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Friday, November 13, 2009

Cisco Routers. Concepts of forwarding metric in OSPF E1 and E2. When should you use these metric types? Why is OSPF better than RIP?


Basic design considerations between E1 and E2
E1 will include all hops within the path and is an accurate measure. Whereas a E2 (which is the default external type) will present the metric calculation upto the point of redistribution only. So if it traverses new links the metric will not provide additional measures. Consider EC2 as a metric strictly from ASBR towards the destination.

It is preferable to use Type 1 instead of Type 2 when the metrics match

1. Intra-area routes.
2. Inter-area routes.
3. External Type-1 routes.
4. External Type-2 routes.

O E1 are recommended when multiple ASBRs exist and you want to be sure that the best path to the exit point is used
O E1 routes are always preferred over O E2 routes regardless of seed metric
O E1 are seen as more complete information because the seed metric is added to the path cost to reach ASBR

If there are scenarios that has only one exit point, like internet access consider using O E2. The keys for getting the design correct is to understand the number of ASBRs and the number of exit points in your network. The O E1 routes give you more control but might result in an sub-optimized design when inter-area routing is involved.

Advantages of OSPF over RIP
  • With OSPF, there is no limitation on the hop count.
  • The intelligent use of VLSM is very useful in IP address allocation.
  • OSPF uses IP multicast to send link-state updates. This ensures less processing on routers that are not listening to OSPF packets. Also, updates are only sent in case routing changes occur instead of periodically. This ensures a better use of bandwidth.
  • OSPF has better convergence than RIP. This is because routing changes are propagated instantaneously and not periodically.
  • OSPF allows for better load balancing.
  • OSPF allows for a logical definition of networks where routers can be divided into areas. This limits the explosion of link state updates over the whole network. This also provides a mechanism for aggregating routes and cutting down on the unnecessary propagation of subnet information.
  • OSPF allows for routing authentication by using different methods of password authentication.
  • OSPF allows for the transfer and tagging of external routes injected into an Autonomous System. This keeps track of external routes injected by exterior protocols such as BGP.
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