CCIE Pursuit Blog

April 22, 2008

Question Of The Day: 22 April, 2008

Topic: OSPF

Which ‘show’ command will give you detailed information about Type-5 LSAs you are receiving?

Click Here For The Answer


Yesterday’s Question

Question Of The Day: 21 April, 2008 

Topic: OSPF

Give two methods of introducing a default route into an OSPF NSSA.

Answer: ‘area nssa default-information-originate’ and ‘area nssa no-summary’

An OSPF NSSA (Not-So-Stubby-Area) will remove Type 5 LSAs while allowing redistribution into the area (Type 7 LSAs which are converted to Type 5 LSAs at the ABR).  The NSSA is the only OSPF stub area that does not originate a default route.  There are two methods to introduce a default route into an NSSA (both are configured on the ABR):

1) area 13 nssa default-information-originate
2) area 13 nssa no-summary

In this example, r1 is the ABR.  We’ve made area 13 an NSSA.  Now we want r1 to advertise a default route into the NSSA.

ABR:
r1
(config)#do sh ip os int br
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/0        100   0               10.1.12.1/24       1     DR    1/1
Fa1/0        100   13              10.1.13.1/24       1     BDR   1/1

Stub:
r3(config-if)#do sh ip os int br
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/0        100   13              10.1.13.3/24       1     DR    1/1

r3(config-if)#do sh ip route os
     100.0.0.0/32 is subnetted, 1 subnets
O IA    100.1.4.4 [110/4] via 10.1.13.1, 00:03:14, FastEthernet0/0
     10.0.0.0/24 is subnetted, 4 subnets
O IA    10.1.12.0 [110/2] via 10.1.13.1, 00:03:54, FastEthernet0/0
O IA    10.1.24.0 [110/3] via 10.1.13.1, 00:03:20, FastEthernet0/0
O IA    10.1.34.0 [110/4] via 10.1.13.1, 00:03:01, FastEthernet0/0

r3(config)#do show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 100)

                Router Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         11          0x80000006 0x0044A6 1
3.3.3.3         3.3.3.3         6           0x80000005 0x00C01C 1

                Net Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.13.1       1.1.1.1         12          0x80000001 0x00B74F

                Summary Net Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.12.0       1.1.1.1         46          0x80000001 0x000813
10.1.24.0       1.1.1.1         46          0x80000001 0x008D80
10.1.34.0       1.1.1.1         46          0x80000001 0x0029D9
100.1.4.4       1.1.1.1         46          0x80000001 0x00B50D

Method 1:
r1(config-router)#area 13 nssa default-information-originate

r3(config-if)#do sh ip route os           
     100.0.0.0/32 is subnetted, 1 subnets
O IA    100.1.4.4 [110/4] via 10.1.13.1, 00:06:06, FastEthernet0/0
     10.0.0.0/24 is subnetted, 4 subnets
O IA    10.1.12.0 [110/2] via 10.1.13.1, 00:06:45, FastEthernet0/0
O IA    10.1.24.0 [110/3] via 10.1.13.1, 00:06:11, FastEthernet0/0
O IA    10.1.34.0 [110/4] via 10.1.13.1, 00:05:52, FastEthernet0/0
O*N2 0.0.0.0/0 [110/1] via 10.1.13.1, 00:01:10, FastEthernet0/0
            
r3(config-if)#do show ip ospf database | b Type-7
                Type-7 AS External Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum Tag
0.0.0.0         1.1.1.1         84          0x80000001 0x002173 0

Note that the default route is advertised in a Type-7 LSA and that the IA routes are not filtered from the routing table.

Method 2:
r1
(config-router)#area 13 nssa no-summary

r3(config)#do sh ip route os
O*IA 0.0.0.0/0 [110/2] via 10.1.13.1, 00:00:46, FastEthernet0/0

r3(config)#do show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 100)

                Router Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         669         0x80000004 0x005C8E 1
3.3.3.3         3.3.3.3         672         0x80000004 0x00D605 1

                Net Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.13.3       3.3.3.3         672         0x80000001 0x0047B5

                Summary Net Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         1.1.1.1         77          0x80000001 0x001B17

Note that the OSPF IA routes are gone from the routing table.  We can see that the Summary Net Link States (LSA Type 3) are replaced with a default route.

When deciding between which method to use, look for phrase in the task like “introduce a default route but still allow inter-area routes to appear in the routing table” or “the default route should be advertised in a Type 7 LSA”.

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3 Comments »

  1. […] Click Here For The Answer […]

    Pingback by Question Of The Day: 21 April, 2008 « CCIE Pursuit — April 22, 2008 @ 8:02 am | Reply

  2. the command to sould be :
    rtr03#show ip ospf database external

    Comment by lessaid — April 22, 2008 @ 4:10 pm | Reply

  3. […] Question Of The Day: 22 April, 2008  […]

    Pingback by Question Of The Day: 23 April, 2008 « CCIE Pursuit — April 23, 2008 @ 6:15 am | Reply


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