Posts Tagged ‘ospf’


Edit: Thanks for playing! You can find the official answer and explanation here.

I had an interesting question come across my desk today which involved a very common area of confusion in OSPF routing logic, and now I’m posing this question to you as a challenge!

The first person to answer correctly will get free attendance to our upcoming CCIE Routing & Switching Lab Cram Session, which runs the week of June 1st 2015, as well as a free copy of the class in download format after it is complete.  The question is as follows:

Given the below topology, where R4 mutually redistributes between EIGRP and OSPF, which path(s) will R1 choose to reach the network, and why?

Bonus Questions:

  • What will R2′s path selection to be, and why?
  • What will R3′s path selection to be, and why?
  • Assume R3′s link to R1 is lost.  Does this affect R1′s path selection to If so, how?

Tomorrow I’ll be post topology and config files for CSR1000v, VIRL, GNS3, etc. so you can try this out yourself, but first answer the question without seeing the result and see if your expected result matches the actual result!


Good luck everyone!

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INE’s long awaited CCIE Service Provider Advanced Technologies Class is now available! But first, congratulations to Tedhi Achdiana who just passed the CCIE Service Provider Lab Exam! Here’s what Tedhi had to say about his preparation:

Finally i passed my CCIE Service Provider Lab exam in Hongkong on Oct, 17 2011. I used your CCIE Service Provider Printed Materials Bundle. This product makes me deep understand how the Service Provider technology works, so it doesn`t matter when Cisco has changed the SP Blueprint. You just need to practise with IOS XR and finding similiar command in IOS platform.

Thanks to INE and keep good working !

Tedhi Achdiana
CCIE#30949 – Service Provider

The CCIE Service Provider Advanced Technologies Class covers the newest CCIE SP Version 3.0 Blueprint, including the addition of IOS XR hardware. Class topics include Catalyst ME3400 switching, IS-IS, OSPF, BGP, MPLS Layer 3 VPNs (L3VPN), Inter-AS MPLS L3VPNs, IPv6 over MPLS with 6PE and 6VPE, AToM and VPLS based MPLS Layer 2 VPNs (L2VPN), MPLS Traffic Engineering, Service Provider Multicast, and Service Provider QoS. Understanding the topics covered in this class will ensure that students are ready to tackle the next step in their CCIE preparation, applying the technologies themselves with INE’s CCIE Service Provider Lab Workbook, and then finally taking and passing the CCIE Service Provider Lab Exam!

Streaming access is available for All Access Pass subscribers for as low as $65/month! Download access can be purchased here for $299. AAP members can additionally upgrade to the download version for $149.

Sample videos from class can be found after the break: Continue Reading

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One of our most anticipated products of the year – INE’s CCIE Service Provider v3.0 Advanced Technologies Class – is now complete!  The videos from class are in the final stages of post production and will be available for streaming and download access later this week.  Download access can be purchased here for $299.  Streaming access is available for All Access Pass subscribers for as low as $65/month!  AAP members can additionally upgrade to the download version for $149.

At roughly 40 hours, the CCIE SPv3 ATC covers the newly released CCIE Service Provider version 3 blueprint, which includes the addition of IOS XR hardware. This class includes both technology lectures and hands on configuration, verification, and troubleshooting on both regular IOS and IOS XR. Class topics include Catalyst ME3400 switching, IS-IS, OSPF, BGP, MPLS Layer 3 VPNs (L3VPN), Inter-AS MPLS L3VPNs, IPv6 over MPLS with 6PE and 6VPE, AToM and VPLS based MPLS Layer 2 VPNs (L2VPN), MPLS Traffic Engineering, Service Provider Multicast, and Service Provider QoS.

Below you can see a sample video from the class, which covers IS-IS Route Leaking, and its implementation on IOS XR with the Routing Policy Language (RPL)

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Hi Brian,

What is the major difference in using an E1 route over an E2 route in OSPF?

From what I’ve observed, if you redistribute a route into OSPF either E1 or E2, the upstream router will still use the shortest path to get to the ASBR regardless of what is shown in the routing table.

The more I read about this, the more confused I get. Am I missing something?


Hi Matt,

This is actually a very common area of confusion and misunderstanding in OSPF. Part of the problem is that the vast majority of CCNA and CCNP texts teach the theory that for OSPF path selection of E1 vs E2 routes, E1 routes use the redistributed cost plus the cost to the ASBR, while with E2 routes only use the redistributed cost. When I just checked the most recent CCNP ROUTE text from Cisco Press, it specifically says that “[w]hen flooded, OSPF has little work to do to calculate the metric for an E2 route, because by definition, the E2 route’s metric is simply the metric listed in the Type 5 LSA. In other words, the OSPF routers do not add any internal OSPF cost to the metric for an E2 route.” While technically true, this statement is an oversimplification. For CCNP level, this might be fine, but for CCIE level it is not.

The key point that I’ll demonstrate in this post is that while it is true that “OSPF routers do not add any internal OSPF cost to the metric for an E2 route”, both the intra-area and inter-area cost is still considered in the OSPF path selection state machine for these routes.

Continue Reading

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OSPF and MTU Mismatch

Dear Brian,

What is the difference between using the “system mtu routing 1500” and the “ip ospf mtu-ignore” commands when running OSPF between a router and a switch?



