EIGRP Flashcards
What type of routing protocol is EIGRP?
Enhanced Distance Vector Protocol
What metric does EIGRP use for its calculations?
Bandwidth and Delay
What load balancing feature does EIGRP allow that other protocols don’t?
Unequal-cost load balancing
What algorithm does it to use to select best paths?
Diffusing Update Algorithm (DUAL)
What is reported distance?
Also referred to as Administrative Distance, it is the cost between the next-hop and the destination
What is feasible distance?
This the cost between our local router and the next-hop plus the next-hop’s RD to the destination
What is a successor?
This is the neighboring router that has the best-path to the destination that is guaranteed to not be in a routing loop (lowest FD)
What is a successor route?
This is the best route that goes into the routing table
What is the feasibility condition?
This is the condition that the reported distance of the feasible successor must be lower than the feasible distance of the current successor to be considered a feasible successor
What does RD lower than FD confirm?
This confirms that there is no loop in the topology because the route must have a RD less than the current FD of the current successor route
What happens if our current successor route goes down, and we have a feasible successor?
This feasible successor route replaces the current successor route in the routing table
When are feasible successors selected?
They are selected at the same time as the successor route, but are kept in the topology table (not routing table) as backups
What is included in the EIGRP table?
Neighbor Tables, Topology Table and the Routing Table
What is in the neighbor table?
Contains EIGRP neighbor addresses
What is in the Topology table?
Contains all destinations advertised by neighboring routers
What is in the routing table?
Contains EIGRP successor routes. EIGRP routes are denoted by the letter D. The next hop address, and the time since last update packet received, and the local router exit interface to destination
What is a passive route?
A route that isn’t being recalculated and is in the ‘passive’ table state
What is an active route?
This is a route that is undergoing re-computation. This means we lost our best-path to a destination and we are finding an alternative
What underlying protocol does EIGRP use for reliable transmission?
Reliable Transport Protocol (RTP)
Why is RTP used instead of TCP/IP?
EIGRP does not use TCP/IP because IPX and Appletalk were in use when EIGRP was created
What packets are not acknowledged in EIGRP?
Hello and ACK packets
What do the numbers (90/40514506) mean in the EIGRP routing table?
First number is the administrative distance (AD) and the second number is the feasible distance (FD) (metric)
What is a metric?
A value that is assigned to a particular route/interface/etc. that is used to determine the cost of it.
What are the 5 EIGRP packets?
Hello, Request, Update, Query, Reply
What is the EIGRP Hello packet used for?
Used for discovery of EIGRP neighbors
What is the EIGRP Request packet used for?
Used to request specific information
What is the EIGRP Update packet used for?
Used to transmit routing information
What is the EIGRP Query packet used for?
Sent out to search for another path to a destination during convergence
What is the EIGRP Reply packet used for?
Sent in response to a query packet
At what number does EIGRP automatically assume that an update message is being looped around the network?
At greater than 100
When is MTU used in EIGRP route selection?
Normally it is not used unless there is a tie between two different routes with the same cost.
Does EIGRP use the slowest bandwidth in it’s metric calculation?
Yes it does
What does EIGRP use the lowest of in path selection?
EIGRP always uses the lowest bandwidth
What is the formula for Calculated Bandwidth?
BW = reference BW / slowest BW (kbps)
What is EIGRP delay?
This is a static value based on the link speeds. Every router adds its delay before passing the packet on to the neighbor. It is better to change the delay rather than the bandwidth, since other protocols use bandwidth but not delay
How is EIGRP link reliability measured?
Dynamically as a fraction of 255, higher fraction means better reliability
How is EIGRP link load measured?
Dynamically as a fraction of 255, lower fraction means less load. Typically not used in EIGRP metric calculation
What are wide metrics?
Due to some high speed links resolving to the same cost according to EIGRP, we enable wide metrics which scales the metric values so higher link speeds have lower metric values
What is the command to enable unequal-cost load balancing?
variance multiplier <1-128>
1 for equal cost
What does EIGRP do if the variance multiplier is greater than 1?
EIGRP will install multiple loop-free routes (feasible successors) as candidates to load-balance
What does variance multiplier (value) do?
It tells EIGRP to use feasible successor routes that have a feasible distance less than (x) times the feasible distance of the current route
How often are EIGRP hello packets sent?
