EIGRP


  • EIGRP
  • (Enhanced Interior Gateway Protocol)
  • Hybrid Protocol / Classless / Hello Packet Updates / AD of 90

Benefits

Rapid ConvergenceUses Diffusing Update Algorithm (DUAL) to achieve rapid convergence.  If no appropriate backup route exists in the local routing table, EIGRP queries its neighbors to discover an alternative route

Reduced bandwidth usageEIGRP does not send periodic updates.  It uses partial updates when the path or the metric to a destination changes.  When the route changes, DUAL sends an update about ONLY that link.

Multiple network layer support – Supports IP, IPX and AppleTalk using protocol-dependant modules (PDMs) that are responsible for protocol requirements specific to the network layer.

Seamless connectivity across all data link layer protocols and topologies – EIGRP does not require special configuration to work across any Layer 2 protocols.  EIGRP supports all WAN topologies; dedicated links, point-to-point links and nonbroadcast multi-access (NBMA) topologies.

EIGRP Features

  1. Rapid Convergence
  2. Reduced bandwidth usage
  3. Support for multiple network-layer protocols
  4. Support for LANs and WANs
  5. Advanced distance vector capabilities
  6. 100 percent loop-free
  7. Easy configuration
  8. Incremental updates
  9. Support for VLSM, discontiguous networks, and classless routing
  10. Compatibility with IGRP

Facts

  1. The EIGRP routing process is a transport layer function.  IP packets carrying EIGRP information use protocol 88 in their IP header.

Frame

Header

Frame Payload

C

R

C

IP Header

Protocol Number

Packet Payload

88 – EIGRP

6 – TCP

17 – UDP

  • EIGRP’s operational traffic is multicast and unicast, rather than broadcast.
  • EIGRP supports route redistribution from other routing protocols.
  • If routers running EIGRP do not for neighbor relationships, those routers do not exchange EIGRP updates with each other.
  • By default, hello packets are sent every 60 seconds on a T1 or slower multipoint interfaces and every 5 seconds on LANs and other serial interfaces.
  • The EIGRP multicast address is 224.0.0.10
  • Hello packets report the hold-time value
  • K Values are carried in hello packets.
  • The hold time is the amount of time a router considers a neighbor up without receiving a hello or some other EIGRP packet from that neighbor.
  • The hold-time interval is by default set to 3 times the hello interval.
  • The hold time is not automatically adjusted after the hello interval change.
  • Hello intervals and hold-time values can be set independently on different routers.
  • Peer relationships are not formed if the neighbor resides in a different autonomous system or if the K-values are misaligned on that link.
  • All packets carrying routing information (update, query, and reply) are sent reliably.
  • Reliable Transport Protocol (RTP) is responsible for a guaranteed, ordered delivery of EIGRP packets to all neighbors.
  • If the Retransmission Timeout (RTO) expires before an ACK packet is received, the EIGRP process transmits another copy of the reliable packet, up to a maximum of 16 times or until the hold time expires.
  • By default, split horizon is enabled on all interfaces.
  • The EIGRP delay value is the sum of the delays in the path, in tens of microseconds, multiplied by 256.
  • EIGRP represents its metrics in a 32-bit format instead of the 24-bit representation by IGRP.
  • The EIGRP metric value ranges from 1 to 4,294,967,296.
  • If the EIGRP topology table has many entries that have an equal-cost FD to a destination network, all successors (up to 4 by default) for that destination network are installed in the routing table.
  • The routing table is a subset of the topology table; the topology table contains more detailed information about each route, backup routes, and information used exclusively by DUAL.
  • The mathematical formula to ensure that the FS is loop-free requires that the AD of the backup route be less that the FD of the Successor.
  • When using serial links, (Frame Relay, SMDS, etc.), define the link’s bandwidth for the purposes of sending routing update traffic on the link.  Set the bandwidth to match the line speed.  For Frame Relay P2P interfaces, set the bandwidth to the CIR, for P2MP connections set the bandwidth to the sum of all CIRs.  If you have PVCs and they have different CIRs, set the bandwidth to the lowest CIR multiplied by the number of PVCs for the connection.
  • When you configure the ip default-network command, a static route is generated in the router’s configuration; however, the IOS does not display a message to indicate this has been done.
  • EIGRP automatically summarizes on the major network boundary by default.
  • IP EIGRP summary routes are given an AD value of 5.  Standard EIGRP routes are given an AD of 90 and External EIGRP routes are given an AD of 170.
  • By default the Cisco IOS balances between a maximum of 4 equal cost paths for IP.  You can request that up to 6 equally good routes be kept in the routing table.
  • Load balancing is performed only on traffic that passes through the router, not on traffic generated by the router.
  • Only paths that are feasible are used for load balancing.
  • By default, EIGRP uses up to 50% of the bandwidth declared on an interface or subinterface.
  • When configuring interfaces (especially for Frame Relay), the bandwidth is shared equally by all neighbors.
  • When a router loses a route and does not have an FS is its topology table, it looks for an alternative path to the destination.  This is known as going active on a route.  If the router does not receive a reply to all the outstanding queries within 3 minutes (the default time), the route goes to the stuck-in-active (SIA) state.  When the route goes to SIA state, the router then resets the neighbor relationships for the neighbors that failed to reply.
  • Only remote routers are configured as stub routers.
  • The stub feature does not prevent routes from being advertised to the remote router.
  • Stub routers are not queried.  Instead, hub routers connected to the stub router answer the query on behalf of the stub router.
  • If there is a communication problem over a WAN link between a hub router and a spoke router, an EIGRP SIA condition can occur and cause instability elsewhere in the network.
  • The sequence number in EIGRP increments each time a query, update, or reply packet is sent, whereas the serial number increments each time the topology table changes.

