CCNA / Lesson 3 - EIGRP

EIGRP defaults

• ·         Routers running EIGRP need to have the same process ID# (AS#) to form adjacencies or neighbor relationships.
• ·         Equal cost load balancing is on by default. Unequal cost in configurable
• ·         Auto summarization is on by default.

EIGRP messages

• ·         Hello – unreliable, discover neighbor and establishes adjacencies every 5 seconds.
• ·         Reply – reliable, sent as a replay fro queries unicast
• ·         Query – reliable, sent when searching for networks multicast
• ·         Updates – reliable, link updates bounded and partial multicast or unicast
• ·         ACK – reliable used when reliable messages are sent unicast.

EIGRP headers

• ·         Opcode field – tells which kind of EIGRP packets (e.g. hello, reply etc)
• ·         Autonomous system number – ID# / process for EIGRP

EIGRP data

• ·         TLV field – internal and external routes are sent over this payload including the subnet mask data.

Successor and feasible successor routes

• ·         Successor (best route found by DUAL) – is the best loop free route which is installed in the routing table
• ·         Feasible successor – a backup route or feasible successor route can be installed in the topology database if the route meets the feasibility condition.
• ·          Feasible distance (FD, my destination to given network) – the load router’s composite metric to a destination network.
• ·         Reported distance (RD, get report to me from other routers) – the received metric of a neighbor’s route to a destination network
• ·         Feasibility condition (FC) – if neighbor router’s reported distance (RD) to a destination network is less than the local router’s feasible distance to that same destination network.

Administrative distance

·         Internal route – 90

·         External route – 170

·         Summary route – 5