as-number – (Optional) Autonomous system number.
ip-address – (Optional) IP address. When specified with a mask, a detailed description of the entry is provided.
mask – (Optional) Subnet mask.
active – (Optional) Displays only active entries in the EIGRP topology table.
all-links – (Optional) Displays all entries in the EIGRP topology table.
pending – (Optional) Displays all entries in the EIGRP topology table that are waiting for an update from a neighbor or are waiting to reply to a neighbor.
summary – (Optional) Displays a summary of the EIGRP topology table.
zero-successors – (Optional) Displays available routes in the EIGRP topology table.
Cisco CCNA Successor Routes
Successor route is used by EIGRP to forward traffic to a destination.
A successor routes may be backed up by a feasible successor route.
Successor routes are stored in both the topology table and the routing table.
EIGRP relies on four fundamental concepts: neighbor tables, topology tables, route states, and route tagging.
When a router discovers a new neighbor, it records the neighbor’s address and interface as an entry in the neighbor table. One neighbor table exists for each protocol-dependent module. When a neighbor sends a hello packet, it advertises a hold time, which is the amount of time that a router treats a neighbor as reachable and operational. If a hello packet is not received within the hold time, the hold time expires and DUAL is informed of the topology change.
The topology table contains all destinations advertised by neighboring routers. The protocol-dependent modules populate the table, and the table is acted on by the DUAL finite-state machine. A topology-table entry for a destination can exist in one of two states: active or passive. A destination is in the passive state when the router is not performing a recomputation; it is in the active state when the router is performing a recomputation. If feasible successors are always available, a destination never has to go into the active state, thereby avoiding a recomputation.
Cisco CCNA EIGRP Convergence
A recomputation occurs when a destination has no feasible successors. The router initiates the recomputation by sending a query packet to each of its neighboring routers. The neighboring router can send a reply packet, indicating that it has a feasible successor for the destination, or it can send a query packet, indicating that it is participating in the recomputation. While a destination is in the active state, a router cannot change the destination’s routing-table information. After the router has received a reply from each neighboring router, the topology-table entry for the destination returns to the passive state, and the router can select a successor.
EIGRP supports internal and external routes. Internal routes originate within an EIGRP Autonomous System (AS). Therefore, a directly attached network that is configured to run EIGRP is considered an internal route and is propagated with this information throughout the EIGRP AS. External routes are learned by another routing protocol or reside in the routing table as static routes. These routes are tagged individually with the identity of their origin.
Cisco CCNA Choosing Routes
Like IGRP, EIGRP uses only bandwidth and delay of the line by default to determine the best path to a remote network. Cisco sometimes likes to call these path bandwidth value and cumulative line delay—go figure.
Values other than bandwidth and delay that can be utilized when calculating the EIGRP metric are load, reliability and MTU.
EIGRP uses the following formula to calculate the metric:
metric = [K1 * bandwidth + (K2 * bandwidth) / (256 – load) + K3 * delay] * [K5 / (reliability + K4)]
Where the default K values are as follows:
K1 = 1
K2 = 0
K3 = 1
K4 = 0
K5 = 0
With the default values the formula can be simplified to:
metric = bandwidth + delay
Cisco CCNA Configuring EIGRP for IP
To start an EIGRP session on a router, use the “router eigrp” command followed by the autonomous system number of your network. You then enter the network numbers connected to the router using the network command followed by the network number. Note, unlike OSPF where a process ID is specified and can be different from router to router, the autonomous system number has to be identical between routers in order for the routers to establish an EIGRP neighbor relationship and exchange routes.
Cisco CCNA EIGRP Load Balancing
To control the maximum number of parallel routes an IP routing protocol can support, use the maximum-paths command in router configuration mode. To restore the default value, use the no form of this command.
Cisco CCNA EIGRP MD5 Authentication
OSPF neighbor authentication can be configured such that OSPF routers must agree on predefined passwords prior to exchanging OSPF information.
OSPF routers authenticate the source of each routing update packet that it receives. Authentication comes in two flavors, either plaintext / simple or MD5 authentication. The authentication is accomplished by the exchange of an authenticating key that is known by both the sending and receiving router.