EIGRP Route-Map Filtering

EIGRP supports filtering with access-lists and prefix-lists but you can also use route-maps. In this lesson I’ll show you how to use a route-map to filter in- and outbound route advertisements. We will use the following topology for this:

r1 r2 multiple loopbacks

We only need two routers for this demonstration. R1 has some networks that it will advertise to R2 through EIGRP. Here’s what the routing table of R2 looks like:

R2#show ip route eigrp 
     1.0.0.0/24 is subnetted, 1 subnets
D       1.1.1.0 [90/409600] via 192.168.12.1, 00:00:45, FastEthernet0/0
     172.16.0.0/16 is variably subnetted, 4 subnets, 4 masks
D       172.16.0.0/24 [90/409600] via 192.168.12.1, 00:00:14, FastEthernet0/0
D       172.16.1.0/25 [90/409600] via 192.168.12.1, 00:00:08, FastEthernet0/0
D       172.16.2.0/26 [90/409600] via 192.168.12.1, 00:00:14, FastEthernet0/0
D       172.16.3.0/27 [90/409600] via 192.168.12.1, 00:00:13, FastEthernet0/0
D    192.168.1.0/24 [90/409600] via 192.168.12.1, 00:00:13, FastEthernet0/0

Above you see that R2 has learned all networks behind R1. Let’s start with something simple…let’s say we want to configure R1 so that 192.168.1.0 /24 won’t be advertised to R2. Here’s how we do this:

R1(config)#router eigrp 1
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

We have to use the distribute-list command under the EIGRP process but as you can see it supports a route-map. Let’s use that and give it a name:

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

I’ll call my route-map “FILTER_OUT” and we will choose outgoing updates:

R1(config-router)#distribute-list route-map FILTER_OUT out

Now we can create the route-map:

R1(config)#route-map FILTER_OUT ?      
  <0-65535>  Sequence to insert to/delete from existing route-map entry
  deny       Route map denies set operations
  permit     Route map permits set operations
  <cr>

We will start with a deny statement:

R1(config)#route-map FILTER_OUT deny 10

The route-map will require a match statement. There are a lot of things you can select for the match statement:

R1(config-route-map)#match ?
  as-path           Match BGP AS path list
  clns              CLNS information
  community         Match BGP community list
  extcommunity      Match BGP/VPN extended community list
  interface         Match first hop interface of route
  ip                IP specific information
  ipv6              IPv6 specific information
  length            Packet length
  local-preference  Local preference for route
  metric            Match metric of route
  mpls-label        Match routes which have MPLS labels
  nlri              BGP NLRI type
  policy-list       Match IP policy list
  route-type        Match route-type of route
  source-protocol   Match source-protocol of route
  tag               Match tag of route

Not all of these options are possible when you use the route-map for filtering. Let’s start with a simple example, let’s look at the IP options:

R1(config-route-map)#match ip address ?
  <1-199>      IP access-list number
  <1300-2699>  IP access-list number (expanded range)
  WORD         IP access-list name
  prefix-list  Match entries of prefix-lists
  <cr>

Here we can use an access-list or prefix-list. Let’s try the access-list:

R1(config-route-map)#match ip address NET_192

Don’t forget to create the actual access-list:

R1(config)#ip access-list standard NET_192
R1(config-std-nacl)#permit 192.168.1.0 0.0.0.255

The route-map is almost complete. We have a deny statement that matches everything in our access-list. There’s one problem though, our route-map doesn’t have any permit statements. If we don’t add one then everything will be blocked. Let’s add it:

R1(config)#route-map FILTER_OUT permit 20
R1(config-route-map)#exit

This permit statement doesn’t require any matches. Let me show you an overview of our configuration so far:

R1#show running-config | section eigrp
router eigrp 1
 network 0.0.0.0
 distribute-list route-map FILTER_OUT out FastEthernet0/0
 no auto-summary
R1#show route-map 
route-map FILTER_OUT, deny, sequence 10
  Match clauses:
    ip address (access-lists): NET_192 
  Set clauses:
  Policy routing matches: 0 packets, 0 bytes
route-map FILTER_OUT, permit, sequence 20
  Match clauses:
  Set clauses:
  Policy routing matches: 0 packets, 0 bytes

Above you can see that the route-map is attached to the distribute-list command in EIGRP. Our route-map will deny everything that matches our access-list while everything else is permitted. Let’s take a look at R2 to see if this works:

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You’ve Ever Spent on Your Cisco Career!
  • Full Access to our 740 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)

551 Sign Ups in the last 30 days

satisfaction-guaranteed
100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

Forum Replies

  1. Rene what will happen if both router-map and access-list are deny?

    route-map NAME deny 10
    match ip address 1
    
    ip access-list standard 1
    deny 1.1.1.0
    permit any 
    

    thanks!

  2. correction it’s “route-map” not “router-map”.

    sorry for typo

  3. Hi John,

    That’s a nice trick question, here’s what happens:

    Everything in NAME deny 10 with a permit in the access-list will be denied, so this matches the “permit any”.

    This means that “1.1.1.0” can be processed further down the route-map, however we don’t have any other route-map statements so it will be denied anyway.

    If you would add a “route-map NAME permit 20” with nothing in it then the 1.1.1.0 network would match there and it will be permitted.

    Rene

  4. Hey Rene should this be corrected.

    The topology shows all 172.16.x.x networks as /24 when the route table shows /24 , /25 , /26 , /27
    Also, the outbound prefix list section:
    For example let’s say that we also want to deny all prefixes in the 172.16.0.0 /16 range that use a /26 subnet mask or smaller.
    Should the smaller be changed to larger since the route table below no longer shows the /26 and /27 network

  5. Hi Matt,

    I fixed the image so it now shows /25, /26 and 27.

    The outbound prefix list is correct but it can be confusing…/27 is a smaller subnet than /26 but it’s a higher number. I changed the sentence to “that use a /26 subnet mask or smaller subnet mask”.

    Thanks for sharing this!

    Rene

48 more replies! Ask a question or join the discussion by visiting our Community Forum