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1.9.a EIGRP Address Families, Classic and Named Mode

Blueprint 1.9.a: Troubleshoot EIGRP (classic and named mode): address families (IPv4, IPv6)

  • Classic mode for IPv4 (router eigrp AS) and IPv6 (ipv6 router eigrp AS)
  • Named mode (router eigrp NAME) with IPv4 and IPv6 address families
  • Where each command lives in named mode: address family, af-interface, topology base
  • Router ID selection, and why EIGRP for IPv6 can fail to start
  • Converting classic to named mode with eigrp upgrade-cli

EIGRP has two configuration styles that run the same protocol. Classic mode is the original: one process per address family, numbered by AS, with authentication, timers and summaries scattered across interface configuration. Named mode puts everything for every address family under one router eigrp NAME block. Both send the same packets (IP protocol 88, multicast 224.0.0.10 or FF02::A), and a classic router can form an adjacency with a named-mode router as long as the neighbor checks pass.

The exam expects you to read both styles fluently and to know where a command has to live for it to take effect. A large share of EIGRP tickets are "the command is in the config, but in the wrong place".

Classic mode vs named mode

Classic mode Named mode
Process router eigrp 100 (IPv4), ipv6 router eigrp 100 (IPv6) router eigrp CORP then one address-family per AF
AS number In the process command In the address-family ... autonomous-system 100 command
Interfaces enabled (IPv4) network statements network statements under the IPv4 AF
Interfaces enabled (IPv6) ipv6 eigrp 100 on each interface Every IPv6-enabled interface automatically
Per-interface settings (hello, hold, auth, split horizon, summary) Interface configuration mode, e.g. ip hello-interval eigrp 100 10 af-interface Gi0/1 (or af-interface default) under the AF
Routing table settings (variance, maximum-paths, redistribution, distance, distribute lists) Directly under router eigrp topology base under the AF
Metric 32-bit classic metric only 64-bit wide metric, scaled into the RIB with metric rib-scale (default 128)
Authentication MD5 with a key chain MD5 with a key chain, or HMAC-SHA-256 with a password
Show commands show ip eigrp ..., show ipv6 eigrp ... Same commands work, plus show eigrp address-family ipv4 ... 
graph TB
  P["router eigrp CORP"] --> A4["address-family ipv4 unicast autonomous-system 100<br/>network, eigrp router-id, eigrp stub, metric weights"]
  P --> A6["address-family ipv6 unicast autonomous-system 100<br/>eigrp router-id, eigrp stub"]
  A4 --> I4["af-interface Gi0/1 or default<br/>hello-interval, hold-time, authentication,<br/>passive-interface, summary-address, split-horizon"]
  A4 --> T4["topology base<br/>variance, maximum-paths, redistribute,<br/>distance, distribute-list, offset-list"]
  A6 --> I6["af-interface<br/>shutdown, passive-interface, authentication"]
  A6 --> T6["topology base<br/>variance, maximum-paths, redistribute"]

What stays the same

  • Administrative distance: 90 internal, 170 external, 5 for a summary route to Null0 on the summarizing router.
  • Hello interval 5 seconds and hold time 15 seconds on LAN and point-to-point links. On multipoint interfaces at T1 speed or slower (old Frame Relay style links) the defaults are 60 and 180.
  • Neighbor checks: same AS, same K values, a common subnet for IPv4, matching authentication.
  • DUAL, successors and feasible successors behave identically.
  • Automatic summarization is off by default on current IOS and IOS XE, in both styles.

Exam trap

In named mode, variance and maximum-paths go under topology base, not directly under the address family. hello-interval and authentication go under af-interface, not under the physical interface. A ticket that says "variance 2 is configured but only one path is installed" can be a placement problem. Check the running config section by section.

