EIGRP Flashcards
EIGRP Features
• Advanced distance vector
• Fast convergence
• Support for VLSM and discontiguous subnets
• Partial updates
• Support for multiple network-layer protocols
• Flexible network design
• Multicast and unicast instead of broadcast address
• Manual summarization at any point
• 100% loop-free classless routing
• Easy configuration for WANs and LANs
• Load balancing across equal and unequal-cost pathways
EIGRP
• “Enhanced” Interior Gateway Routing Protocol
• Based on IGRP and developed to allow easy transition from IGRP to EIGRP. (“Like IGRP+”)
• Cisco proprietary, released in 1994
• EIGRP is an advanced distance-vector routing protocol that relies on
features commonly associated with link-state protocols
++++EIGRP is a distance vector routing protocol and suffers from all of the same disadvantages of any other distance vector routing protocol, i.e. routing loops.
What is hybrid routing protocol
“Often described as a hybrid routing protocol offering the best of
distance-vector and link-state algorithms.” - More proper “features of
distance-vector and link-state” not necessarily “the best.”
EIGRP and IGRP compatibility
• Automatic redistribution occurs when the same AS number is used for
EIGRP and IGRP.
• EIGRP will tag routes learned from IGRP, or any outside source, as
external because they did not originate from EIGRP routers.
• IGRP cannot differentiate between internal and external routes.
Features of EIGRP
• Classless Routing Protocol (VLSM, CIDR)
• Faster convergence times and improved scalability
• Multiprotocol support: TCP/IP, IPX/SPX, Appletalk
• Rapid Convergence and Better handling of routing loops – (DUAL) (coming)
• Efficient Use of Bandwidth – Partial, bounded updates: Incremental updates only to the routers that need them. – Minimal bandwidth consumption: Uses Hello packets and EIGRP packets by default use no more that 50% of link’s bandwidth EIGRP packets.
• PDM (Protocol Dependent Module) – Keeps EIGRP is modular – Different PDMs can be added to EIGRP as new routed protocols are enhanced or developed: IPv4, IPv6, IPX, and AppleTalk
• Unequal-cost load balancing same as IGRP (unlike OSPF)
EIGRP Neighbor Table
• Neighbor table– Each EIGRP router maintains a neighbor table that lists
adjacent routers.
• This table is comparable to the adjacency database used by OSPF.
• There is a neighbor table for each protocol that EIGRP supports.
• Whenever a new neighbor is discovered, the address of that neighbor
and the interface used to reach it are recorded in a new neighbor table
entry.
EIGRP Topology Table
• Topology table – Every EIGRP router maintains a topology table for each
configured network protocol.
• This table includes route entries for all destinations that the router has learned.
• EIGRP uses its topology table to store all the information it needs to
calculate a set of distances and vectors to all reachable destinations.
• Not only does the topology table track information regarding route states, but it can also record special information for external routes, including the
administrator tag.
Advertised Distance (AD) or Reported Distance (RD) = cost between the next-
hop router and the destination
Feasible Distance (FD) = cost from local router to destination = AD of next-hop
router + cost between the local router and the next-hop router
EIGRP Routing Table
• Routing table – EIGRP chooses the best routes to a destination from the
topology table and places these routes in the routing table.
• Each EIGRP router maintains a routing table for each network protocol.
• Successor – Current Route
– A successor is a route selected as the primary route to use to reach a destination.
– Successors are the entries kept in the routing table.
• Feasible Successor -A backup route
– A feasible successor is a backup route.
– These routes are selected at the same time the successors are identified, but they are kept in the topology table.
– Multiple feasible successors for a destination can be retained in the topology table.
DUAL FSM
• The centerpiece of EIGRP is DUAL, the EIGRP route-calculation
engine.
– The full name of this technology is DUAL finite state machine (FSM).
– This engine contains all the logic used to calculate and compare routes in an EIGRP network.
• Selects lowest-cost, loop-free paths to each destination
• Lowest-cost = lowest FD
• (Current) Successor = next-hop router with lowest-cost, loop free path
• Feasible Successor = backup router with loop-free path :
AD of feasible successor must be less than FD of current successor route
AD (Feasible Successor) ≤ FD (Successor)
Since EIGRP has a topology table, does this make it a
link-state routing protocol?
• No, the information in the topology table is not in the form of LSAs
describing the complete network topology.
• The EIGRP topology table contains information about paths through
the router’s adjacent neighbors.
• Also, EIGRP does not perform shortest-path calculation by calculating
the shortest-path tree, but instead uses the DUAL algorithm.
EIGRP Key Technologies
• Neighbor discovery/recovery
• Uses hello packets between neighbors
• Reliable Transport Protocol (RTP)
• Guaranteed, ordered delivery of EIGRP packets to all neighbors
• DUAL finite-state machine
• Selects lowest-cost, loop free, paths to each destination
• Protocol-dependent modules (PDMs)
• EIGRP supports IP, AppleTalk, and Novell NetWare.
• Each protocol has its own EIGRP module and operates independently
of any of the others that may be running.
Establishing Adjacencies with Neighbors
• EIGRP routers establish adjacencies with neighbor routers by using small
hello packets.
• Hellos are sent every 5 seconds by default
• K values must be the same between neighbors.
• Hold time tells the router how long it should consider the neighbor alive if it
has not received any EIGRP packets (Hello, EIGRP updates, etc.)
• Hold time is normally three times the configured Hello interval.
Hello Intervals and Default Hold Times
• Hello Time The interval of Hello Packets
• Hold Time The interval to wait without receiving anything from a
neighbor before considering the link unavailable.
Reliable Transport Protocol
• EIGRP is protocol-independent; that is, it doesn’t rely on TCP/IP to
exchange routing information the way RIP, IGRP, and OSPF do.
• To stay independent of IP, EIGRP uses the transport-layer protocol to
guarantee delivery of routing information: RTP.
• RTP supports reliable and unreliable delivery
• RTP supports unicasting and multicasting
• Initial delivery of EIGRP messages are done using multicast packets, that is
data is sent to all neighbors on a segment, and every neighbor is expected to
acknowledge it with a unicast Hello packet.
• After adjacency has been formed and added to neighbor table, routers
exchange routing information which is stored in the topology table. (later)
• RTP is used for EIGRP queries, updates and replies
EIGRP Load Balancing
• Routes with a metric equal to the minimum metric are installed
in the routing table (equal-cost load balancing).
• There can be up to six entries in the routing table for the same
destination:
• The number of entries is configurable.
• The default is four.
• Set to 1 to disable load balancing.
