Saturday, January 10, 2015

Cisco CCNP - 300-101 - Understanding EIGRP

So it's that time again for me to renew my Cisco Certifications. As a result, this post is based on my preparation for the CCNP Route Exam (300-101).

In this post I will be focusing on Understanding (refreshing my memory) EIGRP

    - Neighboring routers must be configured with the same as ASN
        eg. router eigrp 1

    - EIGRP is only enabled on interfaces matched by the network command
    - Neighbors are discovered via multicast EIGRP Hello messages
    - Advertises its connected subnet
    - Uses either wildcard mask or classful network number
    - If no wildcard mask is used, EIGRP is enabled for all configured interfaces within the classful network
    eg of wildcard mask. For network wildcard mask is
    - By default EIGRP uses bandwidth and delay to calculate metric
    interface load and reliability are some of the other available options. NOT RECOMMENDED BY CISCO
    - MTU is not used in metric calculation but can be used as a tiebreaker. In this case the path with the higher MTU is selected
    - EIGRP only uses MD5 for authentication
    - Hold timer is 15 seconds (default) - Frame Relay 180 seconds
    - if no Hello messages are received from the router within the hold time, the route is considered unavailable or the neighbor is dead
    - EIGRP can use Bi-Directional Forwarding Detection (BFD) which provides subsecond detection of failure
    - Ratio for hold timer to hello timer is 3:1
    - Hello timers tell the local router what to do
    - Hold timer tell the neighbor what to do
    - Hold timer and hello timer is done per interface not per neighbor
    - setting hold timer smaller than the hello timer results in flapping routes
    - Hello and hold timers don't have to match for neighborship to be formed
    - Uses "Router ID" to represent each router
    - Router ID is 32 bits
    - Router ID is determined when the process starts
    - can be set with   
        "eigrp router-id x.x.x.x"
        Use the highest IP on a loopback interface
        use the highest IP on non loopback interface
    - Duplicate RIDs do not prevent routers from forming neighborship
    - RIDs are of greater importance when injecting routes

steps used to add routes
    - Discover neighbors via Hello Messages (default 5 seconds on LAN)
    - topology exchange in full. Partial updates thereafter
    - choose routes via the lowest metric
    - Continue to send hello messages

    - uses update messages via multicast IP to send topology information
    - update messages are sent via Reliable Transport Protocol (RTP)
    - Full update send all routes
    - Full updates occur when neighbors first come up
    - Partial update deals with changed routes
Learning topology
    - list neighboring routes
    - hold all topology information learned from EIGRP neighbors
    - Choose the best routes
Preventing unwanted neighborship
    - make the interface passive. This disable sending EIGRP messages
      eg. passive-interface fa0/0
          passive-interface default
    - advertised the connected route via redistribution instead of EIGRP
EIGRP Static neighborship
    - seldom used
    - neighbor x.x.x.x out interface
    - still needs the network command
    - use show ip neighbors detail to see static configuration
    - multicast EIGRP messages are disabled
    - once configured neighbors cannot be learnt dynamically
Configuring K-Values - Metric Component
    - Uses bandwidth and delay by default
    - Formula can be changed to use link load and reliability - NOT RECOMMENDED
    - bandwidth and or delay can also be disabled
    - Available Ks are K1, K2, K3, K4, K5
    - use "metric weight" to set K-values
    - K values must match or neighborship may not be established
    - Will not form neighborship if AS does not match
Verifying EIGRP config
    show ip eigrp interfaces - List EIGRP enabled interfaces
    show ip protocols - shows each routing process
    show ip eigrp topology - list known EIGRP neighbors
    show ip route - shows routing table - D is for EIGRP
    sh ip eigrp traf

CCNP official Cert Guide - CCNP Routing and Switching - Route 300-101

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