Multi-access and broadcast under OSPF

Redlabel

These terms confuse me when used under OSPF. Would anyone elaborate on the issue?
Thanks.

MrfixitRight

Redlabel wrote:

These terms confuse me when used under OSPF. Would anyone elaborate on the issue?
Thanks.

Are you referring to NBMA?(Nonbroadcast multiaccess)

Redlabel

yes please

MrfixitRight

Okay, I'll try. (I'm sure someone will correct me though.)

An NBMA network is a multi-access network where, by default, you can have multiple devices in the same broadcast domain but the technology is not truly a broadcast technology. Examples of NBMA networks are Frame Relay or X.25.

OSPF recognizes three different network types: broadcast multi-access, point-to-point, and non-broadcast multi-access (NBMA). Each has different characteristics and each is handled a bit differently by OSPF. The most notable characteristic of a broadcast multi-access network is the fact that you can have many devices sharing the same media. All devices on the network hear any information sent. An example of a broadcast multi-access network is Ethernet. A point-to-point network type is any technology where only two routers can share a link. An example of a point-to-point is a serial connection.

Hope this helps.

Anyone else want to jump in?

rossonieri#1

hi,

NBMA in routing protocol for short -- you can have many links but your broadcast doesnt get thru -- so your routing protocol doesnt get update or doesnt know its neighbor.

cheers.

tech-airman

Redlabel wrote:

These terms confuse me when used under OSPF. Would anyone elaborate on the issue?
Thanks.

Redlabel,

To help reduce the confusion, think of the related Data Link and Physical layers related to each of the terms. The terms being:

1. Point to Point
2. Broadcast Multi-Access
3. Non-Broadcast Multi-Access

Now think of the "Point to Point" in terms of a Leased line WAN link. The first point being the first router's serial port. The secoind point being the second router's serial port. Linking them together forms the "Point to Point" connection type in OSPF.

Now think of the "Broadcast Multi-Access" in terms of an Ethernet LAN links. Remember that a transparent switch's first reaction to a destination MAC address that is not in it's MAC address table is to BROADCAST the frame out of all ports, except the port the frame was received on. Now, also remember that a switch is a MULTIport bridge. That's how to remember the "MULTI" part of the "Multi-access."

Now think of the "Nonbroadcast Multi-Access" in terms of Frame Relay. A Frame Relay link may be operating at a speed of T-1 or 1.544 Mbps, you probably don't want too many broadcasts going across that WAN link because the broadcasts may not permit other traffic to go across that WAN link, for example e-mail, web traffic, or other important traffic. So that's how to remember "NONBROADCAST." Now, with Frame Relay, you can setup a virtual hub and spoke configuration between the HQ router and the Branch routers. On the HQ router's serial interface, imagine that it has MULTIple Permanent Virtual Circuits (PVCs) connected to it through subinterfaces of the serial interface. So that's how I remember that NonBroadcast Multi-Access has something to do with Frame Relay.

I hope this helps.

MrfixitRight

tech-airman

Redlabel,

To help reduce the confusion, think of the related Data Link and Physical layers related to each of the terms. The terms being:

1. Point to Point
2. Broadcast Multi-Access
3. Non-Broadcast Multi-Access

Now think of the "Point to Point" in terms of a Leased line WAN link. The first point being the first router's serial port. The secoind point being the second router's serial port. Linking them together forms the "Point to Point" connection type in OSPF.

Now think of the "Broadcast Multi-Access" in terms of an Ethernet LAN links. Remember that a transparent switch's first reaction to a destination MAC address that is not in it's MAC address table is to BROADCAST the frame out of all ports, except the port the frame was received on. Now, also remember that a switch is a MULTIport bridge. That's how to remember the "MULTI" part of the "Multi-access."

Now think of the "Nonbroadcast Multi-Access" in terms of Frame Relay. A Frame Relay link may be operating at a speed of T-1 or 1.544 Mbps, you probably don't want too many broadcasts going across that WAN link because the broadcasts may not permit other traffic to go across that WAN link, for example e-mail, web traffic, or other important traffic. So that's how to remember "NONBROADCAST." Now, with Frame Relay, you can setup a virtual hub and spoke configuration between the HQ router and the Branch routers. On the HQ router's serial interface, imagine that it has MULTIple Permanent Virtual Circuits (PVCs) connected to it through subinterfaces of the serial interface. So that's how I remember that NonBroadcast Multi-Access has something to do with Frame Relay.

I hope this helps.

Excellent explanation! Very detailed!

rossonieri#1

tech-airman wrote:

Redlabel,

To help reduce the confusion, think of the related Data Link and Physical layers related to each of the terms. The terms being:

1. Point to Point
2. Broadcast Multi-Access
3. Non-Broadcast Multi-Access

I hope this helps.

and, just for addition..

those types of connection have a different value which dictates how long or how often for an update arise. you will have this material in CCNP - BSCI.

cheers.

MrfixitRight

and, just for addition..

those types of connection have a different value which dictates how long or how often for an update arise. you will have this material in CCNP - BSCI.

cheers.

Further addition---from CCNP Study Guide:

In seconds

---

Broadcast (multiaccess) 10/40

Non-broadcast multi-access 30/120

Point-to-point (with broadcasts) 10/40

Point-to-multipoint (with broadcasts) 30/120

Point-to-multipoint non-broadcast 30/120

Redlabel

thanks for clarifying the matter.