From: Baety Wayne 18 CS/SCBX (Wayne.Baety@xxxxxxxxxxxxx)
Date: Thu Apr 25 2002 - 06:59:45 GMT-3
Routing protocols won't advertise the same network on to the source
network... see below...
R1----10.0.1/24----R2
R1 will _never_ advertise 10.0.1/24 to R2, likewise R2 will never advertise
the same to R1. You can think of this as internal split horizon, except
there's no knob to turn this off. This obviously doesn't apply to LSP type
routing protocols which share complete information about network topology
with each node. In any case, this would rarely be useful to do, except to
inform routers that happen to have more than one network configured for a
particular segment. In practice, even in the case of secondary networks, R1
and R2 do not advertise information about connected networks unto the
connected network, even to other secondary networks configured on the
interface. Even if they did, the information would never be installed in
the routing table since connected networks have a higher administrative
distance than next hop style advertisements. But let's play around with
this scenario just to make sure you completely understand the implications.
Let's say R1's address is 10.0.1.1 and R2 is 10.0.1.2 and a host on that
segment has an address of 10.0.1.3. Let's say that R1 advertises the fact
that it can reach 10.0.1/24 to R2 and R2 advertises the same to R1 and they
both install the route into their respective routing tables. Let's also
suppose a packet arrives with a destination of the host 10.0.1.3. R2 (or
R1) would look in its routing table and realize that network 10.0.1/24 is
reachable by R1 (or R2). It would send the packet to R1 which would make
the same determination about R2. They would bounce the packet back and
forth amongst themselves until a comet from outer space smashes into them or
you pull the plug. Clearly, not a useful thing to do =)
But to make things even weirder for you... see below...
R1-----10.0.1/24
Ospf
Rip
IS-IS
EIGRP
etc...
R1 will never advertise the connected 10.0.1/24 to any other routing process
via redistribution <routing protocol x>. You __ALWAYS__ must use
redistribute connected, no exceptions. For the same reasons noted above,
connected networks are treated special.
These are common misconceptions and well talked about in the archives.
WAYNE BAETY, MCSE, A1C, USAF
Network Systems Trainer
> -----Original Message-----
> From: Jason [mailto:jgraun@attbi.com]
> Sent: Thursday, April 25, 2002 1:01 PM
> To: ccielab@groupstudy.com
> Subject: ISIS and connected networks
>
> I was wondering if anybody has found a way to get the connected network
> between two routers to enter the ISIS database and get sent to other
> routers. For example
>
>
> R1---12net---R2----23net----R3
> ISIS L2 OSPF
>
> When I redistribute ISIS into OSPF on R2 I see any other ISIS routes
> behind R1 but I don't see the 12 network which is connecting the two of
> them. I have seen this on both Frame Relay and Ethernet, I have used a
> redistributed connected matching a route-map to get the network into the
> table but is there any other way?
>
>
> Thanks
>
>
> Jason
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