From: Kenneth Wygand (KWygand@customonline.com)
Date: Tue Jul 06 2004 - 10:50:42 GMT-3
Chris,
You are right - for every single thing you learn, you learn of two more
things you don't know. Take things one step at a time and enjoy the
journey!
As far as an answer to your question, IRDP is definitely an option, and
yes the client must support it. Microsoft clients support IRDP with the
tweak mentioned in the following excerpt:
<SNIP>
Win2K and NT 4.0 SP5 and later include host support for IRDP. However,
you need to add two subkeys to the
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Adaptername\Paramet
ers\Tcpip Registry key. Add the PerformRouterDiscovery subkey with a
REG_DWORD value of 1, and add the SolicitationAddressBcast subkey with a
REG_DWORD value of 0 (for multicast router solicitation) or 1 (for
broadcast).
</SNIP>
If you can't use HSRP, don't forget about Virtual Router Redundancy
Protocol (VRRP), which is essentially the open standard substitute for
Cisco's proprietary HSRP.
If you are prohibited from using both HSRP and VRRP, don't forget about
a feature that is similar to HSRP and VRRP (but -not- HSRP or VRRP
nonetheless), known as Gateway Load Balancing Protocol (GLBP). This
feature is Cisco proprietary and is available as of 12.2(15)T.
<SNIP>
The Gateway Load Balancing Protocol feature provides automatic router
backup for IP hosts configured with a single default gateway on an IEEE
802.3 LAN. Multiple first hop routers on the LAN combine to offer a
single virtual first hop IP router while sharing the IP packet
forwarding load. Other routers on the LAN may act as redundant GLBP
routers that will become active if any of the existing forwarding
routers fail.
GLBP performs a similar, but not identical, function for the user as the
HSRP and the VRRP. HSRP and VRRP protocols allow multiple routers to
participate in a virtual router group configured with a virtual IP
address. One member is elected to be the active router to forward
packets sent to the virtual IP address for the group. The other routers
in the group are redundant until the active router fails. These standby
routers have unused bandwidth that the protocol is not using. Although
multiple virtual router groups can be configured for the same set of
routers, the hosts must be configured for different default gateways,
which results in an extra administrative burden. GLBP provides load
balancing over multiple routers (gateways) using a single virtual IP
address and multiple virtual MAC addresses. Each host is configured with
the same virtual IP address, and all routers in the virtual router group
participate in forwarding packets.
</SNIP>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft
/122t/122t15/ft_glbp.htm
Hope this helps!
Kenneth E. Wygand
Systems Engineer, Project Services
CISSP #37102, CCNP, CCDP, ACSP, Cisco IPT Design Specialist, MCP, CNA,
Network+, A+
Custom Computer Specialists, Inc.
"The only unattainable goal is the one not attempted."
-Anonymous
-----Original Message-----
From: Lord, Chris [mailto:chris.lord@lorien.co.uk]
Sent: Tuesday, July 06, 2004 3:52 AM
To: Kenneth Wygand; ccielab@groupstudy.com; Yasser Aly
Subject: RE: dhcp default-router
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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