[c-nsp] BRAS Redundancy

Anthony McGarry anthony.mcgarry at plannet21.ie
Tue Feb 23 06:06:46 EST 2010


Seems straight forward. Would there be an issue with the default gateway 
assignment from the DHCP server.

Loopback 0

x.x.96.20 - x.x.103.254
options router x.x.96.1

Loopback 0

x.x.104.20 - x.x.111.254
options router x.x.104.1

So if a client logs into BRAS-A and is assigned a static IP from the 
DHCP scope x.x.96.54 with a default gateway of x.x.96.1 there is no problems
If the same client logs into BRAS-B and is assigned the same static IP 
x.x.96.54 with a default gateway of x.x.96.1 how would the client route 
out of his subnet.

client --------------------------- BRAS-B 
---------------------------------- BRAS-A ------------
x.x.96.54 ----------- x.x.104.1 ----- x.x.1.1 -- iBGP -- x.x.1.2 
----------- x.x.96.1 -------

I am not even sure that what I want to do is possible because the DHCP 
server will see the giaddr in the dhcp request from BRAS-B as x.x.104.1 
and will try assign an address from the the BRAS-B scope and my static 
assignment is from the BRAS-A scope.
I use username to assign static address on the DHCP server
host custid_xxxxxxxxxx { option dhcp-client-identifier 
"\xxxxxxxxxxxx at xxxxxxx.xxx"; fixed-address x.x.96.54; }

Maybe I need to revisit how I assign IP to customers. Would you have any 


Arie Vayner (avayner) wrote:
> Anthony,
> Usually for static IP assignments you would have to redistribute the
> connected/static (static for routes) prefixes into the routing protocol
> (I would recommend BGP) so that you advertise them as /32. No magic...
> Arie
> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Anthony McGarry
> Sent: Monday, February 22, 2010 13:50
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] BRAS Redundancy
> Hi,
> I was hoping someone can help me with the following issue.
> I currently have a 7301 acting as my BRAS running on 12.2(33)SRD3. I use
> the ISG feature to terminate PPPoE sessions on QinQ subinterfaces.
> The virtual templates associated with the bba groups use ip unnumbered 
> loopback 0.
> The IP on loopback 0 is x.x.96.1/21
> DHCP is configured for client IP address assignment using DHCP pools as 
> relay agents to a central DHCP server.
> ip dhcp pool DHCP
>    relay source x.x.96.0
>    class DHCP
>       relay target x.x.111.5
> I would now like to install a second 7301 for load balancing/redundancy.
> I currently trunk the QinQ vlans to the existing 7301 so I just do the 
> same for the second 7301.
> On the second 7301 I assign a new /21 network for DHCP assignment.
> This works fine for dynamic IP assignment.
> My problem is that we have multiple customers with static IP address 
> assignment from the DHCP server.
> How can I assign the same IP address to a certain client session if they
> login to either BRAS when each BRAS has a unique network associated with
> the loopback 0 interface.
> I was thinking mobile IP but I have not tested in the lab and not sure 
> if it is a supported solution.
> Anthony
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