CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

- Point-to-point is a type of network topology which consists a direct connection between 2
routers that provides a single communication path either physically (a direct serial connection) or
logically (a point-to-point Frame Relay virtual circuit). This type of topology does not elect a
DR or BDR.
- Point-to-multipoint is a special type of NBMA network topology which consists of a series of
connections between an interface on one router and multiple interfaces on other routers, and all
the interfaces belong to a same network. This type of topology does not elect a DR or BDR.
- Routers running LS routing protocols establish and maintain (during steady state) neighbor
relationships by exchanging periodical small messages called Hello packets. Hello packets are
sent by DR and BDR to all routers with the IP multicast address 224.0.0.5 (AllSPFRouters).
The Hello interval varies upon media types – 30 seconds for NBMA and 10 seconds for others
(faster links receive more-frequent Hellos). As long as a router continues to receive Hello
packets from its neighbors, the link and the neighboring router are considered up and running.
Hello packets serve the same purpose as periodical full routing updates in DV routing protocols
which notify neighbor routers that a router is up and running and the routes through it are valid.
- When a router fails to receive Hellos from a neighbor for the dead interval, the router believes
the silent router has failed. The router will mark the silent router as ‘down’ in its topology
database and then converge by running SPF algorithm to calculate the new routes. Dead timer
interval is 4 x Hello interval, which is 120 seconds for NBMA and 40 seconds for others.
- LS routing protocols do not need loop avoidance features as in DV routing protocols, eg: split
horizon, poison reverse, and holddown. SPF algorithm prevents loops as natural due to the
processing of the topology database and maintaining the knowledge of the network topology.
- OSPF sends LSUs only when a topology change is detected (event-triggered updates). When a
link or router fails, a router that noticed the failure will immediately flood the new status to its
neighbors, and from those routers to their neighbors. Eventually all routers will receive the new
status information. This is similar to triggered updates in DV routing protocols, but this behavior
is just a feature of LS routing protocols and does not have a specific term.
- In DV routing protocols, most of the convergence time is taken by the loop-avoidance features;
whereas in LS routing protocols, there is no such time-consuming loop-avoidance features,
which means LS protocols can converge very quickly.
- With proper design, OSPF can convergence as fast as 5 seconds after a router notices a failure.
Good design is also very important for OSPF scalability in large networks.
- The 3 scalability issues in large OSPF networks are:
i) Large topology database requires more memory on each router.
ii) Calculation and processing grow exponentially with the size of the topology database.
iii) A single interface or link status change (up – down) forces every router to run SPF!
- Unlike RIP, OSPF can operate in a hierarchy basis. Areas are used to establish a hierarchical
network and the largest entity within the hierarchy is an area. Although it is capable of receiving
routes from and sending routes to other areas, OSPF is still an interior gateway routing protocol.
An AS can be divided into a number of areas, which are groups of networks and routers.
- Hierarchical OSPF implementations reduce routing overhead, speed up convergence, confine
network instability to a single area, and scalable to very large networks.
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Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com