A few people have asked for a diagram, so here goes...
We have many instances of this design - this is an example. Don't bother
commenting on the use of address ranges - they were assigned pre-RFC1918
and there has been no compelling reason to change them. It makes address
assignment for new sites easy, and means I get no practice at binary maths
;-)
Addresses in the core are generally in the 1.0.0.0/8 range.
The local switches have addresses 2.0.1.0/24 (e1), 2.0.2.0/23 (e2),
2.0.4.0/23 (e3). They are all in ospf area 2.1.0.0.
The WAN links are frame relay point to point sub-interfaces. The remote
sites each have an address range 2.x.0.0/16, where x is 100 or greater
(the WAN links are included in these ranges). If x is 128 or greater,
then the site is in ospf area 2.2.0.0 (and must be attached to Rtr2).
Sites with address ranges from 2.100.0.0 to 2.127.0.0 are in area 2.1.0.0
and may be attached to either router. If there aren't any sites with an
address range 2.128.0.0 or greater, then ospf area 2.2.0.0 is not used
(it's usually defined, though, to make adding new sites more idiot-proof).
In the diagram below, siteX is in ospf area 2.2.0.0 and other sites are in
ospf area 2.1.0.0 (assume siteA has an address range of 2.102.0.0/16).
Rtr1 has ospf areas 0.0.0.0 (link to core and e0) and 2.1.0.0 (local
ethernets and all WAN links).
Rtr2 has ospf areas 0.0.0.0 (dialup link to core and e0), 2.1.0.0 (local
ethernets and some or all WAN links), and 2.2.0.0 (remaining WAN links -
possibly none).
The area range statements on Rtr2 are...
[various area 0 range statements snipped]
area 2.1.0.0 range 2.0.0.0 255.128.0.0
area 2.2.0.0 range 2.128.0.0 255.224.0.0
On Rtr1 the statements are...
[same area 0 range statements snipped]
area 2.1.0.0 range 2.0.0.0 255.128.0.0
If sw1 or sw2 fails, or the trunk between them fails, SiteD and SiteE will
no longer be able to contact the core.
| :
|to core to core :
| (dialup):
-------- 1.2.100.0/24 --------
| Rtr1 |e0 ------------------- e0| Rtr2 |
| | --- --- | |
| |e1---| |trunk| |---e1| |
| |e2---|sw1| |sw2|---e2| |
| |e3---| | | |---e3| |
| | --- --- | |
| | /|\ /|\ | |
-------- local switches --------
| (dual homed) |
/|\ /|\
/ | \ 2.120.0.0/16 / | \ 2.130.0.0/16
siteA/ | \__siteC siteD__/ | \__siteX
siteB 2.101.0.0/16 siteE
2.109.0.0/16 2.104.0.0/16
JMcL
----- Forwarded by Jenny Mcleod/NSO/CSDA on 04/04/2002 08:50 am -----
"[EMAIL PROTECTED]" <jenny.mcleod
Sent by: [EMAIL PROTECTED]
03/04/2002 12:05 pm
Please respond to "[EMAIL PROTECTED]"
To: [EMAIL PROTECTED]
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Subject: OSPF design [7:40269]
Hi all,
This is actually a real-life scenario, but I think it throws up some
interesting points about OSPF that some people may not have come across.
And it has a couple of bits that I don't understand. Please excuse the
verbosity.
Currently, (part of) this particular network is as described below. It
normally works fine, but during certain types of failures, connectivity
breaks although there is still a physical path. I am contemplating what
the best way to fix it would be, and would be interested in comments.
Set-up - I don't think my ascii art is up to this but I'll give it a go if
the description isn't clear enough:
Two ABRs (Rtr1 and Rtr2), running IOS 12.1, connected to each other by a
direct ethernet cable in area 0, and also by several local ethernet
networks in area 2.1.0.0. The details of the local ethernets can probably
remain a fluffy cloud, but note that failure of a single component can
potentially cause all area 2.1.0.0 neighbour connectivity between Rtr1 and
Rtr2 to be lost, although the local ethernets may remain up on one or both
routers.
Both routers have a connection back to the core of the network (on Rtr2 it
is dialup, so not usually active), which is in area 0. Both routers have
WAN links to several sites (not dual-homed - each site has a link to only
one ABR), in area 2.1.0.0. Rtr2 may also have WAN links to several sites
in area 2.2.0.0, but that's probably not too relevant.
Both ABRs summarise the networks in area 2.1.0.0 to a single summary
network (Rtr2 summarises the networks in 2.2.0.0, if any, to another
summary network).
This usually works fine - traffic from the core to sites connected to Rtr2
(in area 2.1.0.0) travels from Rtr1 to Rtr2 across the local ethernets
(area 2.1.0.0), and in reverse from Rtr2 to Rtr1 across the Area 0
ethernet. This, while perhaps not ideal, is as expected, and works well
under normal circumstances. (If you're not sure why this is expected,
read up on hot potato routing policy - Howard gave a good description in
the context of stub areas in
http://www.groupstudy.com/archives/cisco/200001/msg01579.html)
The problem happens if the area 2.1.0.0 neighbour connections between Rtr1
and Rtr2 are lost. Even though there is still an area 0 link between
them, area 2.1.0.0 sites connected to rtr2 lose connectivity to the core.
Area 2.2.0.0 sites are OK (this is good - I'd be really confused if they
lost it too).
Despite Doyle claiming that partitioned non-backbone areas are not a
problem (he does, on page 462 of Routing TCP/IP Vol 1), it seems they can
be. As far as I can see, it's because when summarising the 2.1.0.0
networks, Rtr1 also installs a route to null0 for the summary route -
which overrides the summary route that Rtr2 generates (and which would
otherwise cover the 'lost' sites).
I can see a couple of possibilities for fixing this...
1) Install a second direct ethernet cable between Rtr1 and Rtr2, in area
2.1.0.0. This may not be particularly elegant, but it should be
comparatively easy to do and effective (there are plenty of spare ethernet
ports). It also has the useful side-effect of getting the through traffic
off the local ethernets.
2) Use the "no discard-route internal" command - this doesn't appear to be
documented but is mentioned at
http://www.cisco.com/warp/public/104/3.html#12.0
I haven't tested it, but I think it should prevent the null0 route from
being installed by Rtr1, so my theory is that then the summary generated
by Rtr2 should come into play. This, of course, goes against all Cisco
recommendations, which say that having the null0 route is A Good Thing to
prevent routing loops.
3) Muck about with the arrangement of switches within the internal
networks. I think this will cause more trouble than it's worth, since any
rearrangement has to be duplicated at twenty sites. In theory at least,
the whole network may be redesigned from scratch over the next year or so,
so a quick and dirty fix isn't necessarily a problem.
BUT... I am also not positive that my understanding of what is happening
and why is correct, because the support guys have told me that this
problem has been around since we were running IOS 11.2 on the ABRs (not
that long ago, believe it or not), and I'm pretty sure that no route to
null0 was being generated then (summarisation was the same).
So can anyone explain to me why connectivity would fail even if no null0
route was being generated? What am I missing?
And does anyone feel like commenting on the options for fixing it?
JMcL
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=40411&t=40269
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