IPv4 configuration

router eigrp CORP
 address-family ipv4 unicast autonomous-system 100
  network 10.0.0.0                       ! classful form, all 10.x interfaces
  network 192.168.5.0 0.0.0.255          ! wildcard form
  eigrp router-id 10.255.255.1
  af-interface Loopback0
   passive-interface
  exit-af-interface
  topology base
   variance 2
  exit-af-topology
 exit-address-family

router eigrp 100
 network 10.0.0.0
 network 192.168.5.0 0.0.0.255
 eigrp router-id 10.255.255.1
 passive-interface Loopback0
 variance 2

A network statement does two things: it enables EIGRP on every interface whose primary address falls inside the range, and it causes that interface's connected subnet to be advertised. It does not decide which prefix length is advertised. The connected route's real mask is always used.

IPv6 configuration

ipv6 unicast-routing
!
router eigrp CORP
 address-family ipv6 unicast autonomous-system 100
  eigrp router-id 10.255.255.1
  af-interface GigabitEthernet0/0
   shutdown                              ! stop EIGRPv6 on the management port
  exit-af-interface
 exit-address-family

ipv6 unicast-routing
!
ipv6 router eigrp 100
 eigrp router-id 10.255.255.1
 passive-interface Loopback0
!
interface GigabitEthernet0/1
 ipv6 eigrp 100
interface Loopback0
 ipv6 eigrp 100

Facts that create IPv6 tickets:

  1. The router ID is still a 32-bit dotted number. EIGRP picks the highest loopback IPv4 address, then the highest IPv4 address on any up interface. An IPv6-only router has nothing to choose from, so the process cannot start until you set eigrp router-id.
  2. ipv6 unicast-routing is required. Without it the router is an IPv6 host, and the EIGRPv6 process does not route.
  3. Neighbors use link-local addresses. Hellos are sourced from the link-local address to FF02::A, and the next hop in show ipv6 route is a link-local address with an exit interface. Two routers can be EIGRPv6 neighbors even if their global prefixes on the link do not match, because there is no common-subnet check for IPv6.
  4. Named mode enables every IPv6 interface automatically, including loopbacks. To stop EIGRPv6 on an interface, use shutdown under its af-interface. In classic mode the reverse applies: nothing runs until you add ipv6 eigrp 100 to the interface.
  5. On older IOS releases the classic ipv6 router eigrp process was created in a shut down state and needed no shutdown. Current IOS XE does not need it, but if you see shutdown under the process in an exhibit, that is the fault.

Exam trap

"IPv6 adjacency is down but IPv4 is fine on the same link" usually points to something that only the IPv6 address family has: a missing router ID on an IPv6-only router, shutdown under the IPv6 af-interface, a classic interface missing ipv6 eigrp 100, or an IPv6 traffic filter that drops FF02::A. IPv4 and IPv6 address families form separate adjacencies and keep separate neighbor tables.

Address families for VRFs

Named mode also carries VRF instances as address families:

router eigrp CORP
 address-family ipv4 unicast vrf TENANT-A autonomous-system 200
  network 172.16.0.0
 exit-address-family

Classic mode uses address-family ipv4 vrf TENANT-A under router eigrp 100 with autonomous-system 200 inside it. In both styles, forgetting the autonomous-system keyword under a VRF address family leaves the instance with no AS, and no neighbors form.

Converting classic to named mode

R1(config)# router eigrp 100
R1(config-router)# eigrp upgrade-cli CORP

The router rewrites the classic process, including interface-level EIGRP commands such as authentication and hello timers, into a named-mode block. The adjacencies stay up during the conversion. After it runs, the interface-level classic commands disappear from the interface configuration and appear under af-interface. Plan this as a one-way change.

Reading the verification output

R1# show ip protocols | section eigrp
Routing Protocol is "eigrp 100"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP-IPv4 VR(CORP) Address-Family Protocol for AS(100)
    Metric weight K1=1, K2=0, K3=1, K4=0, K5=0 K6=0
    Metric rib-scale 128
    Metric version 64bit
    Router-ID: 10.255.255.1
    Topology : 0 (base)
      Active Timer: 3 min
      Distance: internal 90 external 170
      Maximum path: 4
      Maximum hopcount 100
      Maximum metric variance 2

  Automatic Summarization: disabled
  Routing for Networks:
    10.0.0.0
  Passive Interface(s):
    Loopback0
  Routing Information Sources:
    Gateway         Distance      Last Update
    10.1.2.2              90      00:21:05
    10.1.3.3              90      00:21:05
    10.1.5.5              90      00:21:05
  Distance: internal 90 external 170

From one command you can read the AS, whether it is named mode (VR(CORP) and Metric version 64bit), the K values, the router ID, variance, maximum paths, the networks and passive interfaces. It is the first command to run on any EIGRP ticket.

