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:

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, 00:01:01, 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, 00:01:01, FastEthernet0/0
D       172.16.1.0/25 [90/409600] via 192.168.12.1, 00:01:01, FastEthernet0/0
D       172.16.2.0/26 [90/409600] via 192.168.12.1, 00:01:01, FastEthernet0/0
D       172.16.3.0/27 [90/409600] via 192.168.12.1, 00:01:01, FastEthernet0/0

That’s looking good, everything is in the routing table except 192.168.1.0 /24. Now you might be thinking that this was a lot of work just to filter one network…

You are right, this was a lot of work. The power of using a route-map for filtering is that we can use multiple statements and use a mix of filtering techniques.

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 or smaller subnet. We can do this by creating a prefix-list and attaching it to our route-map:

R1(config)#route-map FILTER_OUT deny 20
R1(config-route-map)#match ip address prefix-list SMALL_PREFIXES
R1(config)#ip prefix-list SMALL_PREFIXES permit 172.16.0.0/16 ge 26
R1(config)#route-map FILTER_OUT permit 30

Above I changed route-map entry 20 to a deny statement that checks for our prefix-list called “SMALL_PREFIXES”. The last permit statement (sequence number 30) doesn’t have any match statements and is required to permit all other route advertisements. Here’s what the complete route-map looks like:

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, deny, sequence 20
  Match clauses:
    ip address prefix-lists: SMALL_PREFIXES 
  Set clauses:
  Policy routing matches: 0 packets, 0 bytes
route-map FILTER_OUT, permit, sequence 30
  Match clauses:
  Set clauses:
  Policy routing matches: 0 packets, 0 bytes

Our first sequence number (10) is used to filter with an access-list, the second one (20) uses our prefix-list and the last one (30) permits everything else. Let’s check the result of R2:

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, 00:38:18, FastEthernet0/0
     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
D       172.16.0.0/24 [90/409600] via 192.168.12.1, 00:38:18, FastEthernet0/0
D       172.16.1.0/25 [90/409600] via 192.168.12.1, 00:38:18, FastEthernet0/0

Great, as you can see network 172.16.2.0 /26 and 172.16.3.0 /27 are gone fishing, they have been filtered because of the prefix-list. I think this example should give you a good idea about the flexibility of a route-map, you can use a variety of filtering techniques.

Let’s try one more thing…we can also use a route-map for inbound filtering. Let’s filter network 1.1.1.0 /24 on R2, to keep things interesting i’ll use the route-map in a different way:

R2(config)#ip access-list standard NET_1
R2(config-std-nacl)#deny 1.1.1.0 0.0.0.255
R2(config-std-nacl)#permit any

First we create an access-list. This access-list denies 1.1.1.0 /24 and permits everything else. Now we create the route-map:

R2(config)#route-map FILTER_IN permit 10
R2(config-route-map)#match ip address NET_1

This route-map has only one permit statement. Everything that matches our access-list will be permitted. Let’s attach it to EIGRP:

R2(config)#router eigrp 1
R2(config-router)#distribute-list route-map FILTER_IN in

Now we can check the routing table of R2:

R2#show ip route eigrp 
     172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
D       172.16.0.0/24 [90/409600] via 192.168.12.1, 00:56:08, FastEthernet0/0
D       172.16.1.0/25 [90/409600] via 192.168.12.1, 00:56:08, FastEthernet0/0

Table of Content

Unit 1: Introduction to EIGRP

Unit 2: EIGRP Neighbor Adjacency

Unit 3: EIGRP Filtering

Unit 4: EIGRP Advanced Features