Networking

Overlapping/Duplicate AS-External-LSA IDs – Network Rare

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The LSA uses the network address as an identifier. If one router is generating multiple Type 5 LSAs with the same network number but different masks, then only 1 would be advertised because the LSA ID would be the same.

RFC 2328 Appendix E

I have created 3 static routes. all end up with the same network number and would normally have the same LSA ID:

R1(config)#ip route 192.9.0.0 255.255.0.0 Null0  
R1(config)#ip route 192.9.0.0 255.255.254.0 Null0
R1(config)#ip route 192.9.0.0 255.255.255.0 Null0
R1(config)#router ospf 1
R1(config-router)#redistribute static subnets 

Let’s see what LSA IDs are:

R1#sho ip osp database | inc 192.9
192.9.0.0       1.1.1.1         246         0x80000001 0x00933F 0
192.9.0.255     1.1.1.1         149         0x80000001 0x00933F 0
192.9.1.255     1.1.1.1         234         0x80000001 0x00834F 0
R1#

R1#sho ip ospf database external 192.9.0.0

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

                Type-5 AS External Link States

  LS age: 14
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.9.0.0 (External Network Number )
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000003
  Checksum: 0x8F41
  Length: 36
  Network Mask: /16
        Metric Type: 2 (Larger than any link state path)
        TOS: 0 
        Metric: 20 
        Forward Address: 0.0.0.0
        External Route Tag: 0

The router gives the last 2 networks as broadcast address of that respective network as the Link State ID. The /16 network got the network address as the ID.

Removing two routes to understand the behavior:

R1(config)#no ip route 192.9.0.0 255.255.0.0 Null0
R1(config)#no ip route 192.9.0.0 255.255.254.0 Null0
R1(config)#no ip route 192.9.0.0 255.255.255.0 Null0
R1(config)#ip route 192.9.0.0 255.255.255.0 Null0   

So now /24 is the only there and it is using 192.9.0.0 as its ID:

R1#sho ip osp database external 192.9.0.0

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

                Type-5 AS External Link States

  LS age: 36
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.9.0.0 (External Network Number )
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000001
  Checksum: 0x933F
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0 
        Metric: 20 
        Forward Address: 0.0.0.0
        External Route Tag: 0

What happens if we add a /16 now?

R1(config)#ip route 192.9.0.0 255.255.0.0 Null0

R1#sho ip osp database external 192.9.0.0

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

                Type-5 AS External Link States

  LS age: 12
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 192.9.0.0 (External Network Number )
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000002
  Checksum: 0x9140
  Length: 36
  Network Mask: /16
        Metric Type: 2 (Larger than any link state path)
        TOS: 0 
        Metric: 20 
        Forward Address: 0.0.0.0
        External Route Tag: 0

R1#

The /16 has taken the ID from the /24. This is amazing.

R1#sho ip osp database | inc 192.9       
192.9.0.0       1.1.1.1         45          0x80000002 0x009140 0
192.9.0.255     1.1.1.1         45          0x80000001 0x00933F 0

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