can point to point replaced by multipoint subinterfaces ?

Unanswered Question
Jun 23rd, 2007

if i have a router, lets call it Router A, connected to 3 sites using frame relay.

each with different subnet,then i have 3 subinterfaces configured as point-to-point subintefaces. we can picture this as hub and spoke.

can i replace these 3 subinterfaces(Router A) as multipoint.

if the answer is yes, this lead to my next question :

1.what is the difference between that two configurations? you think point-to-point become useless ? since it can be replaced by multipoint ?

or maybe each subinterface configured require more memory/cpu processing of the router ?

I have this problem too.
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Richard Burts Sun, 06/24/2007 - 04:59

You certainly could replace 3 point to point subinterfaces with 1 multipoint. It requires significant changes in the IP addressing and will change several behaviors. It can be done, but is generally not regarded as a best practice. And it certainly does not make point to point become useless. There are several benefits in using point to point subinterfaces for Frame Relay.

To understand the impact of doing this, the first thing is to understand the IP addressing. As you have noted if you have 3 point to point subinterfaces then each subinterface will have its own unique subnet. If you change to multipoint then there is 1 subnet connecting all 4 routers. Now instead of having 3 unique paths to unique destinations you will need Inverse ARP or some Frame Relay mapping to determine which PCV gets to which remote destination. Also in doing this you may encounter issues with split horizon. In point to point there is no difficulty with split horizon since you have 3 unique subinterfaces. But with multipoint there may be issues of split horizon because all connections are on the same interface.

So yes it is possible to replace 3 point to point subinterfaces with 1 multipoint. But there are complications in doing this. It is generally accepted best practice in live networks to use point to point subinterfaces.



exam_toni888 Sun, 06/24/2007 - 07:02

Thanks Rick,

i knew we can replace it with 1 multipoint,

but what i mean is 3 multipoint, still 3 different subnet.

1 physical interface configured as 3 subinterfaces, each subinterface configured as multipoint, each in their own subnet.

Richard Burts Sun, 06/24/2007 - 12:29

I did not understand that from your original post, but yes you could configure 3 multipoint subinterfaces instead of 3 point to point subinterfaces. I am not sure why you would want to.

If you configure the subinterfaces as multipoint you still have the issue about needing Inverse ARP or some manual Frame Relay mapping. The subinterfaces will still consider split horizon, though with only a single neighbor on each connection this has little impact. Also consider that OSPF operates significantly differently on a multipoint subinterface as compared to a point to point subinterface.

Another aspect to bear in mind if you are considering changing a configuration on an existing router which has 3 point to point subinterfaces to make them multipoint is that you will need to use different subinterface numbers. The IOS will not change an existing point to point subinterface and make it multipoint. The alternative is to remove the subinterfaces, save the config, reboot the router, and then you can configure the subinterfaces as multipoint using the original subinterface numbers.

If this question is oriented toward preparing for some certification exam then it is a proper and logical question. If it is oriented toward configuring routers in a live environment, then I repeat my assertion from my previous post: the accepted best practice is that point to point subinterfaces are better than multipoint subinterfaces.




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