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Cisco ASA Series CLI Configuration Guide
Chapter 1 Routing Overview
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, page 1-4
•
Next Hop Selection Process, page 1-4
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
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.
Summary of Contents for 5505 - ASA Firewall Edition Bundle
Page 28: ...Glossary GL 24 Cisco ASA Series CLI Configuration Guide ...
Page 61: ...P A R T 1 Getting Started with the ASA ...
Page 62: ......
Page 219: ...P A R T 2 Configuring High Availability and Scalability ...
Page 220: ......
Page 403: ...P A R T 2 Configuring Interfaces ...
Page 404: ......
Page 499: ...P A R T 2 Configuring Basic Settings ...
Page 500: ......
Page 533: ...P A R T 2 Configuring Objects and Access Lists ...
Page 534: ......
Page 601: ...P A R T 2 Configuring IP Routing ...
Page 602: ......
Page 745: ...P A R T 2 Configuring Network Address Translation ...
Page 746: ......
Page 845: ...P A R T 2 Configuring AAA Servers and the Local Database ...
Page 846: ......
Page 981: ...P A R T 2 Configuring Access Control ...
Page 982: ......
Page 1061: ...P A R T 2 Configuring Service Policies Using the Modular Policy Framework ...
Page 1062: ......
Page 1093: ...P A R T 2 Configuring Application Inspection ...
Page 1094: ......
Page 1191: ...P A R T 2 Configuring Unified Communications ...
Page 1192: ......
Page 1333: ...P A R T 2 Configuring Connection Settings and QoS ...
Page 1334: ......
Page 1379: ...P A R T 2 Configuring Advanced Network Protection ...
Page 1380: ......
Page 1475: ...P A R T 2 Configuring Modules ...
Page 1476: ......
Page 1549: ...P A R T 2 Configuring VPN ...
Page 1550: ......
Page 1965: ...P A R T 2 Configuring Logging SNMP and Smart Call Home ...
Page 1966: ......
Page 2059: ...P A R T 2 System Administration ...
Page 2060: ......
Page 2098: ...1 8 Cisco ASA Series CLI Configuration Guide Chapter 1 Troubleshooting Viewing the Coredump ...
Page 2099: ...P A R T 2 Reference ...
Page 2100: ......