CCNP / Uncategorized

EIGRP

Basil ThomasLeave a comment

Enhanced Interior Gateway Routing Protocol

For a large enterprise network we use OSPF and EIGRP , both are very strong and efficient routing protocol. EIGRP was cisco proprietary and now it is open for other vendors.

  EIGRP Overview

  Characteristics of Eigrp:

  1.  Fast Convergence

    • In other words if we have multiple path from source to destination and if the primary route fails then the router converge to the backup path.
    • Eigrp is better than other routing protocol

    convergence_eigrpconvergence_eigrp1

  2. Scalable

    • Scalability is affected by:
      • The number of routers that must be exchanged.
      • The number of routers that must know of a topology change.
      • The number of alternate routes to a network.
      • The number of hops from one end of the network to the other .
    • To improve the scalability, summarize routers when possible,try to have a network depth of no more than seven hops, and limit the scope of eigrp queries.
    • Stub routing is one way to limit queries. A stub router is one that is connected to no more than two neighbors and should never be a transit router. When a router is configured as a EIGRP stub, it notifies its neighbors. The neighbors then do not query that router for a lost route. But stub router still receives all routes from neighbors by default.
      • router(config)# eigrp stub ?
    • Router use SIA-Queries and Replay to prevent loss of a neighbor unnecessarily during SIA conditions.
    • Graceful shutdown is another feature that speeds network convergence. Whenever the EIGEP process is shutdown, the router sends a goodbye message to the neighbors. The neighbors can then immediately recalculate any paths that used the router as the next hop, rather than waiting for the hold timer to expire.

  3. Load balancing over unequal cost links

    • This feature is not available in OSFPF
    • In this if there is more than one path from source to destination then EIGRP use the Metric calculation to choose best cost link. In case if load balancing is required EIGRP choose different paths which has different metric value.

      load_balancing
      EIGRP choose both path even if Metric is different.

  4. Classless (VLSM)

    Variable Length Subnet Masking

    • We do not have to advertise network in the classful bountry.
    • We can advertise information about the subnet mask.

  5. Multicast

    • To advertise information we use multicast address
    • In IPv4                         224.0.0.10
    • In IPv6                         FF02::A

 

EIGRP Components

  1.   Neighbor Discovery

    • EIGRP can discover the neighbor eigrp configured router without even configuring in the router.
    • In order to become an EIGRP neighbor, three essential configuration values must be matched; Active hello packets, AS number and K-values.

  2.   Reliable Transmission Protocol

    • RTP guaranties that the packet from the source eigrp is received at the destination successfully and also it ensures that the packets are received in correct order, maybe they are going over different paths.

  3. DUAL ( Diffusing Algorithm)

    • EIGRP is running on this algorithm.
    • This algorithm keeps track of all the route advertised by that router to the neighbors and DUAL identifies which router provides a efficient route to reach the destination, that router is called the successor.
    • Successor : A eigrp router that provide best route from source to destination.And the route is known as Successor route.
    • Feasible Successor  : A eigrp router that provides the second best route for packet delivery from source to destination. This act as backup.

EIGRP Data Structure

     EIGRP Tables Structure

  1. Neighbor Table

    • A neighbor table containing information about the routers eigrp-speaking neighbors.
    • router#  enable
    • router#  show ip eigrp neighbors

  2. Interface Table

    • The interface table contain list of interfaces participating in the eigrp.
    • router#  enable
    • router#  show ip eigrp interfaces

  3. Topology Table

    • This table contain the routes known to the EIGRP.
    • router#  enable
    • router#  show ip eigrp topology

All this commands does not give any information about the Successor or the Passive-Successor in the topology. To view the complete details we need to type :

  • router#  enable
  • router#  show ip eigrp topology all-links

 

EIGRP Timers

 Eigrp can converge very fast, Timers are used when the neighbors are unavailable.

In case if any of the interface of the neighbor router goes down then the source router needs to wait for hold timer to expire. Again waiting for 15 sec by default is not a good idea. By default the Hello time is 5 sec in LAN and the default Hold time is 15 sec.

Configuration

router(config)#     interface serial 1/0
router(config-if)#  ip hello-interval eigrp <1> <2>  /*dont use <,> char */
router(config-if)#  ip hold-time eigrp <1> <6> 
router(config-if)#  exit

Varification

router(config)#                 show ip eigrp interface brief

timers.png

 

Now you can see the hold time changed to 6 and hello-interval is 2.

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