R1# show ipv6 eigrp neighbors
EIGRP-IPv6 VR(CORP) Address-Family Neighbors for AS(100)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
2   Link-local address:     Gi0/3                    12 00:20:51    3   100  0  14
    FE80::5
1   Link-local address:     Gi0/2                    14 00:20:55    2   100  0  22
    FE80::3
0   Link-local address:     Gi0/1                    11 00:20:58    2   100  0  25
    FE80::2

The link-local addresses shown here are statically configured (ipv6 address FE80::2 link-local) so that output is readable. With auto-generated EUI-64 link-local addresses the column shows long FE80 addresses that are hard to match to a router, which is why many engineers set them by hand.

Command Answers
show ip protocols Which process, AS, K values, router ID, networks, passive interfaces, variance
show ip eigrp interfaces [detail] Which interfaces run EIGRP, peer count, hello timer, authentication mode
show ip eigrp neighbors Who is adjacent, uptime, hold time counting down, queue count
show ipv6 eigrp interfaces / neighbors The same for IPv6
show eigrp address-family ipv4 topology Named-mode form of the topology table
show running-config \| section router eigrp Where each command was actually placed

Exam trap

show ip eigrp interfaces does not list passive interfaces. If an interface is missing from that output, either no network statement covers its primary address, or it is passive. show ip protocols lists passive interfaces separately, so use both.

Lab: build the module baseline, then two tickets

This baseline is reused by every lab in the module. Build it once, save it as flash:baseline.cfg on each router, and reload it between tickets.

graph LR
  R5["R5 branch stub<br/>Lo50 10.5.50.0/24"] -- "Gi0/1 10.1.5.0/30" --- R1["R1 hub"]
  R1 -- "Gi0/1 10.1.2.0/30" --- R2["R2"]
  R1 -- "Gi0/2 10.1.3.0/30<br/>R1 delay 100" --- R3["R3"]
  R2 -- "Gi0/2 10.2.4.0/30" --- R4["R4 data centre<br/>Lo40 10.4.40.0/24"]
  R3 -- "Gi0/2 10.3.4.0/30" --- R4

Five IOSv routers in CML. R1 Gi0/3 connects to R5 Gi0/1; R4 Gi0/1 connects to R2 Gi0/2; R4 Gi0/2 connects to R3 Gi0/2. Addressing follows the course convention: the lower router ID is .1 or ::1 on each link, and loopback0 is 10.255.255.N/32 and 2001:db8:ffff::N/128. Link-local addresses are set to FE80::N so neighbor tables are easy to read.

Step 1. Interfaces. R1 shown in full. Repeat the pattern on the others.

hostname R1
ipv6 unicast-routing
!
interface Loopback0
 ip address 10.255.255.1 255.255.255.255
 ipv6 address 2001:db8:ffff::1/128
interface GigabitEthernet0/1
 description to R2
 ip address 10.1.2.1 255.255.255.252
 ipv6 address FE80::1 link-local
 ipv6 address 2001:db8:1:2::1/64
 no shutdown
interface GigabitEthernet0/2
 description to R3 (backup path)
 ip address 10.1.3.1 255.255.255.252
 ipv6 address FE80::1 link-local
 ipv6 address 2001:db8:1:3::1/64
 delay 100
 no shutdown
interface GigabitEthernet0/3
 description to R5 branch
 ip address 10.1.5.1 255.255.255.252
 ipv6 address FE80::1 link-local
 ipv6 address 2001:db8:1:5::1/64
 no shutdown

On R4 add interface Loopback40 with 10.4.40.1 255.255.255.0 and 2001:db8:40::1/64. On R5 add interface Loopback50 with 10.5.50.1 255.255.255.0 and 2001:db8:50::1/64. The loopbacks stand in for LANs; a loopback has a delay of 5000 microseconds, which you will see in every metric in this module.

