.10      fe-0/0/1.101           Full      10.10.10.10      128    38
  Area 101.101.101.101
10.102.0.6       fe-0/0/1.102           Full      10.0.0.6         128    32
  Area 102.102.102.102
10.102.0.5       fe-0/0/1.102           Full      10.0.0.5         128    37
  Area 102.102.102.102
10.102.0.4       fe-0/0/1.102           Full      10.0.0.4         128    32
  Area 102.102.102.102
10.102.0.3       fe-0/0/1.102           Full      10.0.0.3         128    38
  Area 102.102.102.102
10.102.0.2       fe-0/0/1.102           Full      10.0.0.2         128    32
  Area 102.102.102.102
10.102.0.10      fe-0/0/1.102           Full      10.10.10.10      128    34
  Area 102.102.102.102
10.30.30.6       fe-0/0/1.100           Full      10.0.0.6         255    36
  Area 30.30.30.30
10.30.30.5       fe-0/0/1.100           Init      10.0.0.5         255    35
  Area 30.30.30.30
10.30.0.4        fe-0/0/1.100           Full      10.0.0.4         255    35
  Area 30.30.30.30
10.30.30.3       fe-0/0/1.100           Full      10.0.0.3         255    32
  Area 30.30.30.30
10.30.30.2       fe-0/0/1.100           Full      10.0.0.2         255    32
  Area 30.30.30.30
10.30.0.5        fe-0/0/1.100           Full      10.0.0.5         255    33
  Area 30.30.30.30
10.30.30.30      fe-0/0/1.100           Full      99.99.99.99      199    32
  Area 30.30.30.30                      

[edit protocols ospf area 0.0.0.1]
admin@J2300-1# 

6. What is the best route from one side of the area to the other side? (i.e. from R1 to R2 in area 0.0.0.1, R6 to R8 in area 0.0.0.2 or R9 to R10 in area 0.0.0.3?

R2@LR:R2> show route 10.1.0.1               

inet.0: 50 destinations, 50 routes (50 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.1.0.1/32        *[OSPF/150] 00:00:50, metric 5010, tag 0
                    > to 10.1.123.3 via fe-0/0/1.1003

R2@LR:R2> 

Best route is now via J2300-3.

7. Perform a traceroute from one side of the area to the other and confirm the path is more efficient

R2@LR:R2> traceroute 10.1.0.1 no-resolve    
traceroute to 10.1.0.1 (10.1.0.1), 30 hops max, 40 byte packets
 1  10.1.123.3  0.683 ms  0.456 ms  0.532 ms
 2  10.0.13.1  0.649 ms  1.481 ms  0.742 ms
 3  10.1.0.1  1.245 ms  1.176 ms  1.244 ms

R2@LR:R2> 

Trace now goes from J2300-3, to J2300-1, to R1

8. Compare your Router LSA that is being advertised into the backbone with the Router LSA that is being advertised into area 0.0.0.1, 0.0.0.2 or 0.0.0.3

admin@J2300-1> show ospf database router lsa-id 10.0.0.1 area 0 detail 

    OSPF database, Area 0.0.0.0
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *10.0.0.1         10.0.0.1         0x80000048   193  0x22 0xc755  72
  bits 0x3, link count 4
  id 10.0.0.2, data 10.0.12.1, Type PointToPoint (1)
    Topology count: 0, Default metric: 5
  id 10.0.12.0, data 255.255.255.0, Type Stub (3)
    Topology count: 0, Default metric: 5
  id 10.0.0.3, data 10.0.13.1, Type PointToPoint (1)
    Topology count: 0, Default metric: 5
  id 10.0.13.0, data 255.255.255.0, Type Stub (3)
    Topology count: 0, Default metric: 5
  Topology default (ID 0)
    Type: PointToPoint, Node ID: 10.0.0.3
      Metric: 5, Bidirectional
    Type: PointToPoint, Node ID: 10.0.0.2
      Metric: 5, Bidirectional

admin@J2300-1> show ospf database router lsa-id 10.0.0.1 area 1 detail    

    OSPF database, Area 0.0.0.1
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *10.0.0.1         10.0.0.1         0x80000046   212  0x22 0x7906  48
  bits 0x3, link count 2
  id 10.1.111.101, data 10.1.111.1, Type Transit (2)
    Topology count: 0, Default metric: 2500
  id 10.0.0.3, data 0.0.0.68, Type PointToPoint (1)
    Topology count: 0, Default metric: 2500
  Topology default (ID 0)
    Type: PointToPoint, Node ID: 10.0.0.3
      Metric: 2500, Bidirectional
    Type: Transit, Node ID: 10.1.111.101
      Metric: 2500, Bidirectional

admin@J2300-1> 

The PointToPoint link (Type 1) between J2300-1 and J2300-3 is listed in both LSAs, but only the LSA announced to the backbone has Stub route (Type 3) announcing the IP address space of the link.

