Using Conditional Router Advertisements for Enterprise Multihoming

As discussed in one of two main problems to be solved in the enterprise multihoming scenario is the problem of the next-hop (uplink) selection based on the packet source address. For example if the enterprise network has two uplinks, to ISP_A and ISP_B and hosts have addresses from subnet_A and subnet_B (belonging to ISP_A and ISP_B respectively) then packets sourced from subnet_A must be sent to ISP_A uplink while packets sourced from subnet_B must be sent to ISP_B uplink. While some work is being done in the Source Address Dependent Routing (SADR) area, the simplest way to implement the desired functionality currently is to apply a policy which select a next-hop or an egress interface based on the packet source. Most of SMB/Enterprise grade routers have such functionality available currently.

Another problem to be solved in the multihoming scenario is the source address selection on hosts. In the normal situation (all uplinks are up/operational) hosts have multiple global unique addresses and can rely on the default address selection algorithm () to pick up a source address, while the network is responsible for choosing the correct uplink based on the source address selected by a host as described in . However some network topology changes (changing uplink status) might affect the global reachability for packets sourced from the particular prefixes and therefore such changes have to be signaled back to the hosts. For example: An uplink to an ISP_A went down so hosts should not use addresses from ISP_A prefix; A primary uplink to ISP_A which was not operational has come back up so hosts should start using the source addresses from ISP_A prefix; provides detailed explanation on why SLAAC and router advertisements are the most suitable mechanism for signaling network topology changes to hosts and influencing the source address selection. Sending a router advertisement to change the preferred lifetime for a given prefix provides the following functionality: deprecating addresses (by sending an RA with the preferred_lifetime set to 0 in the corresponding POI) to indicate to hosts that that addresses from that prefix should not be used; making previously unused (deprecated) prefix usable again (by sending an RA containing a POI with non-zero preferred lifetime) to indicate to hosts that addresses from that prefix can be used again; To provide the desired functionality, first-hop routers are required to send RA triggered by defined event policies in response to uplink status change event. while sending periodic or solicted RAs, set the value in the given RA field (e.g. PIO preferred lifetime) based on the uplink status. The exact definition of the 'uplink status' depends on the network topology and may include conditions like: uplink interface status change presence of a particular route in the routing table presence of a particular route with the particular attribute (next-hop, tag etc) in the routing table protocol adjacency change etc. In some scenarios, when two routers providing first-hop redundancy via VRRP, the master-backup status can be considered as a conditions for sending RA and changing the preferred lifetime value. See for more details. If hosts are provided with ISP DNS servers IPv6 addresses via RDNSS it might be desirable for the conditional RAs to update Lifetime field of the RDNSS option as well.

This section illustrates how the conditional RAs solution can be applied to most common enterprise multihoming scenarios.

Let's look at the simple network topology when a single router acts as a border router to terminate two ISP uplinks and as a first-hop router for hosts. Each ISP assigns /48 to the network and the ISP_A uplink is a primary one, to be used for all Internet traffic, while the ISP_B uplink is a backup, to be used only when the primary uplink is not operational. To ensure that packets with ISP_A and ISP_B source addresses are only routed to ISP_A and ISP_B uplinks respectively, network administrator needs to configure a policy on R1:

if { packet_destination_address is not in 2001:db8:1::/48 or 2001:db8:2::/48 packet_source_address is in 2001:db8:1::/48 } then { next-hop is ISP_A_uplink } if { packet_destination_address is not in 2001:db8:1::/48 or 2001:db8:2::/48 packet_source_address is in 2001:db8:1::/48 } then { next-hop is ISP_B_uplink } Under normal circumstances it's desirable that all traffic is sent via ISP_A uplink, therefore hosts (the host H1 in the example topology figure) should be using source addresses from 2001:db8:1:1:/64. When/if ISP_A uplink fails, hosts should stop using 2001:db8:1:1:/64 prefix and start using 2001:db8:1:2:/64 until the ISP_A uplink comes back up. To achieve the desired behavior the router advertisement configuration on the R1 device for the interface facing H1 needs to have the following policy:

prefix 2001:db8:1:1::/64 { if ISP_A_uplink is up then preferred_lifetime = 604800 else preferred_lifetime = 0 } prefix 2001:db8:2:1::/64 { if ISP_A_Uplink is up then preferred_lifetime = 0 else preferred_lifetime = 604800 } The similar policy needs to be applied to RDNSS Lifetime filed if ISP_A and ISP_B DNS servers are used.

