blank.gif (43 bytes)

Church Of The
Swimming Elephant

Search:
E.3.3 The OSPF Algorithm Connected: An Internet Encyclopedia
E.3.3 The OSPF Algorithm

Up: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 1812
Up: APPENDIX E Additional Next-Hop Selection Algorithms
Up: E.3 Some Route Lookup Algorithms
Prev: E.3.2 The Variant Router Requirements Algorithm
Next: E.3.4 The Integrated IS-IS Algorithm

E.3.3 The OSPF Algorithm

E.3.3 The OSPF Algorithm OSPF uses an algorithm that is virtually identical to the Router Requirements Algorithm except for one crucial difference: OSPF considers OSPF route classes.

The algorithm is:

  1. Basic match
  2. OSPF route class
  3. Longest match
  4. Weak TOS
  5. Best metric
  6. Policy

Type of service support is not always present. If it is not present then, of course, the fourth step would be omitted

This algorithm has some advantages over the Revised Classic Algorithm:

  1. It supports type of service routing.

  2. Its rules are written down, rather than merely being a part of the Internet folklore.

  3. It (obviously) works with OSPF.

However, this algorithm also retains some of the disadvantages of the Revised Classic Algorithm:

  1. Path properties other than type of service (e.g., MTU) are ignored.

  2. As in the Revised Classic Algorithm, the details (or even the existence) of the Policy step are left to the discretion of the implementor.

The OSPF Algorithm also has a further disadvantage (which is not shared by the Revised Classic Algorithm). OSPF internal (intra- area or inter-area) routes are always considered to be superior to routes learned from other routing protocols, even in cases where the OSPF route matches fewer bits of the destination address. This is a policy decision that is inappropriate in some networks.

Finally, it is worth noting that the OSPF Algorithm's TOS support suffers from a deficiency in that routing protocols that support TOS are implicitly preferred when forwarding packets that have non-zero TOS values. This may not be appropriate in some cases.


Next: E.3.4 The Integrated IS-IS Algorithm

Connected: An Internet Encyclopedia
E.3.3 The OSPF Algorithm

Cotse.Net

Protect yourself from cyberstalkers, identity thieves, and those who would snoop on you.
Stop spam from invading your inbox without losing the mail you want. We give you more control over your e-mail than any other service.
Block popups, ads, and malicious scripts while you surf the net through our anonymous proxies.
Participate in Usenet, host your web files, easily send anonymous messages, and more, much more.
All private, all encrypted, all secure, all in an easy to use service, and all for only $5.95 a month!

Service Details

 
.
www.cotse.com
Have you gone to church today?
.
All pages ©1999, 2000, 2001, 2002, 2003 Church of the Swimming Elephant unless otherwise stated
Church of the Swimming Elephant©1999, 2000, 2001, 2002, 2003 Cotse.com.
Cotse.com is a wholly owned subsidiary of Packetderm, LLC.

Packetderm, LLC
210 Park Ave #308
Worcester, MA 01609