Routing with a distance vector protocol - EIGRP Introducing Routing and Switching in the Enterprise Chapter 5.2 Copyleft 2012 Vincenzo Bruno (www.vincenzobruno.it) Released under Crative Commons License 3.0 By-Sa Cisco name, logo and materials are Copyright Cisco Systems Inc. 1
Overview
Enhanced Interior Gateway Routing Protocol The two main goals of EIGRP are to provide a loop-free routing environment and rapid convergence. The metric used is a composite metric that primarily considers bandwidth and delay EIGRP tags routes learned from another routing protocol as external.
Diffusing Update Algorithm The Diffusing Update Algorithm (DUAL) used by EIGRP guarantees loop-free operation while it calculates routes. When a change occurs in the network topology, DUAL synchronizes all affected routers simultaneously. For these reasons, the administrative distance of EIGRP is 90, whereas the administrative distance of RIP is 120. The lower number reflects the increased reliability of EIGRP and the increased accuracy of the metric. If a router learns routes to the same destination from both RIP and EIGRP, it chooses the EIGRP route over the route learned through RIP.
EIGRP characteristics Its maximum hop count of 255 supports large networks. EIGRP can display more than one routing table because it can collect and maintain routing information for a variety of routed protocols, such as IP and IPX. The EIGRP routing table reports routes learned both inside and outside the local system. EIGRP does not send complete tables in its updates. EIGRP multicasts partial updates about specific changes to only those routers that need the information, not to all routers in the area. These are called bounded updates because they reflect specific parameters. Instead of sending periodic routing updates, EIGRP sends small hello packets to maintain knowledge of its neighbors
Neighbor Table The neighbor table lists information about directly connected neighbor routers. EIGRP records the address of a newly discovered neighbor and the interface that connects to it. When a neighbor sends a hello packet, it advertises a hold time. The hold time is the length of time that a router treats a neighbor as reachable. If a hello packet is not received within the hold time, the timer expires and DUAL recalculates the topology. Since fast convergence depends on accurate neighbor information, this table is crucial to EIGRP operation.
Topology table The topology table lists all routes learned from each EIGRP neighbor. DUAL takes the information from the neighbor and topology tables and calculates the lowest cost routes to each network. The topology table identifies up to four primary loop-free routes for any one destination. These successor routes appear in the routing table. EIGRP load balances, or sends packets to a destination using more than one path. It load balances using successor routes that are both equal cost and unequal cost. This feature avoids overloading any one route with packets.
Backup routes Backup routes, called feasible successors, appear in the topology table but not in the routing table. If a primary route fails, a feasible successor becomes a successor route. This backup occurs as long as the feasible successor has a lower reported distance than the feasible distance of the current successor distance to the destination.
Topology table explained Number of successors Feasible Distance Reported Distance Route Status P = stable, ready to use Destination Network Successor Feasible Successor Feasible Distance of Feasible Successor Reported Distance of Feasible Successor
Routing Table Whereas the topology table contains information about many possible paths to a network destination, the routing table displays only the best paths called the successor routes. EIGRP displays information about routes in two ways: The routing table designates routes learned through EIGRP with a D. EIGRP tags dynamic or static routes learned from other routing protocols or from outside the EIGRP network as D EX or external, because they did not originate from EIGRP routers within the same AS.
In which table?
Neighbors and adjacencies Before EIGRP can exchange packets between routers, it must first discover its neighbors EIGRP routers use hello packets to discover neighbors and establish adjacencies with neighbor routers. By default, hello packets are multicast: every 5 seconds on links greater than a T1 every 60 seconds on T1 or slower links On IP networks, the multicast address is 224.0.0.10 The hello packet contains information about the router interfaces and the interface addresses
Neighbors and adjacencies /2 Generally, the hold time is three times the duration of the hello interval. When the hold time expires and EIGRP declares the route as down, DUAL re-evaluates the topology and refreshes the routing table. Information discovered through the hello protocol provides the information for the neighbor table. A sequence number records the number of the last received hello from each neighbor and time-stamps the time that the packet arrived.
