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EIGRP

Summary

EIGRP is an advanced distance vector protocol, designed by Cisco. Combines the best of link-state and distance vector.

  • Supports VLSM (classless protocol, advertises a subnet mask for each network)
  • No special configurations for different layer 2 protocols
  • 32-bit metric provides granularity and support for unequal cost load balancing

Features

  • rapid convergence - uses DUAL (diffusing update algorithm). Stores its neighbors' routing tables to it can quickly adapt
  • higher scalability - sends partial triggered updates rather than periodic updates. Sends changes only, not whole routing table
  • supports multiple routed protocols - IPv4 and IPv6 support

Communications

  • Uses RTP (Reliable Transport Protocol) for guaranteed, ordered delivery of EIGRP packets
    • 'Ack' can be required for important packets like HELLO
  • EIGRP = IP protocol 88
  • Supports unicast and multicast
    • Multicast IPv4: 224.0.0.10
    • Multicast IPv6: FF02::A

Neighbor table

  • Contains each neighbor that has an established adjacency
  • Primary IP of the neighbor and directly connected interface

Neighborship process

  • Steps to become neighbors
    1. R1 and R2 send HELLO to all of their configured interfaces
    2. R1/R2 recieve each other's HELLO and send an UPDATE to each other which includes all routes EXCEPT for the ones learned on the same interface the HELLO came from (split-horizon). These UPDATE packets have the initialization bit set to indicate this is the initial exchange.
    3. Once the UPDATE has been recieved, each router sends an ACK to the other to indicate the UPDATE was recieved.
    4. The data from the UPDATE packets is placed in the topology table.
    5. successor routes from the topology table are placed into the routing table
  • Routers must belong to the same AS - identifies the routing process and EIGRP domain

Configure interfaces within 192.168.1.0/24 to participate in EIGRP 

R1(config)# router eigrp 250
R1(config-router)# network 172.16.12.0 [0.0.0.255] <-- optional wildcard mask
* If a wildcard isn't specified, classful subnet will be used

Set EIGRP RID R1(config-router)# eigrp router-id 1.2.3.4 * EIGRP RID (Router ID) is a 32-bit value configured like an IPv4 address * Doesn't change unless the EIGRP process is cleared or router ID is manually configured * Automatic selection process 1. Highest address of it's loopbacks 2. If no loopbacks, highest IPv4 address on an active interface

Verify neighbors with show ip eigrp neighbors [detail] 

R1#show ip eigrp ne
EIGRP-IPv4 Neighbors for AS(250)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
0   172.16.12.2             Gi0/1                    13 00:01:02    8   150  0  2
* H - order the sessions were formed in * Address - IP of the peer * Interface - interface the peer is on * Hold - amt of time the router will wait to hear a HELLO * Uptime - amt of time since the neighborship was formed * SRTT - time in milliseconds required for the router to send an EIGRP packet to it's neighbor and recieve an ACK * RTO - amt of time before sending a packet from the retransmission queue * Q - shows the number of packets waiting in the queue (greater than 0 = network congestion) 
R1#show ip ei n d
EIGRP-IPv4 Neighbors for AS(250)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
0   172.16.12.2             Gi0/1                    14 00:01:45    8   150  0  2
   Version 23.0/2.0, Retrans: 1, Retries: 0, Prefixes: 1
   Topology-ids from peer - 0 
   Topologies advertised to peer:   base

Max Nbrs: 0, Current Nbrs: 0
* Retrans - # of times a pakcet has been retransmitted * Retries - # of times an attempt was made to retransmit * Prefixes - # of prefixes recieved

Topology table

  • Contains all destination routes advertised by neighbors
  • each entry is assocated with a list of neighbors advertising that entry
  • Each neighbor+destination combination has an advertised metric - the metric that the neighbor stores in their routing table to the desination
  • advertised metric + link cost = metric* (i.e. this routers metric to the destination)
  • best metric = successor and is placed into the routing table and advertised to other neighbors