DHCP and BNG

[benediktsv]

Hi all 

The setup I'm looking at is two BNG's running on ASR9010

We use them to aggregate all PPPoE and IPoE traffic from multiple locations using AC/Xconnect into a looped interface, so we get all traffic on one BE interface with multiple subinterfaces where we run the BNG

What I would like to do is use one DHCP pool for multiple subinterfaces - and that seems to work quite well

we have a Loopback with the DHCP pool/GI address/default gateway and every subinterface use that loopback as IP unnumbered and we apply the same DHCP proxy profile on each sub. interface.

This leaves me with the redundancy issue

How can I protect this setup, I cannot have the two loopbacks active on each of the ASR's  

 To conserve IP's, I would not want to split the pools up

If using Geo-redunancy, is this possible? - better running the DHCP on the ASR ? or external ?

- Benni

[Aleksandar Vidakovic]

hi Benni,

if you use an external DHCP server you can deploy BNG Geo-Redundancy easily in your scenario. You can create the subscriber redundancy group at subscriber access interface or sub-interface level, depending on the kind of failure scenarios that may happen in the access network. This document should help:

https://supportforums.cisco.com/document/12981676/asr9000xr-using-and-understanding-bng-geo-redundancy

/Aleksandar

[benediktsv]

Hi Aleksandar, thaks for the reply 

One thing I don't quite understand is the GI address 

As we deployed DHCP we use a Loopback with the GI address, the BE access interfaces use this Loopback as unnumbered IP and we can then use many BE access interfaces for the same large pool

As there is a requirement that the GI address is reachable from the DHCP, the DCHP server will have two competing routes to answer to right ?

And they have to be the same as the loopback is actually the default gateway that the DHCP server actually serves - or is the default gateway just ignored ?, if that's the case this might work having the two loopbacks on different IP's, but within the scope.

did I just find the answer to my own question? ;)

- Benni

[xthuijs]

hey benni,

the giaddr is effectively the gateway address. this means that it is a hint to the dhcp server for the address selection (although many dhcp servers can use other infromation like class, option 82 etc also to make a pool selection), regardless of the address offered, the dhcp server *WILL* respond back to the giaddr in unicast to provide his offer.

since many times the address selection is done based on the source of the discover the giaddr is generally used as a pool hint also.

typically also the giaddr since it is in the unnumbered interface to the subscriber, either as primary or one of its secondaries, is also the gateway for the subscriber.

so the loopback with the supporting addresses, also used as GIaddr in dhcp would ideally be routed towards a single BNG (albeit in GEORED we have them duplicated for redundancy, but only advertised conditionally).

hopefully i am still making sense here :)

cheers

xander