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EIGRP

Advanced Networking: Routing & Switching 2 – Chapter 7

Copyleft 2014 Hacklab Cosenza (http://hlcs.it)Released under Creative Commons License 3.0 By-SaCisco name, logo and materials are Copyright Cisco Systems Inc.

Introduction● Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector

routing protocol developed by Cisco Systems.

● As the name suggests, EIGRP is an enhancement of another Cisco routing protocol IGRP (Interior Gateway Routing Protocol).

● IGRP is an older classful, distance vector routing protocol, now obsolete since IOS 12.3.

● EIGRP includes features found in link-state routing protocols.

● EIGRP was initially released in 1992 as a proprietary protocol available only on Cisco devices.

● In 2013, Cisco released a basic functionality of EIGRP as an open standard to the IETF as an informational RFC.

● Advanced features of EIGRP will not be released to the IETF.

Diffusing Update Algorithm● The Diffusing Update Algorithm (DUAL) resides at the center of the EIGRP routing

protocol.

● DUAL guarantees loop-free and backup paths throughout the routing domain.

● Using DUAL, EIGRP stores all available backup routes for destinations so that it can quickly adapt to alternate routes when necessary.

● EIGRP establishes relationships with directly connected routers that are also enabled for EIGRP.

● Neighbor adjacencies are used to track the status of these neighbors.

RTP, Updates and Balancing● The Reliable Transport Protocol (RTP) provides delivery of EIGRP packets to neighbors.

● RTP and the tracking of neighbor adjacencies set the stage for DUAL.

● EIGRP sends Partial and Bounded Updates.

● The term partial means that the update only includes information about the route changes, such as a new link or a link becoming unavailable.

● The term bounded refers to the propagation of partial updates that are sent only to those routers that the changes affect. This minimizes the bandwidth that is required to send EIGRP updates.

● Equal cost load balancing and unequal cost load balancing allow administrators to better distribute traffic flow in their networks.

Protocol-dependent modules● EIGRP has the capability for routing several different protocols, including IPv4 and IPv6,

using protocol-dependent modules (PDMs).

● PDMs are responsible for the specific routing tasks for each network layer protocol, including:

● Maintaining the neighbor and topology tables of EIGRP routers that belong to that protocol suite

● Building and translating protocol-specific packets for DUAL

● Interfacing DUAL to the protocol-specific routing table

● Computing the metric and passing this information to DUAL

● Implementing filtering and access lists

● Performing redistribution functions to and from other routing protocols

● Redistributing routes that are learned by other routing protocols

EIRP Layers

Reliable Transport Protocol● RTP includes both reliable delivery and unreliable delivery of EIGRP packets, similar to

TCP and UDP, respectively.

● Reliable RTP requires an acknowledgment to be returned by the receiver to the sender.

● EIGRP update packet is sent reliably

● An unreliable RTP packet does not require an acknowledgment.

● An EIGRP Hello packet is also sent over RTP, but unreliably. This means that EIGRP Hello packets do not require an acknowledgment.

● RTP can send EIGRP packets as unicast or multicast.

● Multicast EIGRP packets for IPv4 use the reserved IPv4 multicast address 224.0.0.10

● Multicast EIGRP packets for IPv6 are sent to the reserved IPv6 multicast address FF02::A

EIGRP Authentication● EIGRP can be configured for authentication

● It is a good practice to authenticate transmitted routing information.

● Doing so ensures that routers only accept routing information from other routers that have been configured with the same password or authentication information.

● Note: Authentication does not encrypt the EIGRP routing updates.

EIGRP Packet Types● The 5 EIGRP packet types are also called EIGRP packet formats or EIGRP messages.

● Hello packets - Used for neighbor discovery and to maintain neighbor adjacencies.

● Sent with unreliable delivery - Multicast (on most network types)

● Update packets - Propagates routing information to EIGRP neighbors.

● Sent with reliable delivery - Unicast or multicast

● Acknowledgment packets - Used to acknowledge the receipt of an EIGRP message that was sent using reliable delivery.

● Sent with unreliable delivery - Unicast

● Query packets - Used to query routes from neighbors.

● Sent with reliable delivery - Unicast or multicast

● Reply packets - Sent in response to an EIGRP query.

