From: ccie2be (ccie2be@nyc.rr.com)
Date: Sun Mar 27 2005 - 16:08:02 GMT-3
Hi Guys and Happy Easter,
I'm wondering if this config will cause a routing loop? Even if it doesn't
this scenario raises some interesting issues which I'd like to hear your
opinions on.
Lots of Left side networks R1 isdn R2 lots of Right side
networks
Assume there's another path for packets to get back and forth between the
networks on the Left and Right side.
The isdn link is there to serve as a backup if the primary path fails.
Also, assume when the primary path is available
the IGP's are fully converged and all routes are reachable.
R1 and R2 are each configured with a default static floating route pointing
to the other side of the isdn link;
For example,
ip route 0.0.0.0 0.0.0.0 <ip addr of other end of isdn link> 250
Now, there are a few things to think about when configuring this scenario:
1 Assume the primary path is down and the isdn has come up. If R1 has a
packet for which it doesn't have a path, it will send that packet
to R2 based on the default static route. But, suppose R2 when it gets that
packet also doesn't have a path it. Will R2, based on its own static route,
send
the packet back to R1? Or, will R2 drop the packet?
2 If the default behavior is for R2 to send the packet back to R1, is there
a way to prevent that? Enable split-horizon, maybe?
3 Assume that all networks on both sides are subnets of 172.16.0.0/16.
Would there be any benefit of configuring the default static route like
this?
ip route 172.16.0.0 255.255.0.0 <ip addr of remote end of isdn link> 250
If so, what is the advantage?
4 According to Doyle, if the IGP running on both sides of this network are
either rip or eigrp, I don't need to redist the static route into the
routing process. Both of those IGP's do that automatically, true? But, if
the IGP's are either OSPF or ISIS, then I have to use the command, default
info originate. Agree?
5 Regarding interesting traffic, my thinking is that there's no point to
having routing protocol traffic be considered interesting or even allowed to
cross the isdn link (I would use passive interface bri 0) in this scenario.
Nor, if the IGP is ospf is there any reason to use ip ospf demand-circuit.
Does everyone agree?
6 In the example above, I used an AD of 250 on the static route. Does
everyone agree that the actual value used for the AD doesn't matter as long
as it's higher than the AD of any IGP's running in the network but lower
than 255?
I know I've thrown a lot of thoughts and issues in this post, but as you all
know, Cisco doesn't give partial credit on the lab. One tiny mistake and
you lose all the points for that section.
I appreciate any comments, feedback, or insights you might make.
TIA, Tim
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