What is the general process that EIGRP follows when configured?
router eigrp asn command is issued followed by one or more network net-id wildcard-mask statements.

What are the rules for the above commands?

1. Neighboring routers must have the same ASN.

2. EIGRP is enabled on interfaces that match the network command. When enabled, router does the following:

a. Attempts to find EIGRP neighbors by sending multicast EIGRP messages (Hello).

b. Advertises the connected subnet on interface to neighbors.

3. If no wildcard mask is configured on the network command, command will be classful network number.

4. If no wildcard mask is configured, on the network command, EIGRP is enabled on all router’s interfaces directly connected to configured classful number.

5. If wildcard mask is configured, router performs ACL logic when comparing the network-id configured in the network command with each interface’s IP using configured wildcard-mask as an ACL.

Example configuration:

router eigrp 1
network 10.0.0.0 (classful)
network 10.1.0.0 0.0.31.255 (wildcard ACL)

What commands are used to verify EIGRP configuration?

sh ip eigrp interfaces – lists working interfaces where EIGRP is enabled; omits passive interfaces.

sh ip protocols – lists contents of network configuration commands for each routing process and list of neighbor IP addresses.

sh ip eigrp neighbors – List of known neighbors; doesn’t list neighbors with mismatched paramters.

sh ip eigrp topology – lists all successor & feasible successor routes known to the router. Doesn’t list all known topology details.

sh ip route – lists contents of the IP routing table.

What are the three steps EIGRP follows to add routes to the IP routing table?

1. Neighbor discovery: router sends Hello messages to discover neighbors and performs basic parameter checks to determine which routers should become neighbors.

2. Topology Exchange: Neighbors exchange full topology updates when relationship comes up, then partial updates as needed.

3. Choosing Routes: Each router analyzes their EIGRP topology tables to choose the lowest metric route to each subnet.

How does EIGRP exchange topology information with neighbors?

Through Update messages to multicast 224.0.0.10. Sent using Reliable Transport Protocol (RTP).

How does EIGRP calculate the best route?

metric = (( 10^7/least-bandwidth) + cumulative-delay) * 256

Least bandwidth is lowest bandwidth link in the route in kilobits per second.

Cumulative delay is the sum of all delay values in “tens of microseconds”.

What are the features of EIGRP (summary)?

Transport: IP, protocol 88.

Metric: Constrained bandwidth and cumulative delay.

Hello interval: Interval at which EIGRP sends Hello messages on an interface.

Hold timer: Timer used to determine when a neighboring router has failed.

Update destination address: Sent to 224.0.0.10 with retransmissions sent to neighbor’s unicast IP.

Full or partial updates: Full updates when neighbors are discovered; otherwise partial updates are used.

Authentication: Supports MD5 only.

VLSM/classless: EIGRP includes mask with each route allowing it to support discontiguous networks and VLSM.

Route tags: Allows EIGRP to tag routes as they are redistributed into EIGRP.

Next-hop field: Supports the advertisement of routes with a different next-hop router than the advertising router.

Manual route summarization: Allows route summarization at any point in the EIGRP network.

Automatic summarization: EIGRP supports and defaults to use automatic route summarization at classful network boundaries.

Multiprotocol: Supports the advertisement of IPX & AppleTalk routes & IPv6.

What are the steps to learning the best loop-free routes?

1. Establish EIGRP neighbor relationships with other routers on the same subnet.

2. Exchange EIGRP topology data with neighbors.

3. Calculate best IP route for each subnet based on known topology table and add to the IP routing table.

What are EIGRP Hello messages for?

To find neighbors and to confirm link between neighbors still works.

What is the default Hello interval and Hold timer?

5 & 15, respectively.

How would you optimize convergence between routers?

Reduce Hello interval and Hold timer.

How do you configure Hello interval and Hold timer?

Per interface:

interface fastethernet0/1
ip hello-interval eigrp asn integer-value
ip hold-time eigrp asn integer-value

Which command tells the local router what to do and which tells the neighbor what to do?

Hello-interval: tells local router what to do
hold-time: tells neighbor what to do

What command verifies Hello interval?

sh ip eigrp int type number detail

How do you verify hold timer?

Repeatedly issue sh ip eigrp neighbors

How would you disallow neighbors on an interface?

Configure passive interfaces

How do you configure passive interfaces?

Use command on router config mode: passive-interface type number

OR

passive-interface default with no passive-interface type number

What are the two methods to prevent EIGRP neighbors?

Passive interfaces
Don’t enable EIGRP on interfaces and redistribute connected interfaces

How do you verify passive interfaces?

sh ip protocols

How would you control EIGRP neighborships?

Use authenitcation (MD5). Each message is authenticated.

What is the process to enable EIGRP authentication?

1. Create an authentication key chain

Global command: key chain name (doesn’t have to match on routers)

Create one or more keys: key number (doesn’t have to match on routers)

Define authentication key value: key-string value (must match on routers)

Optionally define lifetime

2. Enable EIGRP md5 authentication on interface for EIGRP ASN

ip authentication mode eigrp asn md5

3. Refer to correct key chain on interface

ip authentication key-chain eigrp asn name-of-chain

Which keys are used for sending EIGRP messages?

Lowest key among all currently valid keys.

Which keys are used for receiving EIGRP messages?

Uses all valid keys.

How do you verify EIGRP authentication?

sh ip eigrp neighbors

sh key chain

debug eigrp packet

What should be kept in mind when troubleshooting EIGRP authentication?

Examine the time (sh clock).

Key chain name and key number on two routers don’t have to match.

Key string on neighbors must match.

Check which keys are valid with sh key chain.

MD5 authentication and key chain must be configured on interface.

Why would you statically configure EIGRP neighbors?

To reduce overhead associated with EIGRP multicast messages.

How do you configure a static EIGRP neighbor?

Under EIGRP router subcommand: neighbor ip-address exit-interface

What are the caveats to using EIGRP static neighbors?

EIGRP multicast processing is disabled.

No dynamic neighbors are discovered on the interface which is used for static neighbor.

Existing dynamic neighbors will fail.

What are neighbor requirements for EIGRP?

Routers must be able to send IP packets to each other.

Interface’s primary IP’s must be on the same subnet.

Connected interface must not be passive.

Must use the same ASN.

Must pass EIGRP authentication.

K-values must match.

How does an EIGRP router determine it’s router-id?

1. Use the configured value (eigrp router-id a.b.c.d).

2. Use the highest configured up/up loopback interface IP address.

3. Use the highest IP address on an up/up non-loopback IP address.

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Posted by   (@rowelldionicio)   on April 28th, 2012