From: Lord, Chris (chris.lord@lorien.co.uk)
Date: Tue Jul 06 2004 - 04:52:04 GMT-3
Thanks to everyone for their response. However much I learn through CCIE study I'm always in awe of how much there is still out there to know (Ken).
Assuming HSRP is not an option, it looks like it's down to what features the clients support, right? Assuming they support irdp, Yasser's suggestion of suplementing DHCP with IRDP seems a good solution. At least then you have some control over the switch-over times by tweaking the irdp timers (default 30min).
Thx,
C.
-----Original Message-----
From: Kenneth Wygand [mailto:KWygand@customonline.com]
Sent: 06 July 2004 01:24
To: Lord, Chris; ccielab@groupstudy.com
Subject: RE: dhcp default-router
Chris,
The particular implementation of how a host station uses multiple default gateways is specific to how the TCP/IP stack is implemented by the particular Operating System running on the host station. RFC 2132 very loosly defines how to use multiple default gateways as follows:
<RFC 2132>
3.5. Router Option
The router option specifies a list of IP addresses for routers on the
client's subnet. Routers SHOULD be listed in order of preference.
The code for the router option is 3. The minimum length for the
router option is 4 octets, and the length MUST always be a multiple
of 4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
| 3 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
</RFC 2132>
Link: http://www.networksorcery.com/enp/RFC/Rfc2132.txt
Specifically, Microsoft versions of the TCP/IP stack implement a feature known as "Dead Gateway Detection" which works as follows:
<SNIP>
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 computer's 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.
</SNIP>
Link: http://www.microsoft.com/technet/community/columns/cableguy/cg0903.mspx
For your particular requirements, if you want all traffic to use a second router as a default gateway, I would use some sort of dynamic negotiation on the infrastructure side instead (IRDP, HSRP, GLBP, etc). This client implementation of a second default gateway is shaky at best, and is very ugly when implementing a feature that must be consistent across all clients due to the loose standard set forth in RFC 2132.
Hope this helps!
Ken
________________________________
From: nobody@groupstudy.com on behalf of Lord, Chris
Sent: Mon 7/5/2004 6:46 PM
To: ccielab@groupstudy.com
Subject: dhcp default-router
Please can anybody confirm my understanding of this one .......
Scenario is simply this. Two routers R1 and R2 on the same ethernet segment. R1 is handing out dhcp adressess to clients. Under normal circumstances clients should use R1 as the default router. However if R1 goes down, clients that already have a valid dhcp lease, should use R2 as the default router.
Will the config below achieve this :
ip dhcp pool VLANX
default-router R1-IP-Addr R2-IP-Addr (..... up to a max of 8 addresses)
(other dhcp lines removed)
In other words, does the client take the 1st address as its prefered gateway, followed by the 2nd if the 1st isn't available, and so on until it finds one which exists?
Thanks,
Chris.
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