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Question about OSPF stub networks...

 
 
Stefan Sybydlo
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      02-08-2004
Hello Everyone.

I need to know if the following OSPF configuration is good/bad,
wanted/unwanted.

The basic question is as such (I will omit describing the larger part of the
network). I have two routers, both running OSPF. Between these two routers
I have two network connections. One is a network connection where IP is
configured, a single port on each router is used, and the routers have full
adjacency where one is DR and the other is BDR. In the OSPF database this
shows up as a transit link.

The other network is as follows. Both routers are using a single port to
connect to the network, IP is configured, both interfaces are configured
with the passive-interface command. There is NO adjacency between the
routers on this network. Both interfaces show up as stub networks in the
database. If you look at the interfaces, because of the passive-interface
command, both say they are the DR for that network. Connected to this
network is two switches used for user access.

In terms of OSPF routing, the routing table seems to show the correct
routes, and the network seems to function as desired.

I want to know if this is a legal (according to the OSPF RFC and general
common sense) configuration. Can one have the same network connected to two
routers as stub networks, where both act as DR. Are they both advertising
the network in LSAs? Can OSPF cope with this?


The CISCO OSPF design guide says the following:
Stub network links: This term has nothing to do with stub areas. A stub
segment is a segment that has one router only attached to it.


Cisco says only one router attached only is allowed, but it seems that the
OSPF RFC has provisions for this type config(?).

Does anyone see any obvious problems with this set up? Or is this just an
academic question and both setups are OK.

Another what if: What if the two switches attached to this netwrok were
split so that one was attached to one router and the other switch to the
second router, so that the network was effectively "split" in half?

One last question. What, in this case, would be considered best practice?
The routers involved do not have a problem with memory or CPU capacity.

Thanks in advance,

Stefan




 
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shope
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Posts: n/a
 
      02-08-2004
"Stefan Sybydlo" <(E-Mail Removed)> wrote in message
news:wMnVb.84075$(E-Mail Removed)...
> Hello Everyone.
>
> I need to know if the following OSPF configuration is good/bad,
> wanted/unwanted.
>
> The basic question is as such (I will omit describing the larger part of

the
> network). I have two routers, both running OSPF. Between these two

routers
> I have two network connections. One is a network connection where IP is
> configured, a single port on each router is used, and the routers have

full
> adjacency where one is DR and the other is BDR. In the OSPF database this
> shows up as a transit link.
>
> The other network is as follows. Both routers are using a single port to
> connect to the network, IP is configured, both interfaces are configured
> with the passive-interface command. There is NO adjacency between the
> routers on this network. Both interfaces show up as stub networks in the
> database. If you look at the interfaces, because of the passive-interface
> command, both say they are the DR for that network. Connected to this
> network is two switches used for user access.
>
> In terms of OSPF routing, the routing table seems to show the correct
> routes, and the network seems to function as desired.
>
> I want to know if this is a legal (according to the OSPF RFC and general
> common sense) configuration.


not sure - last time i read the RFCs there was no mention of "passive"
interfaces.

however i use this a lot (mainly where dual layer 3 switches connect to lots
of networks) - the main reasons are to reduce the number of adjacencies, but
not use external routes to describe the user subnets.

Can one have the same network connected to two
> routers as stub networks, where both act as DR. Are they both advertising
> the network in LSAs? Can OSPF cope with this?


if you didnt use passive, and didnt configure the interfaces with OSPF, then
the subnets would be described as external in the database, and that is
supported in the standards and in widely used practice.

"passive" is preferred over external as you can summarise passives at ABRs
(so end up with a block containing passive and other OSPF internal routes).
externals get flooded throughout the AS, apart from in stub networks.
ASBRs cause other LSDB entries to get flooded, so increase overhead in
database size.
both types of setup prevent transit traffic across the subnet, so give you
more control over which way router to router flows go across your network.
>
>
> The CISCO OSPF design guide says the following:
> Stub network links: This term has nothing to do with stub areas. A stub
> segment is a segment that has one router only attached to it.
>
>
> Cisco says only one router attached only is allowed, but it seems that the
> OSPF RFC has provisions for this type config(?).


i think this is backwards - a stub occurs when there is only 1 attached
router - "stub" means "no path crosses this subnet to another router in
OSPF" - so the stub network should not carry traffic in transit.
>
> Does anyone see any obvious problems with this set up? Or is this just an
> academic question and both setups are OK.
>
> Another what if: What if the two switches attached to this netwrok were
> split so that one was attached to one router and the other switch to the
> second router, so that the network was effectively "split" in half?


split subnet - any flows that get delivered to the "wrong" part of the
subnet cannot be delivered, so end up in a black hole.

again, the way to think of this is that you really dont want this kind of
fault, so thedesign should make it unlikely in practice.
2 possibilities are with resilience within the subnet (which may need
spanning tree), or by having each switch in a separate subnet, so the layer
3 devices can "see" a connection break to the subnet and adjust routing to
work around it.
>
> One last question. What, in this case, would be considered best practice?
> The routers involved do not have a problem with memory or CPU capacity.


i prefer that the layer 3 topology maps directly to the physical switch
connections.

One way is a separate subnet per layer 2 switch or stack (real resilient
stacks, with 3750s or nortel stacking, not just gigabit between separate
2950s). If you need multiple subnets - eg. for voip and data, then multiple
subnets.

this has a further advantage - it is much easier to understand - and a lot
of "faults" are people breaking things by accident, so KISS is a good design
principle all on its own.
>
> Thanks in advance,
>
> Stefan

--
Regards

Stephen Hope - remove xx from email to reply


 
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