Operations

1.3.c i General operations

General information on “EIGRP General options”:

• EIGRP will only utilize 50% bandwidth of a link for all protocol messages (HELLOs, UPDATEs, …)
• This behavior can be modified on a per-interface basis

“EIGRP General options” CLI configuration commands:

## Modifying the EIGRP protocol messages bandwidth utilization value (classic mode)
Router(config-if)# [ip | ipv6] bandwidth-percentage eigrp <value>

## Modifying the EIGRP protocol messages bandwidth utilization value (named mode)
Router(config-router-af)# af-interface <if>
Router(config-router-af-interface)# bandwidth-percent <value>

1.3.c ii Topology table

General information on “EIGRP Topology table”:

• The topology table lists all routes learned/advertised via EIGRP
• This information includes:
  • Route status (Active, Passive, …)
  • Feasible Distance, Computed Distance and Reported Distance
  • Outgoing interface
  • …
• Detailed information of a route can be obtained when specifying a prefix

“EIGRP Topology table” CLI show commands:

## Showing the EIGRP topology table (successor and feasible successor routes)
Router# show [ip | ipv6] eigrp topology

## Showing all links of the EIGRP topology table (successor, feasible successor and non-feasible successor routes)
Router# show [ip | ipv6] eigrp topology all-links

## Showing the EIGRP topolofy table for a specific prefix/route
Router# show [ip | ipv6] eigrp topology <route>

1.3.c iii Packet types

HELLO packets

General information about “EIGRP HELLO packets”:

• Used to form and maintain relationships with neighbors
• Sent out on all interfaces on EIGRP initialization
• Are sent to the multicast address 224.0.0.10 (IPv4) or FF02::A (IPv6)
• Unicast is used with static neighbors

Default Timers:

• Hello time: 5 seconds (LAN/fast links), 60 seconds (slow links)
• Hold time: 3 times the hello time (15 seconds on fast links, 180 seconds on slow links)
• The timers don’t need to match between neighbors but odd behavior can be the result
• Timers can be modified on a per-interface basis

Requirements for EIGRP neighborship:

• Primary interface IP must be on the same subnet
• Connected interface must not be passive
• AS number must match
• Authentication must match
• K-values must match

Important: Router ID doesn’t have to be unique but will cause problems if it isn’t, eg. routing updates are not accepted if the origin router ID of an EIGRP UPDATE is the same as the local router ID!

“EIGRP HELLO packets” CLI configuration commands:

## Modifying the EIGRP hello and/or hold timer (classic mode)
Router(config-if)# [ip | ipv6] hello-interval eigrp <asn> <interval>
Router(config-if)# [ip | ipv6] hold-time eigrp <asn> <interval>

## Modifying the EIGRP hello and/or hold timer (named mode)
Router(config-router-af)# af-interface <if>
Router(config-router-af-interface)# hello-interval <interval>
Router(config-router-af-interface)# hold-time <interval>

UPDATE packets

General information about “EIGRP Update packets”:

• Used to send out topology updates
• When EIGRP neighbors first come up, routers exchange full topology tables
• In normal operation, there’s no periodic update flooding
• Instead, only(!) partial updates are sent out when a topology change occurs
• These partial updates only include information on what has changed (not the full table)
• RTP (reliable transport protocol) used to send UPDATE packets
• In the neighbor initialization sequence, unicast is used to exchange topology information
• UPDATE packets are sent as unicast to newly discovered neighbors
• UPDATE packets are sent as multicast when a link or metric changes
• UPDATE packets always require explicit acknowledgement
• UPDATE packets contain prefix, prefix-length, k-values, MTU and hop count

ACK packets

General information about “EIGRP ACK Packets”:

• An ACK packet is a HELLO packet without any data
• Used to confirm the reliable delivery of an EIGRP packet
• ACKs are always sent to the unicast address of a sender
• RTP (reliable transport protocol) used to send ACK packets

QUERY/REPLY packets

General information about “EIGRP QUERY Packets”:

• Used by a router to advertise that a route is in the ACTIVE state
• ACTIVE state means that the route has failed/is dead
• Are multicast packets (or unicast if neighbor is statically configured and/or on point-to-point interfaces)
• If no answer(s) is received by the neighbor(s) the packet will be resent as unicast to the nonresponsive neighbor(s)
• If there’s no reply after 16 attempts, the neighbor relationship will be reset

General information about “EIGRP REPLY Packets”:

• Neighbors will send a REPLY query when it has a loop-free route
• Otherwise the neighbor will forward the query to its neighbors
• REPLY packets are unicast

1.3.c iv Stuck In Active (SIA)

General information about “EIGRP Stuck In Active (SIA)":

• Stuck In Active (SIA) happens when a router with a route in ACTIVE state sends a QUERY but doesn’t get a REPLY from an adjacent neighbor within a certain amount of time
• When EIGRP loses a route and there’s no feasible successor the route will go from PASSIVE (converged) state to ACTIVE (not converged) state
• If this happens, EIGRP will send out QUERIES on all interfaces except from the interface of the successor route asking for an alternate path for that route
• When a route goes into ACTIVE state the active timer will be started (180 seconds by default)
• 90 seconds after a “normal” QUERY packet was sent, a SIA-QUERY packet will be sent
• All neighbors must then respond with a SIA-REPLY packet
• Any router that fails before the active timer expires will be considered SIA
• When the ACTIVE timer expires, the router will drop the neighbor relationship with the router who didn’t send a SIA-Reply and delete all routes from the neighbor
• If a SIA-Reply is received, the neighbor relationship will not be terminated
• The SIA-Reply tells the originating router that it is currently working on the convergence process

1.3.c v Graceful shutdown

General information on “EIGRP Graceful shutdown”:

• Enabled by default, can’t be disabled
• The router on which the EIGRP process is shutdown will broadcast a GOODBYE message (= HELLO packet with all K-values set to 255)
• This allows neighbors to quickly converge because they won’t wait until the holdtime to the particular router expires
• Important: This only applies when shutting down the EIGRP process and not when shutting down an interface connected to an EIGRP neighbor!