9 - EIGRP Concepts Flashcards
What is the Feasibility Condition?
- If a non-successor route’s Reported Distance is less than the Feasible Distance then the route is a Feasible Successor Route
- Basically “is my neighbor closer to that subnet than I am?”
What is the Feasible Distance?
The local routers Composite Metric of its best route to reach a subnet - in other words, the LOCAL ROUTER’s Metric
What is the Reported Distance?
The next-hop router’s metric for the same route.
Successor
- The local router’s route with the best metric, or best Feasible Distance.
- ‘show ip route’ shows the successor routes.
What is a Feasible Successor?
A route who’s Reported Distance is less than the local router’s Feasible Distance, if the best route fails EIGRP switches to the Feasible Successor.
What does DUAL stand for?
Diffusing Update Algorithm.
If a successor route fails what does EIGRP do?
Switches over to the Feasible Successor route, if no Feasible Successor route then DUAL takes over to find a new route.
What does DUAL do?
1.) The local router sends EIGRP QUERY messages to its remaining neighbors looking for a route 2.) the neighbor replys with an EIGRP REPLY message confirming a viable route 3.) The route is then entered in the local routers table.
What 3 steps does EIGRP do when first joining the network?
1.) Neighbor Discovery 2.) Topology Exchange 3.) Chooses Routes
What type of messages are used to discover neighbors?
Hello Messages
What type of messages are used to exchange topology?
EIGRP Update Messages
What address are EIGRP Hello messages sent to ?
224.0.0.10
What address are EIGRP Update Messages sent to ?
The Unicast address of the neighboring router. If multiple neighbors are on the subnet then to 224.0.0.10
What protocol does EIGRP use?
Reliable Transport Protocol (RTP)
What advantage does RTP have?
provides mechanism to resend any messages that were not received
What is the default Hello interval?
5 seconds
What is the Hold Interval
The amount of time a router will wait to hear a Hello from its neighbor before declaring it down.
What is the default Hold Interval
3 times the Hello time or 15 seconds.
How does EIGRP notify its neighbors of a topology change?
EIGRP Partial Update
What are the checks that EIGRP performs to determine if it can be a neighbor?
- passes authentication
- Same Autonomous System number
- Source IP address used by the neighbor to send Hello is on same subnet
- EIGRP K-values must match
What is the formula EIGRP uses to calculate the Composite Metric?
Metric = ((107 / least-bandwidth in path in Kbps) + cumulative delay) * 256
For RIP, what is considered an infinite route?
16
In EIGRP, what is considered an infinite route?
232 - 1
What does OSPF consider to be an infinite route?
224 - 1
In which 2 situations does an EIGRP router send Multicast Updates?
- When the router needs to update multiple routers on the same subnet
- When the router is sending Hello messages
What transport protocol does EIGRP use?
- Reliable Transport Protocol (RTP)
- which is also IP port 88
What is the multicast address used by EIGRP?
224.0.0.10
What is the variance command for?
To allow for load-balancing across multiple paths by making routes who’s feasible distances aren’t exactly equal to the successor’s appear to be equal.
How does the ‘variance’ command work?
- Multiply the successor’s metric by the variance.
- If a feasible successor’s RD is less than the successor’s FD times the variance then treat the feasible successor’s route as though it were an equal path.
Can a route that is not a FS be added to the routing table by using the ‘variance’ command?
No.
