From: hulbertj@comcast.net
Date: Wed Jul 06 2005 - 12:23:54 GMT-3
One more thing to think about. With the below statement from the link Tom provided (thanks):
"In certain configurations, such as the use of the standby use-bia interface configuration command specified on an interface, redirects cannot be sent. In this case, the HSRP groups use the interface MAC address as their virtual MAC address. The router now cannot determine if the default gateway of the host is the real IP address or one of the HSRP virtual IP addresses that are active on the interface."
I would take this as meaning, that if I wanted optimal operation, I would not implement 'standby use-bia', and I would have to rethink my port-security configuration. Does anyone agree or disagree?
I will try to lab this up later today.
Thanks,
Jerry
-------------- Original message --------------
> Hi Tom,
>
> Thanks again for your response.
>
> I read the link you sent me and see I was wrong on at least one point.
>
> Per this link, "When running HSRP, it is important to prevent hosts from
> discovering the interface (or real) MAC addresses of routers in the HSRP
> group. If a host is redirected by ICMP to the real MAC address of a router,
> and that router later fails, then packets from the host will be lost."
>
> But, given the above, how could a packet get to net 3 if R1 is the active
> router and R1 and R2 are in the same group?
>
> R1 R2 -----> R3 net3
> e0 e0
> |----hsrp----|
> |
> other hosts
>
> It seems to me that this isn't possible. To solve this problem it seems
> that 2 groups must be configured and R2 must be the active router for the
> other group. This way for hosts that use R1 as their default gateway will
> be redirected to the virtual mac address of the other group.
>
> Am I on the right track now?
>
> TIA, Tim
>
> -----Original Message-----
> From: Tom Lijnse [mailto:Tom.Lijnse@globalknowledge.nl]
> Sent: Wednesday, July 06, 2005 9:18 AM
> To: ccie2be; Spyros Kranis
> Cc: ccielab@groupstudy.com
> Subject: RE: 3550 port-security and HSRP.
>
> Hi Tim,
>
> Okay, let's see if I can answer your questions:
>
> - "When HSRP is configured this way with the use-bia command, is the
> failover fast enough that any active sessions that hosts might have at
> the time of switch over remain intact?"
>
> As far as I know 'use-bia' has similar convergence to normal HSRP. The
> only extra step is that when the Standby becomes Active it has to send
> out the gratuitous arp-replies and the hosts need to update their
> arp-caches, but as far as I can see that should only add milliseconds to
> the convergence time. I haven't tested very extensively, but when I did
> use it, it never seemed slower than normal HSRP.
>
> - "I'm wondering if it's possible to config port security on the 3550
> and the use-bia on the routers such that a failover is transparent to
> hosts sessions maybe by lowering hsrp timers?"
>
> As far as I can see this comes down to the same issue. (This depends on
> what you mean by transparent). Convergence time for HSRP is determined
> by the time it takes for the Standby to discover that the Active Router
> disappeared, so this is mainly determined by the hello and holdtime.
> Setting these to lower values will decrease the convergence time.
> Whether or not you're using 'use-bia' seems negligible to me.
>
> - " Suppose your topology was like this:
>
>
> R1 R2 -----> R3 net3
> e0 e0
> |----hsrp----|
> |
> other hosts
>
>
> R1 is the active router. R2 is the standby router but packets must get
> to
> R3 to get to net3.
>
> How should HSRP be configured such that the hosts use R1 for all
> destinations except for net3?"
>
> The whole story about ICMP unreachables being disabled when you enable
> HSRP isn't entirely true anymore. You may want to read the following bit
> on the Doc-CD:
>
> http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
> fipr_c/ipcprt1/1cfip.htm#wp1044507
>
> After reading this and possibly quickly labbing it up I think you should
> be able to come up with an answer to your own question. In the end I
> think this would be more educational than me just giving you the answer,
> but let me know when you need a hint.
>
> Regards,
>
> Tom Lijnse
>
> CCIE #11031
> Global Knowledge
>
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