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CompTIA® Network+ : Ospf Routing & Spanning Tree Protocol

OSPF(Open Shortest Path First) Routing Protocol:

OSPF is a link state technology that uses Dijkstra algorithm to compute routing information.

An OSPF area is a collection of networks and routers that have the same area identification.OSPF process identifier is locally significant.

OSPF Area Types

  • Backbone Area (Area 0) - The backbone area is the central area to which all other areas in OSPF connect.

  • Standard Area : Default OSPF area type -Standard areas are defined as areas that can accept intra-area, inter-area and external routes. Intra-area routes refer to updates that are passed within the area. Inter-area routes refer to updates that are passed between areas. External routes refer to updates passed from another routing protocol into the OSPF domain by the Autonomous System Border Router (ASBR).

  • Stub Area : These areas do not accept routes belonging to external autonomous systems (AS); however, these areas have inter-area and intra-area routes. In order to reach the outside networks, the routers in the stub area use a default route which is injected into the area by the Area Border Router (ABR). A stub area is typically configured in situations where the branch office need not know about all the routes to every other office, instead it could use a default route to the central office and get to other places from there. Hence the memory requirements of the leaf node routers is reduced, and so is the size of the OSPF database.

  • Totally Stubby Area : These areas do not allow routes other than intra-area and the default routes to be propagated within the area. The ABR injects a default route into the area and all the routers belonging to this area use the default route to send any traffic outside the area.

  • Not So Stubby Area (NSSA) : This type of area allows the flexibility of importing a few external routes into the area while still trying to retain the stub characteristic.

OSPF router ID determined based on the following criteria:

  • Use the address configured by the ospf router-id command, if not configured, then

  • Use the highest numbered IP address of a loopback interface, if not configured, then

  • Use the highest IP address of any physical interface,

  • If no interface exists, set the router-ID to 0.0.0.0

Ref.:http://packetlife.net/blog/2008/jun/24/ospf-area-types/

OSPF Priority:

The ip ospf priority command is used to set manually which router becomes the DR. The range is 0- 255 and the default is 1. 0 means it will never be DR or BDR. One important criteria in Multi-access NBMA (Non Broadcast Multi Access) networks is the selection of Designated Router (DR) and Backup Designated Router (BDR). DR and BDR are elected for every multi-access net work, using Hello packets as "ballots."

  • The router�s priority field can be set to either ensure that it becomes the DR or prevent it from being the DR.

  • The highest Router ID breaks any ties. The router with the highest Router ID is elected the DR.

Spaning Tree Protocol

Spanning Tree Protocol (STP) 802.1d is used to prevent loops within a switch network. STP uses Spanning Tree Algorithm (STA) to prevent loops ensuring stable network topology (also called convergence, where all the switches in the network converge to a known network topology). The following are the important features of STP:

  • STP is a layer 2 protocol that runs on switches and bridges. The purpose of STP is to remove switching loops.

  • All switches participating in STP exchange info with other switches in the network through messages known as BPDUs.

  • STP port states are Blocked, Listen, Learn, Forward, Disabled

STP Port States:

As mentioned, the purpose of STP is to prevent switching loops throughout a LAN. It is done by controlling the redundant links that connect into the same network segment. Each network segment is only allowed to have a single designated port that is used to forward traffic onto it. All other access points into the same segment are enabled, but in a blocking state that disallows traffic flow. If a failure should occur on the forwarding port, one of the blocking ports will be transitioned into forwarding state to continue to allow access to the network segment. This transition process includes a number of different states, including each of the states shown in the figure below:

Spaning Tree Protocol port states

  • Blocking - A port that would cause a switching loop if it were active. No user data is sent or received over a blocking port, but it may go into forwarding mode if the other port in use fail and the spanning tree algorithm determines the port may transition to the forwarding state. BPDU data is still received in blocking state. Prevents the use of looped paths.

  • Listening - The switch processes BPDUs and awaits possible new information that would cause it to return to the blocking state. It does not populate the MAC address table and it does not forward frames.

  • Learning - While the port does not yet forward frames it does learn source addresses from frames received and adds them to the filtering database (switching database). It populates the MAC Address table, but does not forward frames.

  • Forwarding - A port receiving and sending data, normal operation. STP still monitors incoming BPDUs that would indicate it should return to the blocking state to prevent a loop.

  • Disabled - Not strictly part of STP, a network administrator can manually disable a port

Different port designations are given below:

  • Root : A bridge can have only one root port. The root port is the port that leads to the root bridge. All bridges except the root bridge will have a root port. the root port is in the STP forwarding state.

  • Designated : One designated port is elected per link (segment). The designated port is the port closest to the root bridge. Each designated port is in the STP forwarding state

  • Alternate : Alternate ports lead to the root bridge, but are not root ports. The alternate ports maintain the STP blocking state.

  • Backup: This is a special case when two or more ports of the same bridge (switch) are connected together, directly or through shared media. In this case, one port is designated, and the remaining ports block. The role for this port is backup.

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