DMVPN - Dynamic routing tips reminder

OSPF - Filter redistribution in a Totally NSSA area

In some cases, we can have the ABR which can also be an ASBR. If we are working with a Totally NSSA area we have unnecessary routes. ABR announces a default route and his redistributed routes which are not necessary. In order to solve this case and only announce a default route, we can use the commande below on the ABR:

 area X nssa no-redistribution no-summary

The schema below is anexample of this implementation:

Troubleshoot OSPF neighbors (Hellos check)

In order to become neighbors, routers perform several checks. If this check fails, we have to troubleshoot and find the cause of this issue. You will find below several examples of neighbor failed. I have added logs messages and debug messages in order to easily find the cause.

• Area mismatch:

• Authentication key mismatch:

• Duplicate Router-id:

• Subnet/mask Mismatch:

• Area Type Mismatch:

OSPF Database Reminder in the same area

This is a reminder in order to interpret an ospf database on a Cisco router. I have created the following lab to test the 'show ip ospf datase':

In red, the router ID:

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

The list of all routers in the area:

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         129         0x80000002 0x0057AC 1
2.2.2.2         2.2.2.2         25          0x80000003 0x00536B 2
3.3.3.3         3.3.3.3         24          0x80000002 0x0016B6 2

All the network segments in the area and the DR for each segment. Here, 10.1.2.2 is the IP address of the DR on the segment and 2.2.2.2 is the router id:

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.2.2        2.2.2.2         103         0x80000001 0x0021F5
10.1.4.2        2.2.2.2         25          0x80000001 0x006F9D

Each subnet, even if there is no OSPF neighbor:

R1#show ip ospf database internal

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Stub Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Mask
10.1.2.255      1.1.1.1         355         0x0        0x006D69 /24
10.1.3.255      2.2.2.2         98          0x0        0x00448D /24
10.1.4.255      2.2.2.2         315         0x0        0x003997 /24
10.1.5.255      3.3.3.3         230         0x0        0x0010BB /24