Hour 225: CCIE Mnemonics

Today, I started doing IPexperts labs and ordered the Paper version of their workbooks. My original plan was to start doing the CCIE lab preparation with INE but since CCIE v5 is coming soon, my new plan since is to start with IPexperts and then do the INE labs. Once the new technologies workbooks for v5 come out, I will most definitely switch completely to INE.

Now I would like to to talk a little bit about how I remembered most of the content for the CCIE Written; through mnemonics. Let me give you a description of what a mnemonics are and a list of the ones you will need for the CCIE Written. Continue reading

Hour 200: Interview questions and how I evaluate someone’s knowledge

I recently got asked to do the first round of technical interviews for candidates applying for a core network engineer position in our firm. I would like to share some of the questions I ask the candidates and how I judge their level of knowledge by their response.

The candidate will first come in the interviewer room and get presented to me (the technical engineer), to the human resources recruiter and to the project architect. The candidate will be asked to briefly present his job history to the room and walk us step by step through his resume. Once this is completed, the project architect will present the job requirements and the tasks involved. It’s also very important for the project architect to sell the job in some way to keep the candidate motivated because after he is done answering his questions, the candidate will be going through a hard technical selection process. Let’s get to the technical interview part. Continue reading

Hour 45: OSPF Review Part 2

See Part 1 Here.

Consult the symbols legend at the end of the post for information on symbols.

OSPF LSA’s:

LSA1 Router:

  • [show ip ospf database router] Describes router interfaces in an area. Lists neighboring routers on each interface. LSIA = RID
  • Routing Bit Set on this LSA means that the route is in the routing table
  • V – (Virtual-link bit) set to one when the router is an endpoint of one or more fully adjacent virtual link
  • E – (External bit) set to one when the router is an ASBR
  • B – (Border bit) set to one when the router is an ABR
  • OSPF advertises host routes /32 as stub networks. Loopback interfaces are also considered stub networks and are advertised as host routes regardless of net mask, issue [(IF)ip ospf network point-to-point] to reflect correct net mask.

LSA2 Network:

  • [show ip ospf database network] Describes transit networks for which DR has been elected
  • Originated only by DR
  • LSID = DR’s interface address

LSA3 Network summary:

  • [show ip ospf database summary]  Lists LSA Type 3 in the database
  • ABR’s do not forward LSA1 and LSA2
  • ABR sends LSA3 with LSA1 and LSA2 subnets (simple vector –network and ABR’s cost to reach that net)
  • LSID is the network number
  • If an ABR knows multiple routes to the destination within its own area, it originates a single LSA3 into the backbone with the lowest cost of the multiple routes
  • ABR’s in the same area (non-backbone) ignore each-others LSA3 + cost to ABR
  • Routers in other areas perform 2-step cost calculation: cost in LSA3 + cost to ABR
  • If one network changes inside one area, all routers in this area perform full SPF calculation, but outside that area, only cost is updated by ABR
  • If router wants to remove the network it sets age to Maxage and re-floods LSA Continue reading

Hour 40: OSPF an advanced link-state protocol

EIGRP is often called an advanced distance vector protocol because it has distance vector routing protocol AND link-state protocol properties. But wait a second… OSPF also has a mix of link-state and distance-vector protocol properties… So why is OSPF not called an “advanced link-state routing protocol”?

Let’s determine the properties of EIGRP that makes it an advanced distance protocol and then compare it to OSPF.

EIGRP distance-vector properties:

  • An EIGRP router only advertises its best route to its neighbor, not every route that it is aware of.
  • An EIGRP router does not have a complete map of the topology , it is only aware of what its neighbors have told it ( routing by rumor )

Now to prove that OSPF also has distance-vector properties, let’s look at OSPF’s Network Summary LSA (Type 3) characteristics:

In OSPF, when an ABR originates a Type 3 LSA and knows multiple paths to a destination, it will only advertise the lowest cost route into the backbone. When a router receives a Type 3 LSA from an ABR, it does not run the SPF algorithm. Rather, it simply adds the cost of the route to the ABR and the cost included in the LSA. Depending on another router instead of determining the full route to the destination is a distance-vector protocol behavior.

Now let’s compare these characteristics to the EIGRP distance-vector properties:

  • Type 3 LSA’s in an OSPF router only advertise its best route to the backbone, not every route that it is aware of.
  • Type 3 LSA’s in an OSPF router do not have a complete map of the topology, since it does not run the SPF algorithm. It is only aware of what the ABR originating the type 3 LSA told it.

