Implementing EIGRP

Hybrid Routing Protocol

  • Rapid convergence (Diffusing Update Algorithm) DUAL
  • Stores all available backup routes
  • No periodic updates, sends partial updates when path or metric changes
  • DUAL sends path updates about a link rather than about the entire route table
  • Multi protocol support (appletalk, ipv4, ipv6, IPX)
  • Classless
  • Uses multicast and unicast
  • Support unequal metric load-balancing
  • Supports summarisation anywhere within the network rather than at major network boundaries only

Has three tables: Neighbour Table, Topology Table and Routing Table

Topology Table

Shows the route entries for each destination learned.  Best routes are chosen and put into the routing table.

Neighbour Table

Shows the adjacent eigrp routers

Successor Route = Best Route

Feasible Successor = Backup Route

Advertised distance – metric for an eigrp neighbour to reach a network

Feasible distance – advertised distance for a network learned from a neighbour plus the metric to reach that neighbour.

The router compares all the feasible distances to reach a network and selects the lowest feasible router and puts it into the route table.

Topology databse contains all the routes that are known to each eigrp neighbour.

If a route becomes invalid DUAL checks for a feasible successor to the destination.  If none is found then a re computation must occur

EIGRP requires an AS, similar to the process ID for OSPF.

EIGRP automatically summarises  on the classful boundary.

EIGRP Load Balancing

EIGRP uses a metric to compute the costs to a given network, by default EIGRP uses two criteria to determine a metric:

  • Bandwidth
  • Delay

You can use the following but they are not recommended because they usually result in frequent recalculation of the topology table:

  • Reliability
  • Load

For IP, IOS load balances across 4 equal-cost paths by default.  I can be configured for up to 16 equal-cost paths.

Setting maximum-path to 1 disables load balancing.

Unequal cost load balancing is controlled with the variance command.  Values from 1 to 128.

Eg: Variance will determine a feasible successor if the metric is within the variance.

Variance  = 2 best path metric = 20 Backup Route = 10, then 10×2 = 20, therefor there is equal cost on both links.

EIGRP Authentication

MD5 Authentication

Each key has an ID, the combination of the key ID and the interface associated with the message identifies the MD5 key in use.

Manage Keys Using Key chains

Troubleshooting EIGRP

  • Troubleshoot Neighbour Relationships
  • Troubleshoot Route Table Issues
  • Troubleshoot Authentication

Troubleshoot EIGRP Neighbour Relationships

  • All Interfaces up/up?
  • Getting, not a common subnet error?
  • All EIGRP neighbors configured?
  • Does Hello info match neighbour?

Both router and neighbour must share a directly connected subnet.  EIGRP Hello packets must patch the AS and K values to neighbor up.  Debug EIGRP packets to troubleshoot hello information.

Troubleshooting EIGRP Routing Tables

  • Advertising all Required Networks
  • Route Filters blocking Networks?
  • Duplicate EIGRP Router ID’s?
  • Is there a discontigious network?

Route table Key:

D = intra-AS Route, D EX = External AS Route

Route ID is formed from the highest IP address assigned to a loopback interface, if no loopback interface, the active interface with the highest IP is used.

Troubleshoot EIGRP Authentication

Use debug eigrp packets to verify authentication information.


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