OSPF Convergence

Resiliency and redundancy to circuit failures are provided by the convergence capabilities of OSPF at layer 3.

There are two components to OSPF routing convergence: detection of topology changes and recalculation of routes.

Detection of topology changes is supported in two ways by OSPF. The first, and quickest, is a failure or change of status  on  the physical interface, such as Loss of Carrier. The second is a timeout of the OSPF hello timer. An OSPF neighbor is deemed to have failed if the time to wait for a hello packet exceeds the dead timer, which defaults to four times the value of the hello timer.

The default hello timer is set to 10 seconds for broadcast and 30 sec for non-broadcast with a dead timer 4x the hello timer.

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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

IP OSPF mtu-ignore alternative

I came across the a command I think would make a great CCIE lab question.

Assume you busy with the lab, and previously a task in the switching section required you to do a dot1q tunnel where you had to change the SYSTEM-MTU on SW1 to 1504.  No beegy.
But you now at the OSPF section, where you have to setup ospf between R1 and SW1, BUT with the following restriction:
(you are not allowed to use the mtu-ignore command)

The usual fix on R1’s interface is prohibited
#interface Fa0/0
#ip ospf mtu-ignore

Hmmm, now what? R1 wont form an adjacency with SW1, due to a MTU mismatch. We obviously cant change the SYSTEM-MTU on SW1, cause that would break a previous question.

Typical behaviour when you have a OSPF MTU mismatch, is a neighbor finite state getting to EXSTART, retrying and eventually giving up.
We can see this on R1 if we do a “debug ip ospf adj”


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OSPF Default Route Alternatives


Assume R3 is a breakout for your network, and lets assume for a second you want to advertise a default route into the OSPF domain, but on the condition that you have reachability to the Gateway (GW)

The usual and a really easy way to do this is by originating a default-route based on a route-map with a condition that a specific route is in the routing table.

interface Gi0/0
ip address
ip prefix-list GW seq 5 permit
route-map DEF-GW permit 10
match ip address prefix-list GW
router ospf 1
default-information originate route-map DEF-GW

This will work, but there is a design flaw to this particular setup. This setup is based on the prefix being in the routing table, but since Gi0/0 is a connected interface, that prefix will always be in the routing table, whether or not the GW router is up or down. Remember this is a not a serial link, where the link state could determine point-to-point reachability. The only time that prefix will not be in the routing table and as a result, not originate the default-route, is when the physical local connection to the switch dies. Inevitably this will create a black-hole, which is no good.

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OSPF Areas

One topic that has been discussed many times over, yet when it comes to configuring and recognizing the area types and there specific requirement,  I still find guys not really knowing which is which.

ospf-areas2R5 is attached to Area 0,1 and 2 making R5 by definition a ABR. We will configure Area 1 as a NSSA (Not-So-Stubby) and Area 2 as a STUB, and then explore the extensions of these two areas called, Totally-NSSA and Totally-Stubby.


Firstly by configuring Area 2 as a Stub-Area:

  • Type 4 (ASBR Summary) LSA’s and type 5 (AS External) LSA’s will no longer be flooded into the area.
  • All type 3 (Network Summary) LSA’s will be flooded to advertise inter-area routes
  • Limitations: NO redistribution is allowed
  • A default route will automatically be generated by the ABR (R5) into area 2.

Below config is required on all routers with Area 2 as all Area 2 routers must agree on setting the OSPF STUB flag.
router ospf 1
area 2 stub

In the routing table you will see the Inter-Area default-route, marked as O*IA


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Incremental SPF is more efficient than the full SPF algorithm, thereby allowing OSPF to converge faster on a new routing topology in reaction to a network event.

So how does this work?
OSPF uses Dijkstra’s SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type-1 or Type-2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged.

Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster OSPF convergence and saves CPU resources. Note that if the change to a Type-1 or Type-2 LSA occurs in the calculating router itself, then the full SPT is performed.

Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.

How do you enable it?
R1(config)#router ospf 1

How to verify its is enabled?
R1#sh ip ospf
Routing Process "ospf 1" with ID
Start time: 2d09h, Time elapsed: 00:07:52.356
Supports only single TOS(TOS0) routes
Supports opaque LSA
Supports Link-local Signaling (LLS)
Supports area transit capability
Router is not originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Incremental-SPF enabled <<----
--output omitted----