CCNP - IP Helper-address

Author

IP Helper-address

Mat P

2001-10-10, 12:12 pm

I understand the principles behnd IP helper-address, not too sure on the implementation.

My understanding is that when you use the command it automatically forwards the 8 default ports.

Do I then need the command ip forward-protocol in addition or is this just if I have other ports to add???

Where does IP directed-broadcast come into the equation?? I know it's off by default - does it need enabling on every interface a directed broadcast will pass (which is wrong according to p90 of cisco press BSCN book), or just the first port?

Thanks.

MadChef

2001-10-11, 4:51 am

You only need to use the forward protocol command if you intend to forward addtional broadcasts from default ports (and I thought there were only 7 by default).
ip directed-broadcast would come into play if you forwarded to a broadcast address rather than specific addresses. If you list individual address, you will forward broadcast traffic as unicast packets. If you forward to say, 192.168.0.255 (assuming a 24 bit mask), and have ip directed broadcast enabled on the target network, the router will convert traffic destined to 192.168.0.255, the IP broadcast address, into layer 2 broadcasts. Turn ip directed broadcast off and the receiving router won't convert layer 3 broadcast traffic to layer 2 broadcasts. It's only a factor on the router that hosts the 192.168.0.0 /24 network, not the intermediate routers. With classless networking, they would have know way of knowing if the destination network was maybe 192.168.0.0 /23, in which case 192.168.0.255 would be a valid host address.
IP directed-broadcast is enabled in code prior to 12.0, I believe, so don't assume it's turned off.

MadChef

Mat P

2001-10-11, 7:47 am

Thanks,
It is 8 - I just checked, counting bootp and dhcp which have the same port number as 1.
You confirmed what I thought in that, IP forward-protocol is only needed if more (or less using no IP forward-protocol)than default is required.
As for the ip directed-beoadcast, this is only on the target router.
Thanks again.

MadChef

2001-10-11, 9:21 am

quote:
Originally posted by Mat P
Thanks,
It is 8 - I just checked, counting bootp and dhcp which have the same port number as 1.

Can you really count those as seperate protocols being that DHCP is just an extension of bootp and interoperate? The headers are virtually identical, even.

MadChef

Mat P

2001-10-11, 12:49 pm

I obviously worded it wrong, what I mean is they both have the same port number, the actual list is :
37 Time
49 TACACS
53 DNS
67 BOOTP(and DHCP) Server
68 BOOTP(and DHCP) client
69 TFTP
137 Netbios name service
138 Netbios datagram service

By enabling BOOTP, DHCP is automatically enabled.

Thanks for your help,

Mat P.

Mat P

2001-10-12, 5:49 am

Madchef,
I woke up in the middle of the night (sad I know) and clicked, it is 7 protocols but 8 port numbers - bootp is there as a server and client.

Anybody know wether for the purpose of the exam it's 7 or 8??

Mat P.

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