need help urgent plssss 3

[dan0923]

what is the advantage of using a multipoing interface instead of point to poing subinterfaces when configuring a fram rely hub and spoke topology??

i need to know weather it is answer A or B pls.

A)it avoid split horizon issues with distance routing protocols.

B)IP address can be conserved if VLSM is not being used for subnetting.

Thank you

 

[senk1s]

sorry for my 'dumb' question ... are u taking a test?

 

[burtsbees]

Well, dan, what do YOU think it is? I know what it is, but I want to make sure you have something to back up the answer you think it is. I am saying this because the correct answer is very obvious, if you understand what a multi point configuration is, and what issues lie with distance vector routing protocols, like being prevented from sending out routing updates out the same interface from which it received it...

Burt

 

[Cluebird]

Answer is B. Multipoint interfaces place all endpoints on the same IP subnet.

Point-to-point subinterfaces fix the split horizon problem but use an IP subnet for each subinterface...VLSM with /31 masks is used here.

 

[senk1s]

burtbees answer is much better !!

 

[paella33]

Cluebird, make that /30

 

[dan0923]

hi guys well i know it is very obvious to be honest i didnt read the material before so after burt respond i went and read the chap and yes cluebird it right and well it was really a easy question.

thanx guys and planing to write my test next week and hope i shout be ok.


suban.s

 

[Cluebird]

paella33,

It is a /31. The new IOS supports /31 on point-to-point links just as load-balancing can now be done on up to 16 equal-cost paths.

Regards.

 

[burtsbees]

Cluebird---I have heard this, and I can imagine this may be possible using the subnet zero and all 1's subnet, but what will the router use for the wire and broadcast if there are only 2 IP's available?

Burt

 

[Cluebird]

A point to point link is like a straw. What you shove in one end will push out the other side. As such, there is no need for a broadcast or wire address so Cisco has tweaked their IOS so the router will be able to use the formerly reserved addresses for wire and broadcast as legitimate host addresses on these type of links. It is for address conservation to improve efficiency with IP addresses in a Cisco shop. However, if you try to use the /31 on a multiaccess link like Ethernet, the router goes stupid.

 

[paella33]

You

 

[burtsbees]

Oh, okay. Only on ptp links. I can see no need for a broadcast, but what if
fa0(R1)---s0(R1)---------------s0(R2)---fa0(R2)

for some reason, R1 needs to broadcast from its fa0 subnet all the way to fa0's subnet on R2---I know this is ethernet, but does the router therefor need the ip-helper address command, or can it not be done? This is with s0 of both routers having the /31 mask...would the fa0 links then have vlsm addresses in the same subnet?

Burt

 

[Cluebird]

This is from Cisco:

http://www.cisco.com/warp/public/701/8.html#using31bit

"Using 31-Bit Prefixes on IPv4 Point-to-Point Links
RFC 3021 describes using 31-bit prefixes for point-to-point links. This leaves 1 bit for the host-id portion of the IP address. Normally a host-id of all zeros is used to represent the network or subnet, and a host-id of all ones is used to represent a directed broadcast. Using 31-Bit prefixes, the host-id of 0 represents one host, and a host-id of 1 represents the other host of a point-to-point link.

Local link (limited) broadcasts (255.255.255.255) can still be used with 31-bit prefixes. But directed broadcasts are not possible to a 31-bit prefix. This is not really a problem because most routing protocols use multicast, limited broadcasts, or unicasts."

If you're just trying to forward a directed broadcast across the p2p link, ip helper will do the trick since routers won't forward any broadcasts by default.

 

[burtsbees]

Thanks!

Burt