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:

R1 R2 2001 DB8 0 12

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:

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Forum Replies

  1. Hi Rene,

    I think telnet traffic filter in line vty NOT per interface ? correct me if I wrong.

  2. Hi @hussien.samer,

    It’s working fine here:

    R2(config)#ipv6 access-list R1_TRAFFIC   
    R2(config-ipv6-acl)#deny tcp any any eq telnet
    R2(config-ipv6-acl)#permit any any
    
    R2(config)#interface GigabitEthernet 2
    R2(config-if)#ipv6 traffic-filter R1_TRAFFIC in
    

    This blocks telnet traffic and permits everything else:

    R1#telnet 2001:DB8:0:12::2
    Trying 2001:DB8:0:12::2 ... 
    % Connection timed out; remote host not responding
    
    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 rat
    ... Continue reading in our forum

  3. I tested this on Cisco VIRL. You might also want to try it on some real hardware…could be a IOL quirk :slight_smile:

  4. Maybe, in the end I wanted to know the correct behavior, thanks for your feedback.

  5. Hello Allen

    Yes, that is counter-intuitive! Can you share with us a little more of your configuration? Can you include your topology, the application of the access list on the interface(s) as well as what seq 10 was originally?

    Laz

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