Cisco ASA 5505 - Manual (Page 510)

27-2
Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter 27 Configuring EIGRP
Licensing Requirements for EIGRP
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 includes a hold time. This is the time in which the ASA can expect to
receive a hello packet from that neighbor. If the ASA does not receive a hello packet from that neighbor
within the hold time advertised by that neighbor, the ASA considers that neighbor to be unavailable.
The EIGRP protocol uses four key algorithm technologies, four key technologies, including neighbor
discovery/recovery, Reliable Transport Protocol (RTP), and DUAL, which is 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 ASA waits for three
minutes to receive a response from its neighbors. If the ASA 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.
Note EIGRP neighbor relationships are not supported through the IPsec tunnel without a GRE tunnel.
Licensing Requirements for EIGRP
The following table shows the licensing requirements for this feature:
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 firewall mode. Transparent firewall mode is not supported.
Model License Requirement
All models Base License.