From: R. Benjamin Kessler (ben@xxxxxxxxxxxxxxxxxxxxx)
Date: Thu Dec 20 2001 - 19:14:31 GMT-3
OK, to answer your specific questions -
> if an Ethernet interface on R2 was to go down, how does that
> affect the routing from R1 to the 10.1.1.0 network?
Again, assuming no IP route-cache; R1 would still load-balance across both
R2 and R3. R1 would have no way of knowing that R2's Ethernet interface
went down.
> For instance, will R1 drop half the traffic?
R1 won't "drop" the traffic, it will forward half of it to R2's address and
the other half to R3's. If R2 is off-line, R1 will have no way of
knowing...until...
> How does the ARP cache on R1 impact routing, or
> rather, how is routing impacted by the ARP cache?
Yes, eventually, if R1 doesn't see any traffic from R2 it will age-out its
ARP entry.
> Will the static route through R2 get dropped so to speak?
No, R1 will maintain both static routes. Once the ARP entry for R2 is
aged-out though, every other packet will cause an "encapsulation failed"
message on R1 (if you're debugging IP packets). At this point, you could
argue that R1 is "dropping" half of the packets but it is only because it
doesn't have a L2 address to match the IP next-hop address for R2 - it can't
"encapsulate" the IP datagram in the Ethernet packet because it doesn't know
the destination MAC address. Because R1's interface is still up/up though,
both of the static routes will stay in the routing table.
This scenario would be quite different if R1 was connected to R2 and R3 via
point-to-point serial links; if R2's interface went down, so (typically)
would R1's in which case the static route would be removed from the routing
table.
Does this help clear things up?
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
vr4drvr .
Sent: Thursday, December 20, 2001 10:37 AM
To: ccielab@groupstudy.com
Subject: OT: Simple static route Q
Here's a static routing question that I need answered. I do have theories,
but I need a proof positive answer. Simple scenario.
R2---10.1.1.0/24
R1----|
R3---10.1.1.0/24
3 routers are connected to an ethernet segment. R1 has 2 static routes to
the 10.1.1.0/24 network pointing to the IP address of the next hop ethernets
on R2 and R3, thereby providing load balancing and fault tolerance. My
question is... if an ethernet interface on R2 was to go down, how does that
affect the routing from R1 to the 10.1.1.0 network? For instance, will R1
drop half the traffic? How does the ARP cache on R1 impact routing, or
rather, how is routing impacted by the ARP cache? Will the static route
through R2 get dropped so to speak?
TIA.
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