IPv6 Access-list on Cisco IOS

As explained in my first tutorial that introduces access-lists, we can use access-lists for filtering (blocking packets) or selecting traffic (for VPNs, NAT, etc).

This also applies to IPv6 access-lists which are very similar to IPv4 access-lists. There are two important differences however:

• IPv4 access-lists can be standard or extended, numbered or named. IPv6 only has named extended access-lists.
• IPv4 access-lists have an invisible implicit deny any at the bottom of every access-list. IPv6 access-lists have three invisible statements at the bottom:
  • permit icmp any any nd-na
  • permit icmp any any nd-ns
  • deny ipv6 any any

The two permit statements are required for neighbor discovery which is an important protocol in IPv6, it’s the replacement for ARP.

When you use a deny ipv6 any any at the bottom of your access-list, make sure you also add the two permit statements for neighbor discovery just before the final statement or this traffic will be dropped.

Having said that, let’s take a look at the configuration.

Configuration

For this demonstration we only need two routers:

I’ll use subnet 2001:DB8:0:12::/64 in between R1 and R2. To demonstrate the access-list, I’ll create one inbound on R2 and we will try to filter some packets from R1. Let’s take a look at the access-list:

R2(config)#ipv6 access-list ?
  WORD        User selected string identifying this access list
  log-update  Control access list log updates

As you can see above the only option is the named access-list. There’s also no option for standard or extended access-list. Let’s create that access-list:

R2(config)#ipv6 access-list R1_TRAFFIC

I’ll call it “R1_TRAFFIC”. Here are our options when we create a statement:

R2(config-ipv6-acl)#permit ?
  <0-255>             An IPv6 protocol number
  X:X:X:X::X/<0-128>  IPv6 source prefix x:x::y/<z>
  ahp                 Authentication Header Protocol
  any                 Any source prefix
  esp                 Encapsulation Security Payload
  host                A single source host
  icmp                Internet Control Message Protocol
  ipv6                Any IPv6
  pcp                 Payload Compression Protocol
  sctp                Streams Control Transmission Protocol
  tcp                 Transmission Control Protocol
  udp                 User Datagram Protocol

This is similar to IPv4 access-lists. You can pick any protocol you like. Let’s see if we can permit telnet traffic from R1 and deny everything else:

R2(config-ipv6-acl)#permit tcp ?
  X:X:X:X::X/<0-128>  IPv6 source prefix x:x::y/<z>
  any                 Any source prefix
  host                A single source host

Let’s permit telnet traffic from R1:

R2(config-ipv6-acl)#permit tcp host 2001:db8:0:12::1 ?
  X:X:X:X::X/  IPv6 destination prefix x:x::y/
  any                 Any destination prefix
  eq                  Match only packets on a given port number
  gt                  Match only packets with a greater port number
  host                A single destination host
  lt                  Match only packets with a lower port number
  neq                 Match only packets not on a given port number
  range               Match only packets in the range of port numbers

After specifying the source IP I also have to select the destination IP, let’s do that:

R2(config-ipv6-acl)#permit tcp host 2001:db8:0:12::1 any eq 23

This should permit telnet traffic from R1. Let’s take a look at our access-list:

R2#show access-lists 
IPv6 access list R1_TRAFFIC
    permit tcp host 2001:DB8:0:12::1 any eq telnet sequence 10

Above you see our statement. One cosmetic difference with IPv4 access-lists is that the sequence number is behind the statement. Let’s apply this access-list on the interface:

R2(config)#interface FastEthernet 0/0
R2(config-if)#ipv6 traffic-filter R1_TRAFFIC in

Instead of using the access-group command you have to use the ipv6 traffic-filter command. Let’s see if it works:

R1#telnet 2001:db8:0:12::2
Trying 2001:DB8:0:12::2 ... Open

R1 is able to telnet to R2. Let’s see if we find any matches on our access-list:

R2#show access-lists 
IPv6 access list R1_TRAFFIC
    permit tcp host 2001:DB8:0:12::1 any eq telnet (10 matches) sequence 10

There we go, we see it matches the access-list. Anything else should be dropped…let’s try a simple ping:

R1#ping 2001:db8:0:12::2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:0:12::2, timeout is 2 seconds:
AAAAA
Success rate is 0 percent (0/5)

The AAAAAs that you see above indicate that the destination is administratively unreachable, it means that an access-list is dropping our packets.

Usually, this output indicates that an access list is blocking traffic. For security reasons it might be a bad idea to tell someone that traffic has been dropped. If you want you can disable this:

R2(config)#interface FastEthernet 0/0
R2(config-if)#no ipv6 unreachables 

Use the no ipv6 unreachables command to disable this. When we send another ping now you will see this:

R1#ping 2001:db8:0:12::2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:0:12::2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

R2 is no longer informing R1 that the packets have been dropped. That’s all I have for now, have fun configuring IPv6 access-lists.

hostname R1
!
ipv6 unicast-routing
!
interface FastEthernet0/0
 no ip address
 ipv6 address 2001:DB8:0:12::1/64
!
end

hostname R2
!
ipv6 unicast-routing
!
interface FastEthernet0/0
 no ip address
 ipv6 address 2001:DB8:0:12::2/64
 no ipv6 unreachables
 ipv6 traffic-filter R1_TRAFFIC in        
!
ipv6 access-list R1_TRAFFIC
 permit tcp host 2001:DB8:0:12::1 any eq telnet
!
end

You can also add an IPv6 access-list on a switchport (L2) interface with the PACL.

If you have any questions just leave a comment.