How to Filter Prefixes with Distribute-list

Prefixes that are advertised by routing protocols like OSPF, EIGRP or RIP can be filtered. One way of doing this is by using a distribute-list. In this lesson I’ll give you an example of how to filter certain prefixes with a distribute-list.

Filtering can occur inbound or outbound. If you have an inbound route filter we will first check if the network is permitted or not before we will accept it. Let’s take a look at a network topology so I can give you a demonstration:

route filtering two routers

R1 and R2 are connected to each other and running EIGRP. On R2 I have added a couple of loopback interfaces with prefixes that we will advertise in EIGRP. Here is the configuration:

R1(config)#router eigrp 12
R1(config-router)#no auto-summary
R1(config-router)#network 192.168.12.0
R2(config)#router eigrp 12
R2(config-router)#no auto-summary
R2(config-router)#network 192.168.12.0
R2(config-router)#network 172.16.0.0 0.0.3.255

Above you can see that we advertise all prefixes in EIGRP.

R1#show ip route eigrp 
     172.16.0.0/24 is subnetted, 4 subnets
D       172.16.0.0 [90/156160] via 192.168.12.2, 00:01:07, FastEthernet0/0
D       172.16.1.0 [90/156160] via 192.168.12.2, 00:01:07, FastEthernet0/0
D       172.16.2.0 [90/156160] via 192.168.12.2, 00:01:07, FastEthernet0/0
D       172.16.3.0 [90/156160] via 192.168.12.2, 00:01:07, FastEthernet0/0

If we look at the routing table of router R1 we can see all those networks on the loopback interfaces as it should be. Now we’ll see if we can do some filtering.

R1(config)#router eigrp 12
R1(config-router)#distribute-list ?
  <1-199>      IP access list number
  <1300-2699>  IP expanded access list number
  WORD         Access-list name
  gateway      Filtering incoming updates based on gateway
  prefix       Filter prefixes in routing updates
  route-map    Filter prefixes based on the route-map

Go to the configuration of the EIGRP process and use the distribute-list command to see your options. As you can see we can choose between an access-list, a prefix-list or a route-map. Let’s start with the access-list. You are probably familiar with the concept of access-lists if you studied CCNA.

R1(config-router)#distribute-list 1 ?
  in   Filter incoming routing updates
  out  Filter outgoing routing updates

If you specify an access-list number you can choose if this route filter has to be inbound or outbound.

R1(config-router)#distribute-list 1 in ?
  Async              Async interface
  BVI                Bridge-Group Virtual Interface
  CDMA-Ix            CDMA Ix interface
  CTunnel            CTunnel interface
  Dialer             Dialer interface
  FastEthernet       FastEthernet IEEE 802.3
  Lex                Lex interface
  Loopback           Loopback interface
  MFR                Multilink Frame Relay bundle interface
  Multilink          Multilink-group interface
  Null               Null interface
  Port-channel       Ethernet Channel of interfaces
  Tunnel             Tunnel interface
  Vif                PGM Multicast Host interface
  Virtual-PPP        Virtual PPP interface
  Virtual-Template   Virtual Template interface
  Virtual-TokenRing  Virtual TokenRing
  <cr>

If you want you can choose the interface where to apply the inbound route filter to. If you don’t specify an interface it will apply to all interfaces.

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Forum Replies

  1. Hey, what is the difference in using route-maps or access-lists or prefixlists when applied to distribute-list ? they all do the same in regards to filtering routes in ospf or eigrp ? are there any minor difference between those ? can you please explain in detail. Thank you.

  2. Hi Sri T,

    All three of them will filter routers but there are some big differences…

    If you haven’t seen my distribute-list with prefix-list tutorial then take a look at this link:

    http://networklessons.com/ip-routing/how-to-configure-prefix-list-on-cisco-router/

    It will explain the prefix-list in detail.

    The route-map is also very powerful…while access-lists and prefix-lists are both used to match on IP addresses or network addresses, route-maps can match on many other things including:

    Incoming interface
    Next hop IP address
    Source of route
    Metrics
    Route-type (

    ... Continue reading in our forum

  3. Rene , this is wrong lesson link
    you have to links referring to the same lesson

    this lesson should be for EIGRP filtering with prefix-list

  4. Hi Rene, I have a question regarding recusive routing and the filtering of routing interfaces.
    I have a network similar to the one shown in the EIGRP Route-MAP Filtering lesson. The difference I have in my network is that the Tunnel areas has two ASAs so that if the primary route fails routing information can be sent over a tunnel to the remote site via the internet. Routing on this network is provided bt EIGRP. Currently when the primary route fails the link that goes over the internet goes into recursive routing and fails.

    interface Tunnel0
     ip address 192.1
    ... Continue reading in our forum

  5. Hi @Robhorseman101,

    The EIGRP route-map filtering lesson only has two routers with a single link so I think you had something else in mind? :smile:

    In your config, I see the tunnel interfaces of your routers:

    interface Tunnel0
     ip address 192.168.100.2 255.255.255.0
     tunnel source GigabitEthernet2/0.40
     tunnel destination 10.164.56.33
    

    And:

    interface Tunnel0
     ip address 192.168.100.1 255.255.255.0
     tunnel source GigabitEthernet2/0.40
     tunnel destination 10.164.58.33
    

    Not sure on which router you get the recursive routing error but for example, if it’s the first one th

    ... Continue reading in our forum

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