9. Change your router to prefer BGP routes more than OSPF external routes (Hint: BGP has a default preference of 170 in Junos)

admin@J2300-1> show route protocol ospf                 

inet.0: 49 destinations, 49 routes (49 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

3.3.3.0/24         *[OSPF/150] 00:05:53, metric 5110, tag 3
                    > to 10.0.13.3 via fe-0/0/1.13
4.4.4.0/24         *[OSPF/150] 22:39:22, metric 1000, tag 4
                    > to 10.1.111.101 via fe-0/0/1.1001
5.5.5.0/24         *[OSPF/150] 17:19:20, metric 3515, tag 5
                    > to 10.0.12.2 via fe-0/0/1.12
7.7.7.0/24         *[OSPF/150] 17:19:20, metric 1000, tag 7
                      to 10.0.12.2 via fe-0/0/1.12
                    > to 10.0.13.3 via fe-0/0/1.13
10.0.0.2/32        *[OSPF/10] 01:19:36, metric 5
                    > to 10.0.12.2 via fe-0/0/1.12
10.0.0.3/32        *[OSPF/10] 01:19:36, metric 5
                    > to 10.0.13.3 via fe-0/0/1.13
10.0.0.4/32        *[OSPF/10] 01:19:36, metric 10
                    > to 10.0.12.2 via fe-0/0/1.12
                      to 10.0.13.3 via fe-0/0/1.13
10.0.0.5/32        *[OSPF/10] 01:19:36, metric 10
                    > to 10.0.13.3 via fe-0/0/1.13
10.0.0.6/32        *[OSPF/10] 01:19:36, metric 15
                                        
admin@J2300-1> edit 
Entering configuration mode

[edit]
admin@J2300-1# set protocols ospf external-preference 171 

[edit]
admin@J2300-1# commit and-quit 
commit complete
Exiting configuration mode

admin@J2300-1> show route protocol ospf    

inet.0: 49 destinations, 49 routes (49 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

3.3.3.0/24         *[OSPF/171] 00:00:07, metric 5110, tag 3
                    > to 10.0.13.3 via fe-0/0/1.13
4.4.4.0/24         *[OSPF/171] 00:00:07, metric 1000, tag 4
                    > to 10.1.111.101 via fe-0/0/1.1001
5.5.5.0/24         *[OSPF/171] 00:00:07, metric 3515, tag 5
                    > to 10.0.12.2 via fe-0/0/1.12
7.7.7.0/24         *[OSPF/171] 00:00:07, metric 1000, tag 7
                    > to 10.0.12.2 via fe-0/0/1.12
                      to 10.0.13.3 via fe-0/0/1.13
10.0.0.2/32        *[OSPF/10] 01:19:58, metric 5
                    > to 10.0.12.2 via fe-0/0/1.12
10.0.0.3/32        *[OSPF/10] 01:19:58, metric 5
                    > to 10.0.13.3 via fe-0/0/1.13
10.0.0.4/32        *[OSPF/10] 01:19:58, metric 10
                    > to 10.0.12.2 via fe-0/0/1.12
                      to 10.0.13.3 via fe-0/0/1.13
10.0.0.5/32        *[OSPF/10] 01:19:58, metric 10
                    > to 10.0.13.3 via fe-0/0/1.13
10.0.0.6/32        *[OSPF/10] 01:19:58, metric 15
                                        
admin@J2300-1> 

10. Advertise your loopback interface out as part of your Router LSA

admin@J2300-1> edit   
Entering configuration mode

[edit]
admin@J2300-1# set protocols ospf area 0 interface lo0 passive 

[edit]
admin@J2300-1# commit 
commit complete

[edit]
admin@J2300-1# 
[edit]

11. Is your router still sending External LSAs out advertising your Loopback interface?

admin@J2300-1# run show ospf database external lsa-id 10.0.0.1 
    OSPF AS SCOPE link state database
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Extern  *10.0.0.1         10.0.0.1         0x80000006   269  0x22 0xa8d   36

[edit]
admin@J2300-1# 
Yes, because the export policy is still applied.

Yes, because the export policy is still applied.

Back to the Route Preference Lab

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Last page update 6 December 2102

OSPF Deep Dive by Blackhole Networks is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
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