● Sent with reliable delivery - Unicast

EIGRP Encapsulation

Hello packets● EIGRP Hello packets are sent as IPv4 or IPv6 multicasts, and use RTP unreliable

delivery.

● The reserved EIGRP multicast address for IPv4 is 224.0.0.10

● The reserved EIGRP multicast address for IPv6 is FF02::A

● EIGRP routers discover neighbors and establish adjacencies with neighbor routers using the Hello packet.

● By defaultEIGRP Hello packets are sent as multicast packets every five seconds.

● However, on multipoint, nonbroadcast multiple access (NBMA) networks, such as X.25, Frame Relay, and Asynchronous Transfer Mode (ATM) interfaces with access links of T1 (1.544 Mb/s) or slower, Hello packets are sent as unicast packets every 60 seconds.

● By default, the hold time is three times the Hello interval

Update and Acknowledgment Packets● EIGRP sends Update packets to propagate routing information only when necessary.

● EIGRP updates contain only the routing information needed and are sent only to those routers that require it.

● EIGRP sends incremental updates only when the state of a destination changes.

● Partial update only includes information about the route changes.

● Bounded update refers to the propagation of partial updates that are sent only to those routers that the changes affect.

● Update packets are sent using reliable delivery as a multicast when required by multiple routers, or as a unicast when required by only a single router.

● EIGRP sends Acknowledgment (ACK) packets when reliable delivery is used.

● An EIGRP acknowledgment is an EIGRP Hello packet without any data sent as an unreliable unicast.

Query and Reply Packets● DUAL uses query and reply packets when searching for networks and other tasks.

● Queries and replies use reliable delivery.

● Queries can use multicast or unicast

● Replies are always sent as unicast.

● When a router has lost connectivity to a network it sends out queries to all EIGRP neighbors searching for any possible routes to the network.

● All neighbors must send a reply, regardless of whether or not they have a route to the downed network.

Type, length, value (TLV)● The EIGRP parameter’s message includes the weights that EIGRP uses for its composite

metric.

● By default, only bandwidth (K1) and delay (K3) are weighted.

● The IP internal routes message is used to advertise EIGRP routes within an AS.

● Important fields include the metric fields (delay and bandwidth), the subnet mask field (prefix length), and the destination field.

● Delay is calculated as the sum of delays from source to destination in units of 10 microseconds.

● Bandwidth is the lowest configured bandwidth of any interface along the route.

● The subnet mask is specified as the prefix length or the number of network bits

● The IP external route message is used when external routes are imported into the EIGRP routing process. For example when we import or redistribute a default static route into EIGRP.

● Opcode: EIGRP Packet Type: Update (1), Request (2), Query (3), Reply (4), Hello/ACK (5)

● Autonomous System Number: ID for this EIGRP routing process

Autonomous System Numbers● The autonomous system number used for EIGRP configuration is only significant to

the EIGRP routing domain.

● It functions as a process ID to help routers keep track of multiple, running instances of EIGRP.

● EIGRP uses the router eigrp autonomous-system command to enable the EIGRP process.

● All routers within the EIGRP routing domain must use the same autonomous system number.

● Global autonomous system numbers are assigned by IANA, the same authority that assigns IP address space.

● The local regional Internet registry (RIR) is responsible for assigning an autonomous system number to an entity from its block of assigned autonomous system numbers.

● Prior to 2007, autonomous system numbers were 16-bit numbers ranging from 0 to 65,535.

● Today, 32-bit autonomous system numbers are assigned.

EIGRP Router ID● The Router ID is used to uniquely identify each router in the EIGRP routing domain.

● EIGRP for IPv4 uses the 32-bit router ID to identify the originating router for redistribution of external routes.

● Cisco routers derive the router ID based on three criteria, in the following precedence:

1. Use the IPv4 address configured with the eigrp router-id ipv4-address router configuration mode command.

2. If the router ID is not configured, the router chooses the highest IPv4 address of any of its loopback interfaces.

3. If no loopback interfaces are configured, the router chooses the highest active IPv4 address of any of its physical interfaces.

● Note: The EIGRP router ID is not changed, unless the EIGRP process is removed with the no router eigrp command or if the router ID is manually configured with the eigrp router-id command.

The Classful Network Command● To enable EIGRP routing on an interface, use the network router configuration mode

command and enter the classful network address for each directly connected network.