Let's look at a bit more complex scenario when two border routers are terminating two ISP uplinks (one each), acting as redundant first-hop routers for hosts. The topology is shown on the Fig.2

In this scenario R1 sends RAs with PIO for 2001:db8:1:1::/64 (ISP_A address space) and R2 sends RAs with PIO for 2001:db8:2:1::/64 (ISP_B address space). Each router needs to have a forwarding policy configured for packets received on hosts-facing interface:

if { packet_destination_address is not in 2001:db8:1::/48 or 2001:db8:2::/48 packet_source_address is in 2001:db8:1::/48 } then { next-hop is ISP_A_uplink } if { packet_destination_address is not in 2001:db8:1::/48 or 2001:db8:2::/48 packet_source_address is in 2001:db8:1::/48 } then { next-hop is ISP_B_uplink } In this case there is more than one way ensure that hosts are selecting the correct source address based on the uplink status. If VRRP is used to provide first-hop redundancy and the master router is one with the active uplink, then the simplest way to use VRRP mastership as a condition for router advertisement. So, if ISP_A is the primary uplink, the routers R1 and R2 need to be configured in the following way: R1 is VRRP master by default (when ISP_A uplink is up). If ISP_A uplink is down then R1 becomes a backup. Router advertisement on R1 interface facing H1 needs to have the following policy applied:

prefix 2001:db8:1:1::/64 { if vrrp_master then preferred_lifetime = 604800 else preferred_lifetime = 0 } R2 is VRRP backup by default. Router advertsement on R2 interface facing H1 needs to have the following policy applied:

prefix 2001:db8:2:1::/64 { if vrrp_master then preferred_lifetime = 604800 else preferred_lifetime = 0 } If VRRP is not used or interface status tracking is not used for mastership switchover, then each router needs to be able to detect somehow the uplink failure/recovery on the neighboring router so RAs with updated preferred lifetime value are triggered. Depending on the network setup various triggers like a route to uplink interface subnet or a defaul route received from the uplink can be used. The obvious drawback of using the routing table to trigger the conditional RAs is that some additional configuration would be required. For example if a route to the prefix assigned to the ISP uplink is used as a trgger then the conditional RA policy would have the following logic: R1:

prefix 2001:db8:1:1::/64 { if ISP_A_uplink is up then preferred_lifetime = 604800 else preferred_lifetime = 0 } R2:

prefix 2001:db8:2:1::/64 { if ISP_A_uplink_route is present then preferred_lifetime = 0 else preferred_lifetime = 604800 }

Let's look at the example topology shown on but with both uplinks used simultaneously. In that case R1 would send RAs containing PIOs for both prefixes, 2001:db8:1:1::/64 and 2001:db8:2:1::/64, changing the preferred lifetime based on particular uplink availability. If the interface status is used as a uplink availability indicator The policy logic would look like:

prefix 2001:db8:1:1::/64 { if ISP_A_uplink is up then preferred_lifetime = 604800 else preferred_lifetime = 0 } prefix 2001:db8:2:1::/64 { if ISP_B_uplink is up then preferred_lifetime = 604800 else preferred_lifetime = 0 } R1 needs a forwarding policy to be applied to forward packets to the correct uplink based on the source address as described in .

In this scenario the example topology is similar to one shown on but both uplinks can be used at the same time. It means that both R1 and R2 have to have the corresponding forwarding policy to forward packets based on their source addresses. Each router would send RAs with POI for the corresponding prefix. setting preferred_lifetime to non-zero value when the ISP uplink is up and deprecating the prefix by setting the preferred lifetime to 0 in case of the uplink failure. The uplink recovery would trigger another RA with non-zero preferred lifetime to make the addresses from the prefix preferred again. The example RA policy on R1 and R2 would look like: R1:

prefix 2001:db8:1:1::/64 { if ISP_A_uplink is up then preferred_lifetime = 604800 else preferred_lifetime = 0 } R2:

prefix 2001:db8:2:1::/64 { if ISP_B_uplink is up then preferred_lifetime = 604800 else preferred_lifetime = 0 }

For simplicity reasons all topologie sbelow show the ISP uplinks terminated on the first-hop routers. Obviously the proposed approach can be used in more complex topologies when dedicated devices are used for terminating ISP uplinks. In that case VRRP mastership or inteface status can not be used as a triiger for conditional RAs and route presence should be used instead as describe above.