Every 5 or 60 seconds (60 for slow links T1 speeds)
Why do we maintain adjacencies with hello packets?
This is to ensure that the router is still alive, otherwise EIGRP routers do not speak to eachother unless there is a routing update
How many consecutive Hello packets must be missed for EIGRP to assume a router is offline?
3 consecutive packets.
What is the Hold Time?
The hold time is the amount of time a router has specified that you wait for another Hello packet before deeming it missed. (default 15 or 180 seconds on on slow links)
When do EIGRP routers reply immediately to a hello packet?
This is when the routers receive a hello packet from a new neighbor.
How does EIGRP convergence work?
When EIGRP detects that it has lost a successor for a path, an FS replaces it (if available). EIGRP then sends out an update packet for that path because of the new EIGRP path metrics
When EIGRP does not have a backup route, what happens?
The DUAL Computation is used which sets the route from passive to active, and EIGRP query packets are sent to all neighbors looking for an alternate route. EIGRP then waits for replies from all neighbors before selecting its new best path
What does an EIGRP query include?
This query includes the network prefix with the delay set to infinity so other routers are aware that this path has gone active
What happens if EIGRP receives two routes with the same Feasible Distance in response to the query?
Equal-cost load balancing is used.
Why do we summarize routes in EIGRP?
We summarize routes because EIGRP routers won’t send queries for a route that is part of a summary
When does EIGRP stop advertising a summary?
When there are no more routers part of a summary route
What are discontinuous subnets?
subnets separated by another subnet with a different classful boundary
What do Hello Messages contain?
They contain Autonomous System IDs, and routers must use the same AS number or a neighbor relationship will not be formed.
Why does EIGRP Named Mode accomplish?
Eliminates the complexity by unifying the configuration of IPv4 and IPv6 EIGRP under one section specified by the name.
How does named mode EIGRP keep IPv4 and IPv6 configurations separate?
By using address-families ‘address-family [ipv4/ipv6] …’
How do we select all interfaces to issue a command to under named mode EIGRP?
Used ‘af-interface default`
Can we configure EIGRP authentication on regular eigrp configurations?
No we must use named mode EIGRP to use EIGRP authentication
What are the 4 ways we can route filter using EIGRP?
Access Control Lists, IP Prefix Lists, Route Maps and Gateway IP Addresses
What is the general way we want to filter routes use access control?
We want to generally filter routes outbound from the router so that there is less overhead
What do ACLs have at the end?
They have an implicit deny, so if there is no specific permit for a packet/route, it is dropped.
What is traffic steering?
It is the directing of traffic through a specific path
How do we enable traffic steering?
We do this by modifying the bandwidth or delay of EIGRP interfaces.
What is EIGRP Stuck-In-Active?
This refers to the unreasonable amount of time a poorly configured EIGRP topology takes to converge after a route is marked active and query packets are sent
What is a query boundary?
This is when EIGRP reaches the end of a network, or reaches a distribution/summarization boundary
What is an active timer?
This is the timer that has a default of 3 minutes (180 seconds) that marks a neighbor dead, after that all routes are deleted learned from that neighbor
How long does EIGRP wait before sending a Stuck-In-Active Query? (SIA)
It waits half the timer, by default 180/2 = 90 seconds. When it reaches this point it sends a SIA Query and is supposed to receive a SIA Reply. If no SIA Reply is received the neighbor is marked as dead
What are the 3 solutions for Stuck-In-Active EIGRP States?
Limit query range through route summarization, Configure remote routers as EIGRP stub routers, Filter routes from being advertised to neighbors by using ACLs
What is a stub router?
This is a router connected to the network core/hub that transit traffic (traffic to other networks not through the hub) should not flow on
Is a stub router ever queried for routes?
They are never queried for routes because stub routers contain no other routes than their own, asking for another networks routes would avail to nothing
What are the 4 stub router modes?
Connected: Only EIGRP configured directly connected routes are advertised
Summary: Only summary routes on EIGRP configured interfaces are advertised
Static: Only static routes in the EIGRP config will be advertised
Receive-only: Router only receives routes, does not advertise any
What percentage of bandwidth does EIGRP use at maximum?
50 percent of max bandwidth
What is split horizon?
It is a policy that states that a router should not advertise a route out the same interface it learned it from
When does split horizon become problematic?
When we have a multi-access interface or DMVPN interface, and we should disable split horizon on these interface(s)