EIGRP packets When a neighbor adjacency is established, EIGRP uses various types of packets to exchange and update routing table information. Neighbors learn about new routes, unreachable routes, and rediscovered routes through exchange of these packets: Update (unicast if new neighbor found, multicast reporting routing change) Query (unicast request info to neighbor, multicast looking for new successor) Reply (unicast respons to a query) Acknowledgement (response to others, unicast hello packet with no data) When a route is lost, it moves to an active state and DUAL searches for a new route to the destination. When a route is found, it is moved to the routing table and placed in a passive state. These various packets help DUAL gather the information it requires to calculate the best route to the destination network.
Acknowledgement packet An acknowledgement packet indicates the receipt of an update, query, or reply packet. Acknowledgement packets are small hello packets without any data. These types of packets are always unicast.
Update packet An update packet sends information about the network topology to its neighbor. That neighbor then updates its topology table. Several updates are often required to send all the topology information to the new neighbor.
Query and Reply packets Whenever DUAL places a route in the active state, the router must send a query packet to each neighbor. Neighbors must send replies, even if the reply states that no information on the destination is available. The information contained in each reply packet helps DUAL to locate a successor route to the destination network. Queries can be multicast or unicast. Replies are always unicast.
Reliable Transport Protocol (RTP) As a routing protocol, EIGRP operates independently of the Network Layer. Cisco designed RTP as a proprietary Layer 4 protocol. RTP guarantees delivery and receipt of EIGRP packets for all Network Layer protocols. RTP can be used as both a reliable and best effort transport protocol, similar to TCP and UDP. Reliable RTP requires an acknowledgement packet from the receiver to the sender. Update, query, and reply packets are sent reliably; hello and acknowledgement packets are sent best effort and do not require an acknowledgement.
Protocol Dependent Module Each Network Layer protocol works through a Protocol Dependent Module (PDM) responsible for the specific routing task. Each PDM maintains three tables. For example, a router running IP, IPX, and AppleTalk has: three neighbor tables, three topology tables, and three routing tables.
EIGRP metric EIGRP uses a composite metric value to determine the best path to a destination. This metric is determined from the following values: Bandwidth Delay Reliability Load Maximum Transmission Unit (MTU) is another value included in routing updates, but is not a routing metric. Metric = [K1 * bandwidth + (K2 * bandwidth) / (256 load) + K3 * delay] * [K5 / (reliability + K4) ] By default, K1 and K3 are set to 1. K2, K4, and K5 are set to 0. The value of 1 designates that bandwidth and delay have equal weight in the composite metric calculation.
Bandwidth The bandwidth metric is a static value and is displayed in kbps. Most serial interfaces use the default bandwidth value of 1544 kbps. This metric reflects the bandwidth of a T1 connection. Sometimes the bandwidth value may not reflect the actual physical bandwidth of the interface. Bandwidth influences the metric calculation and, as a result, the EIGRP path selection. If a 56 kbps link is advertised with a 1544 kbps value, it could interfere with convergence as it struggles to cope with the traffic load.
Delay, reliability, and load The delay metric is a static value that is based on the type of exit interface. The default value is 20,000 microseconds for Serial interfaces and 100 microseconds for Fast Ethernet interfaces. The delay metric does not represent the actual amount of time packets take to reach the destination. Changing the delay value associated with a specific interface alters the metric but does not physically affect the network. Reliability measures how often the link has experienced errors. Unlike delay, reliability updates automatically, depending on the link conditions. It has a value of between 0 and 255. A reliability of 255/255 represents a 100 percent reliable link. Load reflects the amount of traffic using the link. A lower load value is more desirable than a higher value. As an example, 1/255 would be a minimally loaded link, and 255/255 would be a link that is 100 percent utilized.
DUAL Algorithm DUAL uses feasible distance (FD) and advertised distance (AD), or reported distance (RD) to determine successors and feasible successors. Feasible distance is the best EIGRP metric along the path to the destination from the router. Advertised distance is the best metric reported by a neighbor. The loop-free route with the lowest feasible distance becomes a successor. There can be multiple successors for a destination, depending on the actual topology. A feasible successor is a route with an advertised distance that is less than the feasible distance of a successor. DUAL converges quickly after a change in the topology. DUAL keeps feasible successors in the topology table and promotes the best one to the routing table as a successor route if the original route fails. If no feasible successor exists, the original route moves into active mode, and queries are sent to find a new successor.
Feasible and advertised distance
End of lesson