● When using the network command and an IPv4 network address all interfaces on the router that belong to that classful network address are enabled for EIGRP:

● Router(config-router)# network 172.16.0.0 is used on a router to include both interfaces in subnets 172.16.1.0/24 and 172.16.3.0/30

● By default, the eigrp log-neighbor-changes router configuration mode command is enabled. This command is used to:

● Display any changes in EIGRP neighbor adjacencies.

● Help verify neighbor adjacencies during configuration of EIGRP.

● Advise the network administrator when any EIGRP adjacencies have been removed.

The Classless Network Command● To configure EIGRP to advertise specific subnets only, use the wildcard-mask option with

the network command:

● Router(config-router)# network network-address [wildcard-mask]

● Some IOS versions also let you enter the subnet mask instead of a wildcard mask.

● To calculate the inverse of the subnet mask, subtract the subnet mask from 255.255.255.255 as follows:

255.255.255.255

- 255.255.255.252 Subnet mask

----------------

0. 0. 0. 3 Wildcard mask

Passive Interface● The passive-interface command can be used to prevent the neighbor adjacencies.

● It disables the transmission and receipt of EIGRP Hello packets on these interfaces.

● There are two primary reasons for enabling the passive-interface command:

● To suppress unnecessary update traffic, such as when an interface is a LAN interface, with no other routers connected

● To increase security controls, such as preventing unknown rogue routing devices from receiving EIGRP updates

● Example:

● To verify whether any interface on a router is configured as passive, use

● show ip protocols privileged EXEC mode command

Verifying EIGRP: Examining Neighbors● Use the show ip eigrp neighbors command to view the neighbor table and verify

that EIGRP has established an adjacency with its neighbors.

● The show ip eigrp neighbors command output includes:

● H column - Lists the neighbors in the order that they were learned.

● Address - IPv4 address of the neighbor.

● Interface - Local interface on which this Hello packet was received.

● Hold - Current hold time. When a Hello packet is received, this value is reset to the maximum hold time for that interface, and then counts down to zero. If zero is reached, the neighbor is considered down.

● Uptime - Amount of time since this neighbor was added to the neighbor table.

● Smooth Round Trip Timer (SRTT) and Retransmission Timeout (RTO) - Used by RTP to manage reliable EIGRP packets.

● Queue Count - Should always be zero. If more than zero, then EIGRP packets wait to be sent.

● Sequence Number - Used to track updates, queries, and reply packets.

EIGRP Tables● Each EIGRP router maintains a neighbor table, which contains a list of routers on shared

links that have an EIGRP adjacency with this router.

● Each EIGRP router maintains a topology table for each routed protocol configured, such as IPv4 and IPv6.

● The topology table includes route entries for every destination that the router learns from its directly connected EIGRP neighbors.

● When a router receives an EIGRP routing update, it adds the routing information to its EIGRP topology table

– and replies with an EIGRP acknowledgment.

● After receiving the EIGRP update packets from a neighbor, using the information in the topology table, a router updates its IP routing table with the best path to each destination, including the metric and the next-hop router.

EIGRP Composite Metric● By default, EIGRP uses the following values in its composite metric to calculate the

preferred path to a network:

● Bandwidth - The slowest bandwidth among all of the outgoing interfaces, along the path from source to destination.

● Delay - The cumulative (sum) of all interface delay along the path (in tens of microseconds).

● The following values can be used, but are not recommended, because they typically result in frequent recalculation of the topology table:

● Reliability - Represents the worst reliability between the source and destination, which is based on keepalives.

● Load - Represents the worst load on a link between the source and destination, which is computed based on the packet rate and the configured bandwidth of the interface.

● The formula is:

Examining Interface Values

Bandwidth Metric● The bandwidth metric is a static value used by some routing protocols, such as EIGRP and

OSPF, to calculate their routing metric.

● The bandwidth is displayed in kilobits per second (kb/s).

● Always verify bandwidth with the show interfaces command.

● The default value of the bandwidth may or may not reflect the actual physical bandwidth of the interface. On most serial links, the bandwidth metric defaults to 1544 kb/s.

● If actual bandwidth of the link differs from the default bandwidth value, the bandwidth value should be modified.

