EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP)

An advanced distance vector Dynamic Routing protocol. Originally Cisco proprietary, much of the protocol has been released for all manufacturers to use, though most vendors have not implemented it. Considered a more advanced routing protocol than RIP, another distance vector protocol.

EIGRP is the only IGP capable of unequal-cost load-balancing.

Details

• An update to the older IGRP - basically totally non-existent today, as far as I can tell.
• Originally, EIGRP was Cisco proprietary, but Cisco has since published it openly so other vendors can implement it on their own equipment.
  • Some parts of the protocol may still be proprietary. Most vendors haven’t bothered implementing it so it’s effectively still a Cisco-only protocol.
• Considered an ‘advanced’/‘hybrid’ distance vector routing protocol
• Compared to RIP, EIGRP
  • Is much faster than RIP in reacting to changes in the network
  • Does not have the 15 ‘hop-count’ limit of RIP — supports much larger networks
  • Sends messages using multicast address 224.0.0.10 (as opposed to RIPv2’s 224.0.0.9)
• The only IGP that can perform unequal-cost load-balancing (by default it performs ECMP load-balancing over 4 paths like RIP)
  • You can load-balance over paths of different costs, sending more traffic over faster paths and less over slower paths.
• Metric costs tend to be much higher than those of other protocols.
  • Possibly, this is a downside — the numbers are harder to understand
• Router ID
  • Selected semi-automatically
  • Manual configuration > highest loopback interface address > highest physical interface address

Configuration

• Similar to configuring RIP
• Example CLI commands:
  • router eigrp 1
    • The number denotes the Autonomous System number. This must match between routers for them to form adjacencies
  • no auto-summary
    • Works the same as in RIP; disables auto-Classful conversions of advertised network addresses/prefixes
    • auto-summary may be enabled or disabled by default, depending on the router/IOS version. If enabled you should disable it.
  • passive-interface g2/0
    • Tell EIGRP not to send advertisements on interface g2/0
  • network 10.0.0.0
    • assumes classful network if no mask is specified
    • otherwise works like RIP’s; tells it to activate EIGRP on interfaces with IPs that fall in the range of the specified address
  • network 172.16.1.0 0.0.0.15
    • Same as previous but with a /28 mask specified
    • EIGRP uses a Wildcard Mask instead of a regular subnet mask
      • ’0’ in the wildcard mask = must match
      • ’1’ in the wildcard mask = doesn’t have to match (these bits are a wildcard, get it?)
  • You can enable EIGRP on all interfaces at once with network 0.0.0.0 255.255.255.255

Warning

  >This is highly inadvisable in a normal network; you should be more precise with which networks have Dynamic routing enabled!

Metric

• EIGRP’s metric uses bandwidth and delay to calculate metric, by default

$$([K1\ast bandwidth+(K2\ast bandwidth)/(256-load)+K3\ast delay]\ast [K5/(reliability+K4)])\ast 256$$

• The default K values: K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0
• Since you never actually need to calculate this yourself (don’t try and memorize this formula!) you can probably get away with simplifying the formula like this:
  • Metric = bandwidth + delay
    • Specifically, the bandwidth of the slowest link in the path to the destination + the delay of all links on that path

Terminology

• Feasible Distance = This router’s metric value to the route’s destination
• Reported Distance (aka Advertised Distance) = The neighbor’s metric value to the route’s destination
• Successor = the route with the lowest metric to the destination (the best route)
• Feasible Successor = an alternate route to the destination (not the best route) which meets the feasibility condition
  • Feasibility condition: a route is considered a feasible successor if its reported distance is lower than the successor route’s feasible distance