Saturday, November 6, 2010


The metric used in OSPF is the Cost. The Cost is calculated by 100/Bandwidth-In-MBPS or 100000/Bandwidth. A list of common costs is:56k - 1785
64k - 1562
T1 - 65
E1 - 48
Ethernet - 10
Fast Ethernet - 1

Fast Ethernet and above are all considered the cost of 1, that's why networks with Gigabit and 10Gigabit Ethernet would need to have their Cost formula changed.

Paths are always calculated with the router being the center of the world. Routes are calculated by adding costs together. The route with the lowest cost is added to the Routing Table.

In this case, the best route would be 1+65+10=76 and would be added into the Routing Table.

The DR stands for the Designated Router, and the BDR is the Backup Designated Router. DR and BDRs play the role of controlling updates within a shared network segment.

Whenever a Link change is detected, the router that detects it will send the Update out of all links with neighbors. Whoever receives it will send it out to all neighbors except the one he received it from. In a shared segment, this can result in something similar to a broadcast storm!

To resolve this, the DR and BDR concept is added. This effectively turns things into a hub-and-spoke fashion. Whenever a change is detected, it is first sent to the address which DR and BDRs listen to, then the DR then forwards these changes out to where DROTHERS (non-DR and non-BDR routers) listen to.

Every shared segment (Ethernet and certain Frame Relay designs) has a DR. However, on Point-to-Point links, the only address used for communication is

The DR/BDR election is influenced by the Hello field known as the Priority (the same field used for Master/Slave election in the previous article). Ties are broken with the higher Router ID.

Elections are only important in certain Frame Relay topologies. Typically any router regardless of processing power would do well as a DR or BDR. In a shared Ethernet segment, routers will only form full relationships with the DR and BDR. Relationships with other routers will be stuck in 2-WAY state.

There are many OSPF packet types in use in OSPF. Below is a general list:
1) Hello
2) DBD (Database Description)
3) LSR (LS Request)
4) LSA (LS Advertisement)
5) LSU (LS Update)
6) LSACK (LS Acknowledgment)

Hello is used as a neighbor-forming mechanism and keepalive.

After the DBD is exchanged, LSRs are used to request for routes which are sent through LSUs. In each LSU, there are multiple LSAs. Each LSA describe individual routes. Think of LSUs as envelopes and LSAs as documents inside.

Every packet sent (except for Hello and LSACK) is acknowledged with an LSACK.

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