From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Thu Oct 09 2008 - 05:20:25 ART
Bogdan Sass wrote:
> Fake Name wrote:
>> Lan 1 |--R1 ----R2-----R3----R4---R5---| lan 2
>>
>> If a computer on lan 1 wants to talk to Lan 2 and R1 has a route to
>> Lan 2 it
>> via R2 then R2 has a route to Lan 2 via R3 and R3 has a route to Lan
>> 2 Via
>> R4 and R4 has a route to lan 2 vlan R5 then when the traffic returns
>> what is
>> every router but R3 has a return route to lan 1 but R3 will the
>> traffic be
>> black holed? If it will be if R3 was a multi layer switch would it go
>> through because of the multilayer switch cache?
>>
> What cache are you referring to? Anyway, at Layer3, it really
> doesn't matter if the device is a MLS or a router - as long as it does
> not have a route for a specific destination, traffic for that
> destination will be dropped.
>
Switch caches Layer 2 information about the source MAC address, but in
this case that is the mac address of R2. CEF saves time performing
routing lookups, but it uses the routing table and it does not cache L3
source addresses of transit traffic; hence traffic will be blackholed.
Sounds like you really need to get your hands on a lab to try this kind
of thing out. You must have matching routes for both the source and
destination addresses if you want bidirectional communications.
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