Step 2. EIGRP named mode, dual stack. On R1:

key chain BRANCH-KEYS
 key 1
  key-string Br4nchKey1
!
router eigrp CORP
 address-family ipv4 unicast autonomous-system 100
  af-interface default
   authentication mode hmac-sha-256 0 C0reS3cret
  exit-af-interface
  af-interface GigabitEthernet0/3
   authentication mode md5
   authentication key-chain BRANCH-KEYS
  exit-af-interface
  topology base
   variance 2
  exit-af-topology
  network 10.0.0.0
  eigrp router-id 10.255.255.1
 exit-address-family
 !
 address-family ipv6 unicast autonomous-system 100
  eigrp router-id 10.255.255.1
 exit-address-family

On R2, R3 and R4 use the same block without the Gi0/3 section, the key chain and variance. On R5 use the MD5 key chain on Gi0/1 (no HMAC default), and add eigrp stub connected summary under both address families. Each router uses its own Loopback0 address as router ID.

Step 3. Verify the baseline.

R1# show ip route eigrp
D        10.2.4.0/30 [90/15360] via 10.1.2.2, 00:21:05, GigabitEthernet0/1
D        10.3.4.0/30 [90/20480] via 10.1.2.2, 00:21:05, GigabitEthernet0/1
D        10.4.40.0/24 [90/2575360] via 10.1.2.2, 00:21:05, GigabitEthernet0/1
                      [90/3082240] via 10.1.3.3, 00:21:05, GigabitEthernet0/2
D        10.5.50.0/24 [90/2570240] via 10.1.5.5, 00:20:51, GigabitEthernet0/3
(output trimmed)

You should see three IPv4 and three IPv6 neighbors on R1, two paths to 10.4.40.0/24 (the metrics page works out every number in that output), and the branch LAN from R5. Save with copy running-config flash:baseline.cfg on every router.

Ticket 1: "IPv6 traffic to the data centre no longer shares both WAN paths." Inject on R2:

router eigrp CORP
 address-family ipv6 unicast autonomous-system 100
  af-interface GigabitEthernet0/2
   shutdown

Diagnose from R1 and R2 only. show ipv6 eigrp neighbors on R2 shows R1 but not R4. show ipv6 eigrp interfaces on R2 does not list Gi0/2, while show ip eigrp interfaces does. IPv6 traffic to 2001:db8:40::/64 still works through R3 because R3 also connects to R4, but show ipv6 route 2001:db8:40::/64 on R1 now has one path instead of two. Fix with no shutdown under the same af-interface.

Ticket 2: "R3 lost its neighbor to R4 after an address cleanup." Inject on R3:

router eigrp CORP
 address-family ipv4 unicast autonomous-system 100
  no network 10.0.0.0
  network 10.1.3.0 0.0.0.3
  network 10.255.255.3 0.0.0.0

show ip eigrp interfaces on R3 lists Gi0/1 and Loopback0 but not Gi0/2. The neighbor to R4 is gone, and 10.3.4.0/30 is still reachable from R1 only through R2 and R4. The IPv6 adjacency between R3 and R4 is still up, which is the clue: IPv6 named mode does not use network statements. Fix by adding network 10.3.4.0 0.0.0.3 or restoring the classful statement.

Quick check

A router has only IPv6 addresses and address-family ipv6 unicast autonomous-system 100 under router eigrp CORP. show ipv6 eigrp neighbors is empty. What do you check first?

The router ID. EIGRP for IPv6 still needs a 32-bit router ID, and with no IPv4 address there is nothing to select it from. Add eigrp router-id under the IPv6 address family. Then confirm ipv6 unicast-routing is on.

Where do variance 3 and hello-interval 2 belong in named mode?

variance 3 goes under topology base inside the address family. hello-interval 2 goes under af-interface <name> (or af-interface default) inside the address family.

Two EIGRPv6 routers on the same link have global addresses in different /64 prefixes. Will they become neighbors?

Yes, if everything else matches. EIGRPv6 forms adjacencies with link-local addresses and does not run the common-subnet check that IPv4 EIGRP does. Routing still works, because next hops are link-local.

References