CCNP TIPS - EIGRP Part I

**Protocol** · 29-01-2007, 16:32

i am learning CCNP so i want to share you my tips.
thanks for your's replay.

Routing protocols are distinguished by the way that they select the best pathway and the way that they calculate the routing protocol metric.

EIGRP is a CISCO proprietary protocol that combines the advantages of link-state and distance vector protocols.
EIGRP includes the following key features:

Fast convergence:
A router running EIGRP stores all its neighbors routing tables so that it can quickly adapt to alternate routes. If no appropriate route exists, EIGRP queries it's neighbors to discover an alternate route. These queries propagate until an alternate route is found.

VLSM support:
EIGRP is a classless routing protocol, which means that it advertise a subnet mask for each destination network; this structure enables EIGRP to support discontinuous sub-networks and VLSM.

Partial updates:
EIGRP doesn't send periodic updates. Instead, it sends partial
triggered updates; these are sent only when the path or the metric changes for a route, and they contain information about the changed routes only.

Multiple network-layer protocol support:
EIGRP support IP, AppleTalk, and Novell IPX through the use of protocol-dependent modules.

Other features:
EIGRP uses multicast and Unicast, rather than broadcast. The multicast address used for EIGRP is 224.0.0.10.

EIGRP has four key technologies that describe as follow:

Neighbor discovery/recovery mechanism:
Enable routers to dynamically learn about other routers on their directly attached networks. This process is achieved with low overhead by periodically sending hello packets.

RTP:
Responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. For example, on multi-access network that has multicast capabilities, such as Ethernet, it is not necessary to send hello packets reliably to all neighbors individually, so EIGRP sends a single multicast hello packet containing an indicator that informs the receiver that the packet need not be acknowledged. Other types of packets, such as updates, indicate in the packet that acknowledgment is required.

DUAL finite state machine:

Embodies the decision process for all route computations. DUAL tracks all routes advertised by all neighbors and uses distance information, known as a metric or cost, to select efficient, loop free paths to all destinations.

Protocol dependent modules:
EIGRP supports IP, AppleTalk, and Novell NetWare. The IP EIGRP module, for example, is responsible for sending and receiving EIGRP packets that are encapsulated in IP. The IP EIGRP asks DUAL to make routing decisions, as results of which are stored in the IP routing table.

The topology table lists all the learned routes to each destination, while the routing table contains the best route to each destination; this best route is called successor route. A feasible successor route is a backup route to a destination, which is kept in the topology table.
When a neighbor sends 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 expired, and DUAL is informed of the topology changed.
The neighbor table entry also includes information required by RTP like sequence numbers, last sequence number and round trip timers to estimate an optional retransmission interval.

Dual Terminology:

Advertised distance is the cost between the next-hop router and the destination.
Feasible distance = cost from local router = AD of next-hop router + cost between the local router and the next-hop router.

EIGRP topology table:

When the router dynamically discovers a new neighbor, it sends an update about the routes that is knows to its new neighbor and receives the same information from the new neighbor. These updates populate the topology table. The topology table contains all destinations advertised by neighboring routers.
The 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 re-computation; it's in the active state when the router is performing a re-computation.
The re-computation occurs when a destination has no feasible successors. The router initiates the re-computation by sending a query packet to each of its neighboring routers.
All routers compares all FD's to reach a specific network and then selects the route with the lowest FD and places it in the IP routing table; this is the successor route.

EIGRP packets:

EIGRP uses the following five generic packet types:

HELLO:
Routers uses hello packets for neighbor discovery. The packets are sent as multicasts and do not require an ACK,

UPDATE:
Update packet contain route change information. They are sent reliably to the affected routers only. The updated can be unicast or multicast.

Query:
When a router performs route computation and does not have a feasible successor, it sends a reliable query packet to its neighbors to determine if they have a feasible successor for the destination.

REPLAY:
A router sends a replay packet in response to a query packet.

ACK:
The ACK packet acknowledge update, query, and replay packets.

Initial route discovery

EIGRP metric:

EIGRP metric is multiplied by 256. the metric can be based on five criteria, but EIGRP uses only two of these criteria by default :

Bandwidth:
The smallest bandwidth between source and destination.

Delay:
The cumulative interface delay along the path.
.other criteria:

Loading:
The value represents the worst load on a link between source and destination.
MTU:
This criterion represents the smallest MTU in the path.

Reliability:
This value represents the worst reliability between source and destination.