Is it possible to load balance between two ISP links?

Presently it is not possible to load balance traffic between two ISP links on an ASA. The reason being, there can only be one default route configured on the ASA.

Does the ASA support PBR (Policy Based Routing)?

No, the ASA does not support PBR.

Does the ASA support secondary IP address on interfaces?

No, the ASA does not support secondary IP address on interfaces.  This is not to be confused with the standby IP address configured on the interfaces for failover configuration.

What other options do we have?

SLA Route Tracking

With this method we can configure both the ISP links on the ASA and use the primary ISP for all outgoing traffic and then the secondary ISP, if the primary fails. Failure of the primary ISP causes a temporary disruption of traffic. Use this configuration for redundancy or backup purposes only.

Refer this link: http://www.cisco.com/en/US/products/hw/***devc/ps2030/products_configuration_example09186a00806e880b.shtml

PBR on the router outside the firewall

With this method we can configure both the ISP links on the router outside the firewall.  We can translate some traffic to use Primary ISP provided IP address and the rest of the traffic to use Secondary ISP provided IP address.  Now, based on this source address that hits the router, we can configure the router to do policy based routing and route the traffic either via the Primary ISP or via the Secondary ISP.

Let us assume the requirement as below:

1. We would like all the users traffic translated to the ISP1 provided address

2. We would like all the servers traffic translated to the ISP2 provided address

3. The router should look at the translated addresses and based on the address it should set the next hop address and route the traffic via the appropriate  ISP.

ISP1 provided address block is 10.10.10.0/24 and ISP2 provided address block is 172.18.124.0/24. These are not routable addresses. For simplicity reasons we are using RFC 1918 address space.

ASA config:

Translation for all users to take ISP 1
nat (inside) 1 192.168.2.0 255.255.255.0
global (outside) 1 10.10.10.1

Translation for web and e-mail servers to take ISP2
static (inside,outside) 172.18.124.20 192.168.2.20 netmask 255.255.255.255
static (inside,outside) 172.18.124.30 192.168.2.30 netmask 255.255.255.255

route outside 0 0 172.16.12.2

Router config:

ip access-list ext isp1-addr

permit ip 10.10.10.0 0.0.0.255 any

ip access-list ext ips2-addr

permit ip 172.18.124.0 0.0.0.255 any

route-map ISP permit 10

match ip address isp1-addr

set ip next-hop 10.10.10.2

route-map ISP permit 20

match ip address isp2-addr

set ip next-hop 172.18.124.2

int f0/0

ip address 172.16.12.2 255.255.255.0

ip policy route-map ISP in

Allowing outbound via ISP1 and inbound via ISP2

Let us take the same example above. We can use one ISP1 for all outbound connections and use IPS2 for all inbound connections.

Translation for all outbound connections from users and servers to take ISP 1
nat (inside) 1 192.168.2.0 255.255.255.0
global (ISP1) 1 10.10.10.1

route ISP1 0 0 10.10.10.254

Here are the translations for inbound connections to the servers:

Translation for web and e-mail servers to take ISP2
static (inside,ISP2) 172.18.124.20 192.168.2.20 netmask 255.255.255.255
static (inside,ISP2) 172.18.124.30 192.168.2.30 netmask 255.255.255.255

In the previous case even the out bound connections made by the servers would take the ISP2 path but, in this example outbound connections from the web and e-mail servers will take ISP1. ONLY the INBOUND connections will come through ISP2 and will be responded back using ISP2.

Allowing internet access via ISP1 and L2L *** via ISP2

The above example can also be used to allow internet access outbound and inbound via ISP1 with the default route pointing to ISP1 and ××× site to site tunnels can be terminated on the ISP2 interface by configuring specific routes to reach peer networks via IPS2.

Multiple context mode

The last option is to use multiple context mode where we can load balance on a per context basis.  ××× is not supported in this mode and so are dynamic routing protocols.  Please refer this link for the limitations: http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/contexts.html#wp1146747

Context-1 could use ISP1 link and Context-2 could use ISP2 link.