Network Architecture and Design 1

Advanced IP Routing

Open Shortest Path First (OSPF) Protocol

Reference

D. E. Comer, Internetworking with TCP/IP, ISBN 1-13-018380-6, 4th Ed., Vol. 1, Ch. 15.

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OSPF

OSPF is the Open Shortest Path First protocol.

That means it is an “Open” version of the “Shortest Path First” algorithm, it does not mean it tries to open the shortest path first.

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OSPF

OSPF version 2 is defined in RFC 2328.

(Version 1 is no longer in use.)

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OSPF

OSPF is an update-based, link-state, open routing protocol.

These attributes make OSPF the most commonly used IGP in use today.

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OSPF

OSPF sends its entire routing table upon startup, then sends a keep alive every 10 seconds. If a topology change occurs, only the changes are sent.

This is much less bandwidth intensive and much faster than RIP.

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OSPF - Link State Protocol

OSPF is a “link state” protocol.

This means that each router keeps a database of all the links in its “area”, and calculates the “shortest path” to each destination network from that database.

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OSPF - Dijkstra Algorithm

In a nutshell, the Dijkstra Algorithm has each router imagine itself as the root of a tree, and calculates each successive link as a branch in the tree.

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Dijkstra

See ospf.pdf Slides 5 to 16.

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OSPF - Flooding

In each area, every time a link changes state, every router is “flooded” with Link State Advertisements (LSAs) describing the change.

Every router must run the Dijkstra algorithm to re-calculate every route in the area.

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When a Link Changes State

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OSPF - LSAs

Different types of LSAs.

Examples Router link (LSA type 1) Network link (LSA type 2) Network summary (LSA type 3) External (LSA type 5) …..

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Location of Different LSAs

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OSPF - Link Cost

Each link is assigned a cost. By default this cost is 100,000,000 / (speed of link in bps). So, the default cost for a FDDI link is 1, 10BaseT is 10, and a T1 is 64.

Unfortunately, this does not scale well with today’s technology.

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OSPF - Link Cost

Fortunately, the cost of a link can be set manually. In cisco, this is done under each interface:

interface serial 0

ip ospf cost 10

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OSPF – Link Cost

A “total” cost is then calculated for each destination prefix. Each prefix is installed into the routing table with a next hop relating to the lowest cost path.

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OSPF - Convergence

When a link changes state, the LSA flood and recalculation happen in a very short time, usually seconds.

Because a link change is explicitly stated, there are very few routing loops (and for very short periods) in OSPF.

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OSPF - Convergence

This may seem CPU and memory intensive, and it does take a great deal more memory and CPU than RIP, but it also allows OSPF to converge routes in seconds even over large and complex networks.

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OSPF - Areas

To help conserve CPU and RAM, and to limit LSA floods, areas were introduced.

Each router need only know about the links in its area, and the link back to Area 0 (zero).

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OSPF - Areas

Areas are defined as a 32-bit number, either straight decimal (e.g. 123456) or as a “dotted decimal” (e.g. 10.0.0.1).

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OSPF - Area 0

Every area must be directly connected to “Area 0”.

Area 0

Area 2

Area 1

Area 3

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OSPF - Area 0

The routers in Area 0 contain the aggregated prefixes for every area.

Area 0 is sometimes called the “Backbone” area because all inter-area traffic must traverse Area 0.

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OSPF - Area 0

Area 0 places an extreme burden on a network designer using OSPF.

Many networks “grow” in non-elegant ways, making a truly hierarchical network difficult or even impossible.

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OSPF - ABRs

Routers with an interface in Area 0 and an interface in a non-zero area are called “Area Border Routers” or ABRs.

ABRs aggregate the prefixes for a non-zero area and inject the aggregated prefixes into Area 0.

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Different Types of Routers

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Location of Different Routers

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OSPF - Neighbors

OSPF uses neighbor relationships to send routing updates.

If a neighbor relationship cannot be achieved, no routing updates will pass.

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Routing Protocol Packets

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OSPF - DR

On broadcast media (e.g. Ethernet), OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR).

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OSPF - DR

When updates are sent, each router on the LAN sends the updates to the DR (and the BDR), which sends one copy to each router.

This is much better than each router sending a copy of each update to each other router.

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OSPF - BDR

If the DR is disabled or otherwise does not respond to queries, the BDR takes over.

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DR and BDR

DR BDR

Hellos elect DR and BDR to represent segment Each router then forms adjacency with DR and BDR

Hellos elect DR and BDR to represent segment Each router then forms adjacency with DR and BDR

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OSPF - Virtual Links

A tunnel, or “virtual link”, can be used when direct physical connectivity cannot be achieved.

This is not the preferred method.

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OSPF - Multicast

OSPF uses multicast (224.0.0.x) to propagate its routing updates, not broadcast. This reduces the CPU requirement on other hosts on the LAN as they do not have to process the multicast packet if they are not part of the multicast group.

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Third Intermediate Report

OSPF I. Stergiou

BGP A. Sgora

Deadline: 25/02/03

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Third Intermediate Report

Structure Overview of examined technology Focus on open research points Related to open points works - State of the

art behind open points Your own interests - Ideas Conclusions References

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Third Intermediate Report

Report (soft and hard copy) Use of Greek language A related presentation (about thirty

minutes).

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End of Sixth Lecture