OSPF Inter area Filtering / Type 3 LSA Filtering

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May 202012
 

As link state protocols do not exchange routes, Selective route filtering can be done only on an ABR.

Type 3 LSA filtering can be achieved in 2 different ways. This can be done by using:

  • Filter-list
  • Summarization

When using filter-list we could choose which routes should be filtered to which area. We could define the source area and the destination area. In the following example the filter-list is used to filter the 1.1.1.1/32 subnet towards area 23. 

R2#sh ip ospf database self-originate | begin Area 23
                Router Link States (Area 23)

Link ID         ADV Router      Age         Seq#       Checksum Link count
22.22.22.22     R2              324         0x80000005 0x00E547 1

                Summary Net Link States (Area 23)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.0         R2              1825        0x80000001 0x003D99
1.1.1.1         R2              72          0x80000001 0x0033A2
2.2.2.2         R2              256         0x80000001 0x00A03B
10.0.12.0       R2              324         0x80000004 0x004A77
 

Before filtering we could see Type 3 LSA for 1.1.1.1. Now we can filter the LSA as it enters inbound to area 23. For this we first create a prefix list which only denies 1.1.1.1/32.

R2(config)#ip prefix-list A23_FILTER_IN deny 1.1.1.1/32
R2(config)#ip prefix-list A23_FILTER_IN permit 0.0.0.0/0 le 32

 

Now we can apply this prefix list to the OSPF process for the area 23 in inbound direction.

R2(config)#router ospf 1
R2(config-router)#area 23 filter-list prefix A23_FILTER_IN in
 
R2#sh ip ospf database self-originate | begin Area 2 
                Router Link States (Area 2)
 
Link ID         ADV Router      Age         Seq#       Checksum Link count
22.22.22.22     R2              39          0x80000001 0x00B3C4 1
 
                Summary Net Link States (Area 2)
 
Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.0         R2              34          0x80000001 0x003D99
1.1.1.1         R2              34          0x80000001 0x0033A2
10.0.12.0       R2              34          0x80000001 0x005074
10.0.23.0       R2              34          0x80000001 0x00D6E2
 
                Router Link States (Area 23)
 
Link ID         ADV Router      Age         Seq#       Checksum Link count
22.22.22.22     R2              102         0x80000005 0x00E547 1
 
                Summary Net Link States (Area 23)
 
Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.0         R2              1603        0x80000001 0x003D99
2.2.2.2         R2              34          0x80000001 0x00A03B
10.0.12.0       R2              102         0x80000004 0x004A77
 

The databse from area 23 does not have 1.1.1.1/32 subnet anymore.

Now we could use the second method to filter which is summarization. With Summarization filtering we cannot influence the destination area to which the LSAs goes to. 

R2(config)#router ospf 1
R2(config-router)#area 0 range 1.1.1.1 255.255.255.255 not-advertise
 
R2#sh ip ospf database self-originate | begin Area 2
                Router Link States (Area 2)
 
Link ID         ADV Router      Age         Seq#       Checksum Link count
22.22.22.22     R2              479         0x80000001 0x00B3C4 1
 
                Summary Net Link States (Area 2)
 
Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.0         R2              475         0x80000001 0x003D99
10.0.12.0       R2              475         0x80000001 0x005074
10.0.23.0       R2              475         0x80000001 0x00D6E2
 
                Router Link States (Area 23)
 
Link ID         ADV Router      Age         Seq#       Checksum Link count
22.22.22.22     R2              543         0x80000005 0x00E547 1
 
                Summary Net Link States (Area 23)
 
Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.0         R2              32          0x80000002 0x003B9A
2.2.2.2         R2              475         0x80000001 0x00A03B
10.0.12.0       R2              543         0x80000004 0x004A77
 

The summarization method is less preferred as it does not have the granularity to apply filter. Anyway summarization is not mainly used for achieving filtering.