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24-4
Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter 24 Routing Overview
How Routing Behaves Within the ASA
Link-State Versus Distance Vector
Link-state algorithms (also known as shortest path first algorithms) flood routing information to all
nodes in the internetwork. Each router, however, sends only the portion of the routing table that describes
the state of its own links. In link-state algorithms, each router builds a picture of the entire network in
its routing tables. Distance vector algorithms (also known as Bellman-Ford algorithms) call for each
router to send all or some portion of its routing table, but only to its neighbors. In essence, link-state
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, this type of
algorithm is 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, page 24-4
• Next Hop Selection Process, page 24-4
Egress Interface Selection Process
The selection process follows these steps:
1. 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.
2. 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.
3. 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.
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