Terminologies

  • Neighbor Table – Lists adjacent routers.  Comparable to the neighborship (adjacency) database used by link-state protocols.  EIGRP keeps a neighbor table for each network protocol supported.
  • Topology table – Maintains all learned routes to a destination.  EIGRP maintains a topology table for each network protocol.
  • Routing table – Contains the best routes to a destination.  The router maintains one routing table for each routing protocol configured.
  • Successor – The neighbor that provides the primary route used to reach a destination.  Successor routes are kept in the routing table.
  • Feasible Successor – The neighbor that provides a backup route to the destination stored in the topology table.  There can be multiple feasible successors in the topology table.
  • Advertised Distance – The metric used for an EIGRP neighbor router to reach a particular network.
  • Feasible Distance – The metric for this router to reach a particular network.
  • Smooth Round Trip Time (SRTT) – The amount of time it takes (ms) to tx a packet reliably to a neighbor and receive response of ack.
  • Retransmission Timeout (RTO) – The amount of time EIGRP waits before retransmitting a packet from the retransmission queue to a neighbor.

EIGRP Packet Types

  • Hello – Used for neighbor discovery.  They are sent in multicasts and carry an acknowledgement of 0.
  • Update – Sent to communicate the routes that a particular router has used to converge; an update is sent only to affected routers.  They are sent in multicasts when a new route is discovered and when convergence is completed.  To synchronize topology tables, updates are sent unicasts to neighbors during the startup sequence.  Updates are sent reliably.
  • Query – Sent when a router is performing routing computations due to the inability to find a feasible successor.  Always sent multicast and reliably.
  • Reply – Sent in response to a query packet.  Sent unicast to the originator of the query and are sent reliably.
  •  
  • Acknowledge (ACK) – Used to acknowledge updates, queries and replies.  Sent as unicasts and have a nonzero acknowledgement number

EIGRP Router States

  • Passive (P) – The network is available, and installation can occur in the routing table.  Passive in the state for a stable network.
  • Active (A) – The network is currently unavailable, and installation cannot occur in the routing table.  Being active means that this network has outstanding queries.
  • Update (U) – This code applies if a network is being updated (placed in an update packet).  This code also applies if the router is waiting for an acknowledgement for this update packet.
  • Query (Q) – This code applies if this network has an outstanding query packet.  This code also applies if the router is waiting for an acknowledgement for a query packet.
  • Reply (R) – This code applies if the router is generating a reply for this network or is waiting for an acknowledgement for the reply packet.
  • Stuck in Active (SIA) – This code signifies an EIGRP convergence problem for the network with which it is associated.

