My Personal Practices for Handling IPv6 Allocations

This is going to be based on a bit of both practical deployment experience and opinion. Let us start with what is acceptable for BGP announcements. RIRs hand out /48s as the smallest allocation. This is perfectly fine for BGP announcements. If you got a larger allocation, say a /32, you should be announcing that master prefix. This will help curb de-aggregation of routes and keep the IPv6 routing table smaller. Is anyone going to tar and feather you for announcing a few specifics? Most likely not, especially if you are providing some sort of anycasted service. Otherwise, until an RIR starts allocating and handing out something more specific than a /48, put in some global filters to make sure nothing gets leaked like /64s.

When providing someone with an abundance of IPv6 subnets (/48, /56, multiple /64s, etc.), make certain that you aren’t putting them ON-LINK, and instead are routing them to a destination IPv6 address on a downstream router. Either by static routes or accepting and properly filtering BGP advertised routes. If you allocate someone a /48 and configure 2001:db8:1::1/48 on the interface facing their router, you basically do a disservice to them. They would need to hack around with proxy NDP and other headaches, rather than have something straight forward and working in seconds.

As far as I’m aware, only 2 popular providers out there seem incompetent regarding this: OVH and FDC. Either they don’t understand how to set static routes on their routers, or don’t want to learn. Seriously, what is so hard about typing in:

conf t
ipv6 route 2001:db8:2::/48 2001:db8:1:2::2
wr mem

With RA, a host will usually configure their default gateway as the link-local address based on the upstream router’s interface, facing that host. For static IPv6 configurations, the 2001:db8:1:2::/64 network address tends to be reserved for just that purpose, being the network address. Some software out there might not like trying to bind on this address, or perhaps there are some older and possibly poorly designed and deployed networking code that won’t like treating that as a host’s address. My recommendation is just use the first “usable” address out of an allocation as the gateway address for hosts, like 2001:db8:1:2::1/64. However any address will work, as with a /64 you get 18+ quintillion of them to pick from (as long as it doesn’t conflict with the host).

Another thing to consider is using /126 allocations for links between routers. This gives a few IPs to use on the link which can be good for multiple BGP sessions, etc. It will also limit the possibility for a ND table overload by someone trying to hit all possible addresses in a larger range, if the link is configured with something smaller (I’ve oversimplified this explanation).

Sure some players like Sprint will plop down 2600:: as that host’s AAAA record and address. Maybe it is a /127 with 2600::1/127 set as gateway; who knows other than Sprint. Point is, to avoid any potential issues, you are better served treating it as the reserved network address for the allocation.

Basic BGP configuration for IPv6

So you have your ASN and just got your PI (Provider-Independent Address Allocation) straight from an RIR, or perhaps a LoA (Letter of Authority) to announce some PA (Provider-Assigned Address Allocation) space from your ISP. You’ll need to start getting that announced out there to your peers, customers and transits. So lets use some documentation prefixes and ASNs and sort out a basic working config. I’ll base the examples on my experience with Brocade NetIron software on XMRs, which can translate over to Quagga or Cisco IOS with a few tweaks.

Assuming you are familiar with BGP4+ with IPv4, IPv6 is not so different or any more complex when getting started. Lets start off with some numbers:

Upstream ISP ASN: 64500
Your ASN: 64501
Specific /126 configured on interfaces out of allocated /64: 2001:db8:0:1::/126
PA allocation to announce: 2001:db8:1234::/48

Start off with making certain that you’ve configured IPv6 on your upstream facing interface, and they’ve configured your side, and you can ping each other over the link. The upstream provider’s configuration can be done as so:

isp#conf t
isp#(config)ipv6 prefix-list as64501-ipv6-filter seq 1 permit 2001:db8:1234::/48
isp#(config)router bgp
isp#(config-bgp)nei 2001:db8:0:1::2 remote-as 64501
isp#(config-bgp)nei 2001:db8:0:1::2 desc Customer_Name
isp#(config-bgp)no nei 2001:db8:0:1::2 activate
isp#(config-bgp)add ipv6 uni
isp#(config-bgp-af)nei 2001:db8:0:1::2 activate
isp#(config-bgp-af)nei 2001:db8:0:1::2 filter-list as64501-ipv6-filter in
isp#wr mem

So the breakdown of these steps is:

  • enter configuration mode on router
  • build a filter to restrict what the customer (you) are allowed to announce (seq optional but required for multiple entries in list)
  • enter BGP configuration mode
  • create a specific session using target/destination IPv6 address and ASN of customer
  • optionally add a description of the session perhaps to track who it is more clearly
  • you don’t want IPv4 routes going out or coming in over the session, IPv6 routes only
  • change address-family for IPv6 specific BGP settings
  • activate sending and learning IPv6 routes over the session
  • apply the filter for accepting INBOUND routes from you/customer
  • exit & then write out the config

On the customer side it is similar but not exact:

you#conf t
you#(config)ipv6 prefix-list outbound-ipv6-filter seq 1 permit 2001:db8:1234::/48
you#(config)router bgp
you#(config-bgp)nei 2001:db8:0:1::1 remote-as 64500
you#(config-bgp)nei 2001:db8:0:1::1 desc ISP_Name
you#(config-bgp)no nei 2001:db8:0:1::1 activate
you#(config-bgp)add ipv6 uni
you#(config-bgp-af)network 2001:db8:1234::/48
you#(config-bgp-af)nei 2001:db8:0:1::1 activate
you#(config-bgp-af)nei 2001:db8:0:1::1 filter-list outbound-ipv6-filter out
you#wr mem

The differences being:
1) outbound filter list on your session to the ISP
2) network statement for the allocation to be announced by BGP

You’ll also need some sort of anchor route so BGP knows to announce the route. This can be either a local null-route or static-route for the covering prefix, or an IP out of the prefix configured on an interface. So once BGP is configured, and establishes between both routers, the ISP side should see something similar to:

isp#sh ipv6 bgp nei 2001:db8:0:1::2 routes
       There are 1 accepted routes from neighbor 2001:db8:0:1::2
Searching for matching routes, use ^C to quit...
       Prefix             Next Hop        Metric     LocPrf     Weight  Status
1      2001:db8:1234::/48  2001:db8:0:1::2
                                          1          140        0       E
         AS_PATH: 64501

And that should be it. You can obviously play around with peer-groups, route-maps, etc. Communities should work as long as your ISP offers them, so be sure to ask. Ideally there is at least a blackhole community in place. That should allow you to announce a specific range or IP that you want to have null-routed upstream in cases of abuse or attacks. That filter could look like either of the following, depending on how specific they allow you to announce:

ipv6 prefix-list as64501-ipv6-filter seq 1 permit 2001:db8:1234::/48 le 64


ipv6 prefix-list as64501-ipv6-filter seq 1 permit 2001:db8:1234::/48 le 128

With the obvious requirement that the BGP sessions would need to be configured as blackhole community enabled on both sides.