cable bundle with subinterfaces and dhcp

[svenlanckmans]

Hi,

I'm looking for the logic behind using multiple subinterfaces on a bundled interface.

Config example:

interface Bundle1

no ip address

cable arp filter request-send 3 2

cable arp filter reply-accept 3 2

no cable arp

!

interface Bundle1.1

ip address 12.12.12.1 255.255.255.0 secondary

ip address 11.11.11.1 255.255.252.0

cable dhcp-giaddr policy

cable helper-address 13.13.13.1

interface Bundle1.2

ip address 14.14.14.1 255.255.255.252 secondary

ip address 15.15.15.1 255.255.255.248

cable dhcp-giaddr policy

cable helper-address 13.13.13.1

Let's say that a modem boots up and it sends out a DHCP DISCOVER, how does the CMTS handle this packet? Does it send it out to the dhcp server using a giaddr of 11.11.11.1 (under bundle1.1) or 14.14.14.1 (under bundle1.2)? Or does it send it two packets?

Secondly, am I correct in presuming that the helper-address must always be the same under the different subinterfaces?

Many thanks,

Sven.

[alanssie]

Hi Sven

> Let's say that a modem boots up and it sends out a DHCP DISCOVER, how does the CMTS handle this packet?

If the CM is unknow (we never heard about this MAc before) it will be always match the first sub-interface

Once the BOOTreply is coming back the packet is gleaned, hence the CMTS learns under which sub-interface

the CM will be move, this is also working the same way with sub-interface fitted with "ip vrf forwarding  ..."

If the CM is known it will match the interface where it was learned before at the DHCP process

the helper is particulary usefull at that time in the sub-interface (1.2 in your exampel)

Looking at the above configuration there is no advantage, but later one could add specific ACL .. under the particular

sub-interface.

Alain

[dneamtu]

Hi,

In this scenario, do we reference the bundle 1 interface under the cable interface or wideband interface, or we do not reference anything (as we canot reference the bundle sub-interface in the config). Example, would this be ok, or we have to remove the "cable bundle 1" statements:

interface Cable7/0/0
no cable packet-cache
cable bundle 1

interface Wideband-Cable7/0/0:0
description Wideband for CM1
load-interval 30
cable bundle 1

Dan

[alanssie]

Hi Dan

In this case the "bundle" is there for the CMTS to learn the l3

forwarding, so both would need it

interface is definitely needed as you will need to forward DHCP, TFTP ...

Wideband for L3 traffic unicast, multicast

regards

Alain