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Home > Archive > CCNP > November 2001 > Solving : SubInterfaces (OSPF)
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Solving : SubInterfaces (OSPF)
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| Hellas 2001-11-29, 11:05 am |
| Hey you all !
I'm presently working on the OSPF section and I'm stuck trying to understand the subinterfaces that are used with point-to-point and/or frame-relay with or without BMA or NBMA.
Can somebody suggest me something or help me out ?
Thanks | |
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| I know exactly how you feel, I reread my CCNA notes in the relevant section and it was well worth the effort.
I couldn't even remember what NBMA was!! | |
| haseeb_eng 2001-11-29, 2:54 pm |
| point-to-point sub int. u r using if you want both ends to be in same subnet . BMA is used mostly when you have more bandwidth so routing updates does not effect your network performance that much . NBMA is cnfigured when you are having slow WAN links . Sending periodic updates on WAN links can bring your network down | |
| sidodgers 2001-11-29, 3:21 pm |
| quote:
Originally posted by haseeb_eng
point-to-point sub int. u r using if you want both ends to be in same subnet . BMA is used mostly when you have more bandwidth so routing updates does not effect your network performance that much . NBMA is cnfigured when you are having slow WAN links . Sending periodic updates on WAN links can bring your network down
The differences between network access types as far as OSPF is concerned can be subtle, so pay attention... 
BMA (aka shared media) networks such as Ethernet, Broken Ring and FDDI work 'out of the box' by electing one router to be the DR and one to be the BDR per broadcast domain, and having all other routers in the same broadcast domain exchange routing updates with them, and only with them. in addition, the BDR peers with the DR to make sure it isn't dead. While all routers send updates to both (actually, to the AllDR multicast address), only the DR actually sends updates to other routers (to the AllSPFRouters address.) If the DR carks it, the BDR realises this, becomes the DR and a new BDR is elected.
Point to point networks (eg ISDN, Serial 'interface-dlci'-style subinterfaces, etc,) since they only have a maximum of two routers present don't bother with DR/BDR elections, and exchange LSUs with each other directly.
NBMA networks pose a problem, which has two solutions. If the network is configured in a full mesh, and all of the 'frame-relay map' commands have the 'broadcast' keyword at the end, the routers will pretend that the mesh is actually a BMA network and everything will work normally (except for the added cpu utilisation and network utilisation caused by broadcast emulation.) Alternately, you can configure the neighbours by hand (in much the same way that BGP is configured,) but this isn't recommended as it has the same weaknesses as the above strategy.
HTH,
Sid  | |
| Hellas 2001-11-29, 5:20 pm |
| Man, you took a whole lot of weight from my shoulders. Thanks for the advice.
This actually explains a few other things I was confused about as well.
You guys are great, thanks again !!!
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| sidodgers 2001-11-29, 5:42 pm |
| quote:
Originally posted by Hellas
Man, you took a whole lot of weight from my shoulders. Thanks for the advice.
This actually explains a few other things I was confused about as well.
You guys are great, thanks again !!!
Not a stress, glad I could help out.
Take it easy,
Sid | |
| haseeb_eng 2001-11-30, 4:03 am |
| i gave the summary and you gave the details sidodgers . Anyway that was good 2 |
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