These distance-vector properties are similar and we can conclude that OSPF also has distance-vector properties. By this logic, if we can call EIGRP an advanced distance vector protocol because it has link-state properties, can we then not also call OSPF an “advanced link-state protocol” since OSPF has distance vector properties?

Hour 36: OSPF Review Part 1

OSPF

Consult the symbols legend at the end of the post for information on symbols.

Metric:

  • AD of 110
  • Metric is defined as Cost,
  • Cost =  reference bandwidth/ interface bandwidth
  •  [(RTR)auto-cost reference-bandwidth <bw in Mbps>] Default autocost reference: 100000000/BW bps (100 Mbps)
  • [(IF)ip ospf cost <cost>] overrides the cost
  • [(RTR)neighbor <ip> cost <cost>] only for point to multipoint and point-to-multipoint non-broadcast links (spokes with different CIR’s)

Neighbors:

  • Hello is always sourced from interface primary subnet with destination MAC 0100.5E00.0005 and multicast 224.0.0.5. For DR routers multicast address is 224.0.0.6 and destination MAC is 0100.5E00.0006. Protocol is 89.
  • Adjacency
  1. To form adjacency these parameters must match: Authentication, Area, DR/BDR capability, Timers, Stub Area Flag, MTU and subnet (unless p2p that does not check subnet).
  2. Adjacency will not form with secondary addresses but will advertise the prefix
  3. Unnumbered interfaces with different subnets are possible because it does not check the source IP. Also, primary interface must be covered by network statement [(RTR)network <net><wildcard> area <id>] not an interface statement[(IF) ip ospf <process> area <area>].
  • States:
  1. Down: No Hello’s have been received from neighbor. Hello’s sent every Hellointerval.
  2. Attempt – applies only to manually configured neighbors on NBMA networks. A router sends unicast hello packets to a neighbor in at PollInterval instead of HelloInterval (see timers)
  3. Init – Hello packet has been seen from neighbor in the last RouterDeadInterval
  4. 2-Way – Router has seen its own Router ID (RID) in the Neighbor field of the neighbor’s hello packets
  5. ExStart – Routers establish a master/slave relationship and determine the initial Database Descripton (DD) sequence. Highest RID becomes the master. Lower MTU is accepted. Use [(RTR)ip ospf mtu-ignore] if router is stuck on ExStart because of mismatching MTU, only has to be configured on one side.
  6. Exchange – The router sends DD packets
  7. Loading – Router sends LSR and LSU packets Continue reading

Hour 15: Distance Vector vs Link State analogy

One of the questions people new to networking often ask me is the difference between Distance Vector routing and Link State routing. There are many differences between them and you can research the details if you’d like to but I like to use two analogies to explain them. Here they are:

Distance Vector Routing: You are wandering in the Banff mountains of northern Alberta in Canada, a wonderful place to wander if you aren’t lost. But you are lost. You come upon a fork in the trail and you see a sign pointing west, reading “Edmonton 140km”. You have no choice but to trust the sign. You have no clue how the terrain is for the next 140km and you don’t know whether there is a better route or even if the sign is correct. In similar way, distance vector protocols provide road signs to networks. They provide the direction and the distance, but no details about what lies along the route.

Link-state Routing: You are in the biggest mall in Canada; the West Edmonton mall. You need to get to a computer store. You find the map of the mall and there’s a “you are here” dot on it, indicating where you are. From there you plan your way to the nearest computer store.

Just like every map in the mall is the same, the link-state database is the same in all routers within an area of a link-state routing protocol. The one difference between all the maps in the shopping mall is the “you are here” dot and so, the best path to a specific store will be different from each location in the mall. Link-state routing functions the same way, it calculates the best way to every network within the area, from their own perspective, using its own map; the Link-State Database.

I hope you liked these analogies as they’ve been really helpful to me.

On a side note: today I received my Cisco 2511 Access Server. I need to make a couple of roll-over cables to connect them to my home lab. I’m still waiting on 2x NM-1FE-2W WIC modules to be delivered. I need these for my 3600’s series Frame Relay switching router because the NM-1E-2W version does not support WIC-2T interface cards. So be careful if you plan on purchasing a 3600’s series router as only the FE modules support the WIC-2T interface cards.