● Use the following interface configuration mode command to modify the bandwidth metric:

Delay Metric● Delay is the measure of the time it takes for a packet to traverse a route.

● The delay (DLY) metric is a static value based on the type of link to which the interface is connected and is expressed in microseconds.

● Default delay values for various interfaces are:

Calculate the EIGRP Metric● By examining the bandwidth and delay values for all of the outgoing interfaces of the

route, we can determine the EIGRP metric as follows:

Step 1.

● Determine the link with the slowest bandwidth.

● Use that value to calculate bandwidth (10,000,000/bandwidth).

Step 2.

● Determine the delay value for each outgoing interface on the way to the destination.

● Add the delay values and divide by 10 (sum of delay/10).

Step 3.

● Add the computed values for bandwidth and delay, and multiply the sum by 256 to obtain the EIGRP metric.

Diffusing Update Algorithm Concepts● EIGRP uses the Diffusing Update Algorithm (DUAL) to provide the best loop-free path

and loop-free backup paths.

● DUAL uses several terms:

● Successor

● Feasible Distance (FD)

● Feasible Successor (FS)

● Reported Distance (RD) or Advertised Distance (AD)

● Feasible Condition or Feasibility Condition (FC)

● These terms and concepts are at the center of the loop avoidance mechanism of DUAL.

Nota: Feasible → fattibile, praticabile, possibile

Successor and Feasible Distance● A successor is a neighboring router that is used for packet forwarding and is the least-cost

route to the destination network.

● FD is the lowest calculated metric to reach the destination network.

Feasible Successors● An FS is a neighbor that has a loop-free backup path to the same network as the

successor

Reported Distance

FD vs RDFD is in the routing tableRD is only in EIGRP updates

The RD is simply an EIGRP neighbor’s feasible distance to the same destination network.

Feasibility Condition● The FC is met when a neighbor’s Reported Distance (RD) to a network is less than

the local router’s feasible distance to the same destination network.

● If the reported distance is less, it represents a loop-free path.

States of EIGRP routes● PASSIVE. When DUAL is not performing its diffusing computations to determine a path for

a network, the route is in a stable mode, known as the passive state.

● ACTIVE. If DUAL recalculates or searches for a new path, the route is in an active state

● All routes in the topology table should be in the passive state for a stable routing domain.

● debugging can be enabled using the debug eigrp fsm command

Configuring EIGRP for IPv6● EIGRP for IPv6 messages are sent using:

● Source IPv6 address

● This is the IPv6 link-local address of the exit interface.

● Destination IPv6 address

● When the packet needs to be sent to a multicast address, it is sent to the IPv6 multicast address FF02::A, the all-EIGRP-routers with link-local scope.

● If the packet can be sent as a unicast address, it is sent to the link-local address of the neighboring router.

● Note: IPv6 link-local addresses are in the FE80::/10 range. The /10 indicates that the first 10 bits are 1111 1110 10xx xxxx, which results in the first hextet having a range of 1111 1110 1000 0000 (FE80) to 1111 1110 1011 1111 (FEBF).

Configuring IPv6 Link-local Addresses

The same IPv6 link-local address is configured on all of R1’s interfaces.

The same IPv6 link-local address is configured on all of R2’s interfaces.

Enable IPv6 routing on the router● The ipv6 unicast-routing global configuration mode command enables IPv6 routing on the

router.

● This command is required before any IPv6 routing protocol can be configured.

● This command is not required to configure IPv6 addresses on the interfaces, but is necessary for the router to be enabled as an IPv6 router.

Configuring the EIGRP for IPv6 Routing Process

● The eigrp router-id command is used to configure the router ID.

● EIGRP for IPv6 uses a 32 bit value for the router ID. To obtain that value, EIGRP for IPv6 uses the same process as EIGRP for IPv4.

● By default, the EIGRP for IPv6 process is in a shutdown state.

● The no shutdown command is required to activate the EIGRP for IPv6 process.

● Both the no shutdown command and a router ID are required for the router to form neighbor adjacencies.

Enable EIGRP for IPv6 on an interface● EIGRP for IPv6 uses a different method to enable an interface for EIGRP.

● Instead of using the network router configuration mode command to specify matching interface addresses, EIGRP for IPv6 is configured directly on the interface.

End of lesson