From: David Timmons (masterdt@yahoo.com)
Date: Thu Sep 21 2006 - 15:25:52 ART
Hi,
I have seen this used once and only once in my life. The time that I saw it configured it worked. I don't know that Cisco has an implementation that supports redundant gatways provided via DHCP. My testing was limited to providing XP/ MS 2000 clients multiple gateways. I'll see if I can find a link where Cisco supports this via DHCP.
This is brief description of how it should work for MS:
Dead Gateway Detection
Dead gateway detection is used by the TCP component of Windows TCP/IP to detect the failure of the default gateway and to adjust the IP routing table to use the next default gateway when there are multiple default gateways configured.
When a TCP segment for a TCP connection forwarded via the default gateway is retransmitted three times (by default), dead gateway detection changes the Route Cache Entry (RCE) for that remote IP address to use the next default gateway in the list as its next-hop address. An RCE is an entry in the routing cache, which stores the next-hop IP address for a destination address.
When one fourth of the TCP connections routed through the default gateway have had their RCEs adjusted to the next default gateway, dead gateway detection informs IP to change the computerbs default gateway to the one that the adjusted connections are now using. If TCP connections continue to fail, dead gateway detection attempts to use the next default gateway in the list, returning to the first default gateway after cycling through the entire list.
Dead gateway detection monitors only TCP traffic. If connectivity fails for other types of traffic, the default gateway is not switched. Dead gateway detection can cause the default gateway configuration to change when a remote router fails. Remote routers in the path between the host and the destination that fail might also cause TCP connections forwarded along that path to fail and for the host to switch its default gateway. Because dead gateway detection relies on an end-to-end protocol (such as TCP), a host can switch its default gateway even when the current default gateway is fully operational.
The combination of multiple configured default gateways pointing to disjoint networks and dead gateway detection can cause confusing consequences. If you configure default gateways for both Internet and intranet interfaces, the result can be intermittent connectivity to the Internet or your intranet when dead gateway detection changes the active default gateway. For example, when the active default gateway points to the Internet and dead gateway detection switches the active default gateway to the intranet default gateway, DNS-based name resolution for Internet names and Internet-based connections can fail.
More Links:
http://support.microsoft.com/kb/128978/EN-US/
http://support.microsoft.com/?kbid=319778 (This one says failover may have issues with UDP)
http://trevortinsman.com/WinTCP.aspx (This one tells what we may expect on Bista)
----- Original Message ----
From: Radoslav Vasilev <deckland@gmail.com>
To: David Timmons <masterdt@yahoo.com>
Cc: Arun Arumuganainar <aarumuga@hotmail.com>; ccielab@groupstudy.com
Sent: Wednesday, September 20, 2006 5:04:21 PM
Subject: Re: Default-router in DHCP POOL
Hi David,
How is this "reachability" checked by the client?
I have a config where a cisco router acts as dhcp client and choses
only the first default gateway, which is not active on the segment.
Just wondering how reliable solution this configuration option can
actually be... with cisco clients...
Rado
On 9/20/06, David Timmons <masterdt@yahoo.com> wrote:
> Hi,
>
> When we configure more then one Default Gateway on a router, the client will use try the other address in the event that the first one is offline. You should not have to configure anything else. This might be handy if you have gateways that point to devices from different vendors that you don't manage. I don't think it is the best option, but is just another one that you might be tested on....
>
>
> dt
>
> ----- Original Message ----
> From: Arun Arumuganainar <aarumuga@hotmail.com>
> To: ccielab@groupstudy.com
> Sent: Wednesday, September 20, 2006 6:31:59 AM
> Subject: Default-router in DHCP POOL
>
>
> Hi
>
> I am wondering why IOS is allowing us to configure 8 ip address as a
> Default-router address.
>
> Multiple address under DNS-SERVERs , NAME-servers are fine ...But wonder
> whether default-routers makes any sense ...
>
> Will that mean we need to configure IRDP also on the Candidate Gateways !!!
>
> Thanks and Regards
> Arun
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Sun Oct 01 2006 - 16:55:41 ART