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New Member

OSPF and Multicast packet for DR or BDR

I have found this article in net

http://www.ciscopress.com/articles/article.asp?p=99982&seqNum=5

"Point-to-multipoint networks are a special configuration of NBMA networks in which networks are treated as a collection of point-to-point links. Routers on these networks do not elect a DR or BDR, and because all links are seen as point-to-point, all OSPF packets are multicast".

My confusion are

First:

If it is special configuration of NBMA (as link says), how are OSPF packets sent as multicast, because NBMA does not support neither broadcast not multicast ?

Second:

If it can be seen as point-to-point(I do not why), why do we not treat it as point-to-point link ?

3 REPLIES
Cisco Employee

Re: OSPF and Multicast packet for DR or BDR

First:

OSPF messages are indeed exchanged between the hub and spoke sites using the multicast address 224.0.0.5.

You are correct that NBMA do not naturally support multicast or broadcast. Multicast is achieved by replicating the same message to the multiple neighbors.

Second:

The links are not physically point-to-point links but they are from an OSPF topology standpoint.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
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México
New Member

Re: OSPF and Multicast packet for DR or BDR

"Multicast is achieved by replicating the same message to the multiple neighbors."

What is the difference between the above type of multicast (replicating the same message), and normal multicast ?!

Gold

Re: OSPF and Multicast packet for DR or BDR

In normal multicast, a single packet is transmitted on the medium, and received by other systems connected to the same broadcast network. They would normally examine the destination address verses the locally configured MAC address filter on the interface chipset (this filter is automatically configured), and dump multicast traffic the system's not interested in. The way multicast works with a point-to-multipoint link is by creating a single packet, and then replicating it in a special "broadcast queue." This process actually involves creating multiple pointers to the same physical memory location, containing the packet, and transmitting the packet multiple times from the same memory location. This is much the same as the way BGP peer groups work. While it's not a true multicast, it prevents creating the packet multiple times, and also prevents the router from copying a single packet into multiple memory locations, saving a lot of processing power, etc.

From OSPF's perspective, BTW, all of this is transparent. OSPF just treats the link as multicast capable, and sends multicast packets normally.

:-)

Russ.W

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