Enhanced Interior Gateway Routing Protocol --- EIGRP

           
What EIGRP is:

a.  Enhanced Interior Gateway Routing Protocol
       -- Successor to Interior Gateway Routing Protocol (IGRP)
b.  Cisco proprietary "hybrid" protocol
       -- Both Distance Vector and Link State Behaviour
c.  "Classless" protocol
       -- Supports VLSM and summarization

Features of EIGRP and why it is implemented:

•   Guaranteed loop-free topology

     -- Diffusing Update Algorithm (DUAL)

•   Fast convergence

     -- Fastest of all IGPs in certain designs

•   Reliable and efficient updating

     -- Forms active neighbor adjacencies

     -- Guarantees packet delivery with  

         Reliable Transport Protocol (RTP)

     --  Supports partial updates

           --- Not all neighbors need all routes

•   Multiple routed protocol support

     --  IPv4, IPX and Appletalk

     --  Legacy now, but originally important

       in non-converged networks

•   Granular Metric

     --  Hybrid metric derived from multiple

       factors

•   Unequal Cost Load Balancing

     --  Only IGP that supports true load

       distribution

•   Control Plane Security

     --  Supports MD5 based authentication


How EIGRP Works:

• Step 1   -   Discover EIGRP Neighbors
• Step 2   -   Exchange Topology Information
• Step 3   -   Choose Best Path via DUAL
• Step 4   -   Neighbor and Topology Table Maintenance

Step 1   -   Discovering EIGRP Neighbors

• EIGRP uses multicast "HELLO" packets to discover neighbors on EIGRP enabled attached links

      --   Transport via IP protocol 88 (EIGRP)
      --   Destination address 224.0.0.10

• Hello packets contain

      --   Autonomous System Number
      --   Hold Time
      --   Authentication
      --   Metric Weightings (K values)

• Neighbors found are inserted into EIGRP "neighbor table"

      --   show ip eigrp neighbors

• Neighbors that agree on attributes and exchange updates form active "adjacency"

Step 2   -   Exchanging Topology Information

• Once neighbors are found, EIGRP "UPDATE" messages used to exchange routes

      --   Sent as multicast to 224.0.0.10 or as unicast

• RTP uses sequence numbers and acknowledgements (ACKs) to ensure delivery

• Update messages describe attributes of a route

      --   Prefix + Length
      --   Next-Hop
      --   Bandwidth
      --   Delay
      --   Load
      --   Reliability
      --   MTU
      --   Hop Count
      --   External Attributes

• All routes learned from all neighbors make up the EIGRP "topology table"

      --   show ip eigrp topology



Step 3   -   Choosing the Best Path

• Once the topology is learned, DUAL runs to choose loop-free best path to each destination

• Unlike other protocols, EIGRP uses complex "composite" metric to choose best path

• Composite metric is calculated from:

      --   Administrative Weighting
      --   Bandwidth
      --   Delay
      --   Load
      --   Reliability

• Path with lowest composite metric is considered best and installed in IP routing table

• One or more backup routes can also be pre-calculated per destination

• Only best route is advertised to other EIGRP neighbors
  

Step 4   -   Neighbor and Topology Table Maintenance

• Unlike RIP or IGRP, active EIGRP neighbor adjacency reduces convergence time in event of network failure

• Adjacent neighbors' Hello packets contain "Hold Time"

      --   If no hello is received within hold time, neighbor is declared unreachable

• When neighbor is lost:

      --   Paths via that neighbor are removed from topology and routing table
      --   If backup routes exist, they become new best paths and are inserted in routing table

            --  In this case EIGRP can have sub-second convergence
      --   If no backup routes exist, DUAL must run again


DUAL Reconvergence:

• When best path is lost and no backup routes exist, route goes into "active" state and "active timer" starts

      --   Stable routes not in active state are considered "passive"

• EIGRP "QUERY" message is reliably sent to remaining neighbors asking if there is an alternate route

• QUERY is propagated to all neighbors within EIGRP "query domain" or "flooding domain"

• Neighbors respond with EIGRP "REPLY" packet indicating if alternate route is available

      --   If alternate route exists, DUAL recalculates new best path
      --   If no alternate route exists, prefix is removed from topology table
      --   If active timer expires and no REPLY is received, the route is declared "Stuck-In-Active" (SIA) and removed from topology table