Cisco ASA 5505 Configuration Manual page 422

Licensing Requirements for EIGRP
neighbor, it sends its topology table to the neighbor with an initialization bit set. When the neighbor
receives the topology update with the initialization bit set, the neighbor sends its topology table back to
the adaptive security appliance.
The hello packets are sent out as multicast messages. No response is expected to a hello message. The
exception to this is for statically defined neighbors. If you use the neighbor command, or configure the
Hello Interval in ASDM, to configure a neighbor, the hello messages sent to that neighbor are sent as
unicast messages. Routing updates and acknowledgements are sent out as unicast messages.
Once this neighbor relationship is established, routing updates are not exchanged unless there is a change
in the network topology. The neighbor relationship is maintained through the hello packets. Each hello
packet received from a neighbor contains a hold time. This is the time in which the adaptive security
appliance can expect to receive a hello packet from that neighbor. If the adaptive security appliance does
not receive a hello packet from that neighbor within the hold time advertised by that neighbor, the
adaptive security appliance considers that neighbor to be unavailable.
The EIGRP protocol uses four key algorithm technologies, four key technologies, including neighbor
discover/recovery, Reliable Transport Protocol (RTP), and the fourth one, DUAL being important for
route computations. DUAL saves all routes to a destination in the topology table, not just the least-cost
route. The least-cost route is inserted into the routing table. The other routes remain in the topology
table. If the main route fails, another route is chosen from the feasible successors. A successor is a
neighboring router used for packet forwarding that has a least-cost path to a destination. The feasibility
calculation guarantees that the path is not part of a routing loop.
If a feasible successor is not found in the topology table, a route recomputation must occur. During route
recomputation, DUAL queries the EIGRP neighbors for a route, who in turn query their neighbors.
Routers that do no have a feasible successor for the route return an unreachable message.
During route recomputation, DUAL marks the route as active. By default, the adaptive security appliance
waits for three minutes to receive a response from its neighbors. If the adaptive security appliance does
not receive a response from a neighbor, the route is marked as stuck-in-active. All routes in the topology
table that point to the unresponsive neighbor as a feasibility successor are removed.
EIGRP neighbor relationships are not supported through the IPSec tunnel without a GRE tunnel.
Note
Licensing Requirements for EIGRP
Model License Requirement
All models Base License.
Guidelines and Limitations
This section includes the guidelines and limitations for this feature.
Context Mode Guidelines
Supported in single context mode.
Firewall Mode Guidelines
Supported only in routed mode. Transparent mode is not supported.
Cisco ASA 5500 Series Configuration Guide using ASDM
23-2
Chapter 23 Configuring EIGRP
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