EIGRP adjacency using a secondary IP

Consider the following statement from Cisco.com : “Routers do not form EIGRP neighbors over secondary networks.

A Routing-BitsHandbook candidate queried this last week, claiming the statement is misleading and that EIGRP will indeed form an adjacency using a secondary IP address under specific conditions.

Consider the following configuration. R1 connects to R2 using a back-to-back serial connection. Both S1/1 interfaces have a primary and a secondary IP address defined. The EIGRP processes only matches the secondary IP addresses.

interface Serial1/1
ip address secondary
ip address
router eigrp 1
no auto-summary

interface Serial1/1
ip address secondary
ip address
router eigrp 1
no auto-summary

So what do you think will happen in this scenario? Will R1 and R2 become adjacent? Cisco explicitly mentions that a secondary IP address is not used in the EIGRP hello packets, therefore EIGRP neighbors will not become adjacent using secondary IP addresses.
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R&S Quick Notes – IGP’s


  • Know your filters: Offset-list, Distribute-lists, distance command.
  • With filters read carefully: “between 25 & 45” or “from 25 to 45”.
  • Know your prefix-lists or alternatively using ACL’s instead.
  • “passive interface” command, ONLY stops the sending of updates out the interface. Interface will still receive and process those updates. Passive interfaces will still be advertised in other updates.


  • Advertising a default route out one interface: “ip summary-address eigrp [AD]”
  • To see if a neighbor is configured as STUB, “show ip eigrp neighbors [detail]” as look for ‘CONNECTED SUMMARY’
  • On frame-relay multipoint interfaces, don’t forget to disable split-horizon.
  • External EIGRP routes AD (admin distance = 170) can NOT be changed on per prefix basis.
  • Metric weight values:
    1 0 1 0 0 = Default
    0 0 1 0 0 = Only DLY
    1 0 0 0 0 = Only BW
    3 0 1 0 0 = BW has 3 times more weight reference than DLY
  • Metric formula:

Metric = ((107 / BW) + (DLY/10) ) * 256


  • The Neighbor IP used with OSPF distance command is the Neighbors Router-ID.
  • “area range” summarize type 3 LSA’.
  • “summary-address” summarize type 5 & 7 LSA’s.
  • Auto-cost reference BW (Default = 100mb), formula = Ref-BW/Int-Bw.
  • Switches do no support the interface command “ip ospf {pid} area {area-id}” .
  • OSPF path selection: O > O*IA > O*E1 > O*E2.
  • Using E1 metric type : Packets will be routed out the closest exit point of the network.
  • Using E2 metric type : If you want packets to exit your network at the closest point to their external destination.
  • Don’t forget with hub and spoke topology, “ip ospf priority 0”.
  • PITFALL, when forbid to use RID, Loopbacks created later on might change the DR on you network after a reload.
  • PITFALL, when forbid to use RID, Later requested to configure the same loopback on two routers, could break your adjacencies, as two router cant peer with the same RID.
  • “no capability transit” – Mimics OSPFv1 behaviour for all data traffic to pass through Area-0.
  • “max-metric” – Configures OSPF stub configurations
  • “max-lsa” – Limit amount of non-local LSA’s
  • “timers throttle lsa all” – Slow down update rate.
  • “timers pacing lsa-group” – Group more LSA’s together in updates.
  • “no ip ospf flood-reduction” – Disables every 30-min LSA DB refresh.
  • “ip ospf database filter all out” – Breaks RFC, Stop sending LSA’s, but still receive LSA’s

Eigrp Metric Manipulation

One of the great unique features EIGRP brings to the table is the ability to load-balance traffic across unequal links.


To properly configure R1 to load balance traffic on a 5:1 scale to network we need to understand how the EIGRP composite metric.
The composite metric consists of
– BW : the minimum bandwidth of any outgoing interface along a specific path
– Delay : the cumulative delay of all the outgoing interface along a specific path
– Load : effective load of a route on the interface
– Reliability : likelihood of successful packet transmission
– Smallest MTU along a path.

There is a big long formula that takes each of the above metrics into account using specific K-values. But I’m not going to cover that here. The only thing of importance is that the K-values could be changed to en/disable some of the above metrics
By default, only BW & Delay is taken into account when calculating the metric according the the following shorten formula:
Metric = 256 * ((10^7/BW) + (DLY/10))

Let look at a example on R1:


Continue reading “Eigrp Metric Manipulation”