Cisco ASA Series Cli Configuration Manual page 606

How Routing Behaves Within the ASA
algorithms send small updates everywhere, while distance vector algorithms send larger updates only to
neighboring routers. Distance vector algorithms know only about their neighbors. Typically, link-state
algorithms are used in conjunction with OSPF routing protocols.
How Routing Behaves Within the ASA
The ASA uses both routing table and XLATE tables for routing decisions. To handle destination IP
translated traffic, that is, untranslated traffic, the ASA searches for existing XLATE, or static translation
to select the egress interface.
This section includes the following topics:
•
•
Egress Interface Selection Process
The selection process follows these steps:
1.
2.
3.
Next Hop Selection Process
After selecting the egress interface using any method described previously, an additional route lookup
is performed to find out suitable next hop(s) that belong to a previously selected egress interface. If there
are no routes in the routing table that explicitly belong to a selected interface, the packet is dropped with
a level 6 syslog message 110001 generated (no route to host), even if there is another route for a given
destination network that belongs to a different egress interface. If the route that belongs to a selected
egress interface is found, the packet is forwarded to the corresponding next hop.
Load sharing on the ASA is possible only for multiple next hops available using a single egress interface.
Load sharing cannot share multiple egress interfaces.
If dynamic routing is in use on the ASA and the route table changes after XLATE creation (for example,
route flap), then destination translated traffic is still forwarded using the old XLATE, not via the route
table, until XLATE times out. It may be either forwarded to the wrong interface or dropped with a level
6 syslog message 110001 generated (no route to host), if the old route was removed from the old
interface and attached to another one by the routing process.
Cisco ASA Series CLI Configuration Guide
1-4
Egress Interface Selection Process, page 1-4
Next Hop Selection Process, page 1-4
If a destination IP translating XLATE already exists, the egress interface for the packet is determined
from the XLATE table, but not from the routing table.
If a destination IP translating XLATE does not exist, but a matching static translation exists, then
the egress interface is determined from the static route and an XLATE is created, and the routing
table is not used.
If a destination IP translating XLATE does not exist and no matching static translation exists, the
packet is not destination IP translated. The ASA processes this packet by looking up the route to
select the egress interface, then source IP translation is performed (if necessary).
For regular dynamic outbound NAT, initial outgoing packets are routed using the route table and
then creating the XLATE. Incoming return packets are forwarded using existing XLATE only. For
static NAT, destination translated incoming packets are always forwarded using existing XLATE or
static translation rules.
Chapter 1 Routing Overview