Link state protocol question

hankooknarahankooknara Inactive Imported Users Posts: 24 ■□□□□□□□□□
While taking below exam, I ran into below problem.
Here is the excerpt from cisco press icnd book
"Similary, with OSPF, when a router fails to hear Hellos from a neighbor for an interval called the dead interval, the router believes the silent router has failed. The dead interval is typically set to some multiple of the Hello interval. The the router that stopped receiving the hellos runs a Dijkstra to calculate new routes."

What's more,
"Hello packets serve the same purpose as timed,regular full routing updates serve for distance vector protocols"

so how is this answer c?? can someone explain to me?

3. What mechanism is used to address problems in link-state routing protocols?

a. sending periodic updates
b. triggered updates
c. exchanging route summaries at border areas
d. defining infinity as some maximum number
e. split horizon

excerpt from CCNA exam cram 2
"Link state protocols rely on network events to address topology changes. If a router detects a network event (such as one of its neighbors is no longer reachable or a new neighbor appears), it triggers an update." 's explanation
3. Answer C is correct. Link-state protocols may have to be tuned to avoid information overload.

This can mean:

* Limiting the distribution of information (exchanging summaries at border areas)
* Flooding information at vulnerable times (dampening the update frequencies)
* Not bothering all stations on the links they run over (targeting updates to multicast addresses instead of broadcasting)

Distance vector protocols need to avoid the effects of wrong routing information. This is done by:

* Recognizing when information is wrong quicker (defining infinity)
* Not assuming they know more than they really do (split horizon)
* Some distance vector protocols passing on network changes immediately (triggered updates)

Sending periodic updates is a feature of distance vector protocols.


  • david_rdavid_r Member Posts: 112
    3. What mechanism is used to address problems in link-state routing protocols?

    a. sending periodic updates (this is a "feature" of DV protocols)
    b. triggered updates (this addresses overhead and slow convergence problems in DV protocols)

    d. defining infinity as some maximum number (this prevents routing loops in DV protocols)
    e. split horizon (this prevents routing loops in DV protocols)

    c. exchanging route summaries at border areas. This prevents constant triggered updates being sent throughout the entire enterprise when you have one route flapping. Although I would disagree with the authors that it "addresses problems in LS protocols" as it also addresses the same problem in DV protocols that use triggered updates.

    We had a site that would roll onto ISDN at random times because of a bad FR circuit. When this happened, the flood of routing updates would take down a big WAN tunnel. Summarizing it's route like the rest of our ISDN backup sites remedied the problem.
  • computerguy9355computerguy9355 Inactive Imported Users Posts: 81 ■■□□□□□□□□
    Distance-Vector Characteristics :

    1. They waste too much banwidth by sending out complete routing table updates every 30 seconds (RIP) and 90 seconds (IGRP).
    2. They do not build a toplogy "tree" like link-state protocols do.
    3. RIP sends out maximum of 25 routes in each routing update. So that means if you got 90 routes, it will need to send the updates 4 times to its neighbors.

    Link-State Characteristics:

    1. They elect DR/BDR to conduct the flooding process for each segment.
    2. OSPF use the term LSA for advertisements. There are 7 total LSAs. Type 1- 7.
    3. OSPF use the term Area boarder router, internal router, backbone router, autonomous system boarder router.
    4. LSA type 3 is originated by ABRs. They summarize routes in a particular area and advertises them throughout the backbone to the outer ABRs. Thereby reducing the overhead and conserves the resources.

    Here is the complete list of terms i described above

    OSPF router types:

    * Internal router- All the interfaces on a router belong to a single area.
    * Backbone router- Needs to have at least 1 interface in area 0.
    * Area boarder routers- It has to have at least 1 interface in area 0 and another in a non-backbone area.
    * Autonomous system boarder router- It takes routes learned from the different routing protocols and place it into OSPF network.

    * OSPF LSA types:

    * OSPF LSA Type 1: Generated by each router for areas it belongs to. These are flooded to a single area only. LSA type 1 contains the “router link status” for this particular router. LSA type 1 is donated by O in the routing table.
    * OSPF LSA Type 2: Type 2 LSAs are sent out by the DR only. These are flooded to a single area only. LSA type 2 is denoted by O in the routing table.

    - LSA type 1 and type 2 are confined to a single area only. That’s one of the benefits of using the OSPF – conserving the router’s resources.

    * OSPF LSA Type 3: Originated by ABRs. They describe inter-area routes and summarize the networks from one area to another. These entries are flooded throughout the backbone area to the outer ABRs. LSA type 3 is denoted as O in the routing table.
    * OSPF LSA Type 4: Type 4 LSAs are generated only by the ABR. They describe the reachability to the ASBR. These entries are not flooded through totally stubby areas. LSA type 4 is denoted as IA in the routing table.
    * OSPF LSA type 5: Originated by the ASBRs. Type 5 LSAs describe links that are external to the OSPF domain. These links are flooded out to all the areas except stub and total stub areas. LSA type 5 is denoted by either an E1 or E2 in the routing table.
     OSPF LSA type 6: Not implemented by Cisco. Multicast OSPF (MOSPF) enhances OSPF by letting routers use their link-state databases to build multicast distribution trees for the forwarding of multicast traffic.
    * OSPF LSA type 7: Type 7 LSAs are only generated by the ASBR into not-so-stubby areas. Type 7 messages can be flooded throughout NSSAs and translated into LSA type 5 messages by ABRs. Type 7 LSAs are denoted by either an N1 or an N2 in the routing table.

    Don't be overwhelmed with this info. It will help you if you want to pursue your CCNP and beyond.
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