Hi Paul,

Within the scope of the CCIE Lab Exam, it may be acceptable to issue either of these commands to solve a specific lab task. However, it is key to note that there is a difference between ignoring the MTU for the purpose of OSPF adjacency and matching the MTU within a real production network.

Continue Reading

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One of the most important technical protocols on the planet is Open Shortest Path First (OSPF). This highly tunable and very scalable Interior Gateway Protocol (IGP) was designed as the replacement technology for the very problematic Routing Information Protocol (RIP). As such, it has become the IGP chosen by many corporate enterprises.

OSPF’s design, operation, implementation and maintenance can be extremely complex. The 3-Day INE bootcamp dedicated to this protocol will be the most in-depth coverage in the history of INE videos.

This course will be developed by Brian McGahan, and Petr Lapukhov. It will be delivered online in a Self-Paced format. The course will be available for purchase soon for $295.

Here is a preliminary outline:

Day 1 OSPF Operations

●      Dijkstra Algorithm

●      Neighbors and Adjacencies

○   OSPF Packet Formats

○   OSPF Authentication

○   Link-State information Flooding

Continue Reading

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This document is presented as a series of Questions and Answers, discussing various aspects of OSPF protocol designed to prevent inter-area routing loops and related issues. The discussion covers ABR functions, Virtual-Links, OSPF Super-backbone, OSPF Sham-Links, BGP Cost Community. Reader is assumed to know these concepts already, as this publication focuses on complex interaction features arising in MPLS/BGP VPN scenarios. The discussion is culminated by analyzing a number of issues arising in complex multi-area multi-homed OSPF site deployed in MPLS VPN environment. Please download the following document to read the publication: Loop Prevention in OSPF

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I enjoyed Petr’s article regarding explicit next hop.  It reminded me of a scenario where a redistributed route, going into OSPF conditionally worked, depending on which reachable next hop was used.

Here is the topology for the scenario:

3 routers ospf fa blogpost

Here is the relevant (and working :) ) information for R1. Continue Reading

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This publication briefly covers the use of 3rd party next-hops in OSPF, RIP, EIGRP and BGP routing protocols. Common concepts are introduced and protocol-specific implementations are discussed. Basic understanding of the routing protocol function is required before reading this blog post.


Third-party next-hop concept appears only to distance vector protocol, or in the parts of the link-state protocols that exhibit distance-vector behavior. The idea is that a distance-vector update carries explicit next-hop value, which is used by receiving side, as opposed to the “implicit” next-hop calculated as the sending router’s address – the source address in the IP header carrying the routing update. Such “explicit” next-hop is called “third-party” next-hop IP address, allowing for pointing to a different next-hop, other than advertising router. Intitively, this is only possible if the advertising and receiving router are on a shared segment, but the “shared segment” concept could be generalized and abstracted. Every popular distance-vector protocols support third party next-hop – RIPv2, EIGRP, OSPF and BGP all carry explicit next-hop value. Look at the figure below – it illustrates the situation where two different distance-vector protocols are running on the shared segment, but none of them runs on all routers attached to the segment. The protocols “overlap” at a “pivotal” router and redistribution is used to provide inter-protocol route exchange.

Continue Reading

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This goal of this post is brief discussion of main factors controlling fast convergence in OSPF-based networks. Network convergence is a term that is sometimes used under various interpretations. Before we discuss the optimization procedures for OSPF, we define network convergence as the process of synchronizing network forwarding tables after a topology change. Network is said to be converged when none of forwarding tables are changing for “some reasonable” amount of time. This “some” amount of time could be defined as some interval, based on the expected maximum time to stabilize after a single topology change. Network convergence based on native IGP mechanisms is also known as network restoration, since it heals the lost connections. Network mechanisms for traffic protection such as ECMP, MPLS FRR or IP FRR offering different approach to failure handling are outside the scope of this article. We are further taking multicast routing fast recovery out of the scope as well, even though this process is tied to IGP re-convergence.

It is interesting to notice that IGP-based “restoration” techniques have one (more or less) important problem. During the time of re-convergence, temporary micro-loops may exist in the topology due to inconsistency of FIB (forwarding) tables of different routers. This behavior is fundamental to link-state algorithms, as routers closer to failure tend to update their forwarding database before the other routers. The only popular routing protocol that lacks this property is EIGRP, which is loop-free at any moment during re-convergence, thanks to the explicit termination of the diffusing computations. For the link state-protocols, there are some enhancements to the FIB update procedures that allow avoiding such micro-loops with link-state routing, described in the document [ORDERED-FIB].

Even though we are mainly concerned with OSPF, ISIS will be mentioned in the discussion as well. It should be noted that compared to IS-IS, OSPF provides less “knobs” for convergence optimization. The main reason is probably the fact that ISIS is being developed and supported by a separate team of developers, more geared towards the ISPs where fast convergence is a critical competitive factor. The common optimization principles, however, are the same for both protocols, and during the conversation will point out at the features that OSPF lacks while IS-IS has for tuning. Finally, we start our discussion with a formula, which is further explained in the text:

Convergence = Failure_Detection_Time + Event_Propagation_Time + SPF_Run_Time + RIB_FIB_Update_Time

The formula reflects the fact that convergence time for a link-state protocol is sum of the following components:

  • Time to detect the network failure, e.g. interface down condition.
  • Time to propagate the event, i.e. flood the LSA across the topology.
  • Time to perform SPF calculations on all routers upon reception of the new information.
  • Time to update the forwarding tables for all routers in the area.

Continue Reading

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