EIGRP Metric Calculation

  • Bandwidth (Default)– Smallest bandwidth between the source and the destination.
  • Delay (Default)– Cumulative interface delay along the path.
  • Reliability – Worst reliability between the source and destination based on keepalives.
  • Load – Worst load on a link between the source and destination based on the packet rate and the interface’s configured bandwidth.
  •  MTU – Smallest MTU in the path.  (MTU is included in the EIGRP update but is not used in the metric calculation).

Metric K – Values & Equation

K1 – Bandwidth  K2 – Load  K3 – Delay  K4 – Reliability  K5 – MTU

Metric = (K1 * bandwidth) +[(K2 * bandwidth / 256 – Load)] + (K3 * Delay)

If K5 is not equal to 0 then the following operation is performed

Metric = metric * [K5 / (reliability + K4)]

Acronyms

FS

Feasible Successor

ACK

Acknowledgement

HDLC

High-Level Data Link Control

SMDS

Switched Multimegabit Data Service

SRTT

Smooth Round Trip Time

RTO

Retransmission Timeout

RTP

Reliable Transport Protocol

MTU

Maximum Transmission Unit

DUAL

Diffusing Update Algorithm

AD

Advertised Distance

FD

Feasible Distance

CIR

Committed Information Rate

SIA

Stuck In Active

Show Commands

sh ip eigrp neighbors Displays the EIGRP neighbor table.
sh ip eigrp topology Displays only the Successors and Feasible successor routes.
sh ip eigrp topology all-links Displays all of the IP entries in the topology table.
sh ip route eigrp Displays the current EIGRP entries in the IP routing table.
sh ip eigrp traffic Displays the number of EIGRP packets sent and received.

Debug Commands

debug eigrp packets Displays the types of EIGRP packets sent and received.
debug eigrp neighbors Displays the neighbors discovered by EIGRP and the contents of the hello packets
debug ip eigrp Displays the packets that are sent and received on an interface. (Only use during light traffic)
debug ip eigrp summary Displays summarized version of EIGRP activity.  Also displays filtering and redistribution numbers and neighbor and distance information.

Configuration Commands

ip eigrp hold-time as-number seconds Adjusts the EIGRP hold time.
router eigrp as-number Enables EIGRP and assigns an autonomous system
bandwidth kilobits Defines the bandwidth on an interface.
ip default-network network-number Creates a default route in the router.  Also known as the gateway of last resort.
no auto-summary Turns off automatic summarization (allows for classless routing)
ip summary-address eigrp as-number address-mask Manually creates a summary route at an arbitrary bit boundary
maximum-paths maximum-path Sets the maximum (equally good) paths kept in the routing table.
variance multiplier Controls the amount of load balancing that is performed.  The multiplier must be between 1 and 128.
traffic-share balanced Distributed router traffic proportionately to the ratios of the metric associated with the different routes
traffic-share min Allows the router to use only routes with minimum costs.
ip bandwidth-percent eigrp as-number percent Sets the percentage of bandwidth used on an interface or subinterface.
timers active-time [time-limit | disabled] Changes the active-state time limit from the default of 3 minutes.
eigrp log-neighbor-changes Enables logging of neighbor adjacency changes to monitor the routing system’s stability and to help detect problems related to SIA.
eigrp stub [receive-only | connected | static | summary] Configures the EIGRP router to Stub.

  • receive-only – restricts the router from sharing any of its routes with any other router in the EIGRP AS.
  • connected – Permits the EIGRP stub routing feature to send connected routed.  This is enabled by default.
  • static – Permits the EIGRP stub routing feature to send static routes.
  • summary – Permits the EIGRP stub routing feature to send summary routes.
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