OSPF with Multiple ABR & ASBR

Fri Nov 14 18:46:44 UTC 2008

OSPF (on cisco anyway) will balance 6 paths automatically assuming you
haven't messed with bandwidth or cost settings, and the paths have the
same iftype.  If not, setting the bandwidth equally will also do the
trick.  (I don't like messing with cost directly, just me)

I would also point out that you would probably be better off not using a
multi-area config for this solution.  My rule of thumb, which may be a
bit antiquated now, is create multiple areas when you have either more
than 2000 routes, or more than 300 interfaces total, or 200 on a single
router within a single area.  This was just a general rule of thumb
based on some problems with certain hardware scaling past that point and
behaviors during reconvergence.  If you can use a single area, go for
it.  If you are the type of company that might get acquired someday, or
hopes to, you can save yourself time and select an AREA ID that is based
on address space you own, that way there will be no AREA ID overlaps and
you would have the possibility of connecting to someone else's area 0
(acquiring company).

Also, don't use per packet load balancing.  It may work in the short
term, but it almost always screws with an applications performance.

My $0.02.

Steve

-----Original Message-----
From: Darden, Patrick S. [mailto:darden at armc.org] 
Sent: Friday, November 14, 2008 9:28 AM
To: devang patel
Cc: nanog at nanog.org
Subject: RE: OSPF with Multiple ABR & ASBR

It is my understanding of OSPF that, if you have paths with equal
distance and cost to a destination, load balancing happens automatically
for up to four (or is it 6 for OSPF?) clear paths.  In your diagram R1
to R8 load balancing should happen naturally, unless you have weighted
one of the paths.  You have much more than 4 paths here, so you should
weight the ones you want.  E.g. 1-2-4-6-8, 1-3-5-7-8 would be the most
straightforward, and barring some type of natural concentration of
bandwidth (e.g. R3 having 10X the hosts connected that R2 has) it would
be the easiest to implement.  This only applies to coequal routing (e.g.
all links are T1s).  If you are doing unequal routing I think you are
out of luck.  I would stick to two paths if possible for simplicity's
sake.  OSPF can become a quagmire if you let it.

So, first step is weight your chosen paths equally, and make sure they
are preferred over other possible paths.

Second step is to decide what kind of load balancing you want: per
packet, or endpoint.  If you set it up per packet, you get equal load
balancing with the chance of out-of-order packets on the other end.  It
can also take up a lot of the router's cpu resources.  If you decide on
endpoint load balancing you will almost always have one path taking the
majority of the traffic--e.g. all traffic to the file sharer will take
path1 and all traffic to the ntp server will take path2, and path1 will
definitely be more heavily loaded.  To properly balance by endpoint
takes some micromanagement.

Depending on your router, you turn ip route cache on for endpoint
balancing, and turn it off to enable per packet balancing.

Cisco has something called CEF which I have never used, which supposedly
enhances OSPF load balancing--uses special algorithms to speed it up. 

--p

-----Original Message-----
From: devang patel [mailto:devangnp at gmail.com]
Sent: Friday, November 14, 2008 10:52 AM
To: Darden, Patrick S.
Cc: nanog at nanog.org
Subject: Re: OSPF with Multiple ABR & ASBR

Sorry about that!!!

1.  Do these remote areas have multiple paths to the central area for
failover?  E.g. a 10Mbps Metro Ethernet primary link, and a 1.5Mbps DSL
secondary?
2.  Does the central area have multiple routers for failover?  E.g. a
Cisco 7200 for the incoming Metro Ethernet primary connections, and a
Cisco 3660 for the slower secondary connections?
3.  Are there any tie-ins between the remote sites that bypass the
central site?  E.g. site1 and site2 both communicate to the central site
via Metro Ethernet, and they also communicate to eachother via DSL.

Answers:
 I have two T1 line to the non-backbone area and both T1s are terminated
to the two different routers on non-backbone area as well as to backbone
area, and I dont want to achieve primary and secondary role, I want to
go for the load sharing kind of scenario. All sites are connected with
the central site.

ABR means Area border router only.

I am attaching one generalized diagram, please look at that one.
Now I want to achieve the load balancing between the traffic going from
R1 to R8, I want to achieve some of the networks on R1 should be
reachable via R2 and some of them via R3 for the traffic coming from the
R8.  assume all links are same. 

regards
Devang Patel

On Fri, Nov 14, 2008 at 9:29 AM, Patrick Darden < darden at armc.org>
wrote:

First, without any details, it sounds like you might be better off with
static routes than with OSPF.  I'm not trying to be patronizing, but you
don't mention many details, and some of the details you omit are the
crucial ones for OSPF.

1.  Do these remote areas have multiple paths to the central area for
failover?  E.g. a 10Mbps Metro Ethernet primary link, and a 1.5Mbps DSL
secondary?
2.  Does the central area have multiple routers for failover?  E.g. a
Cisco 7200 for the incoming Metro Ethernet primary connections, and a
Cisco 3660 for the slower secondary connections?
3.  Are there any tie-ins between the remote sites that bypass the
central site?  E.g. site1 and site2 both communicate to the central site
via Metro Ethernet, and they also communicate to eachother via DSL.

If none of the above are true, then static routes would be better for
you (for the remote area/s in question).  E.g. area1 has multiple paths,
so ospf is warranted; however, area2 has just one path so a static
approach would usually be better.

Your language seems to indicate that OSPF is warranted (area0, area1,
two ABRs).  I am assuming you mean Area Border Router not Associative
Based Routing (vs. OSPF).  I am also assuming this is a non-public
system (internal network, probably a MAN or WAN).

If so, without any further details, I would set it up for
bandwidth/failover.  Weight the paths appropriately.  Keep it as simple
as you can.  OSPF can become a morass.

If you sketch your situation out more, we can be more helpful....
Campus?  MAN?  How public?  Multi-pathed?  Multi-homed?  Multiple
interlinks?  Are there some lines with reliability problems where the
lower bandwidth links are actually preferred?  Do you have any
decentralized concentration points that might have problems due to
multiple remote sites shuttling traffic through it (due to multiple
interlinks)?

--p 

devang patel wrote:

Hi All,

I am not sure is this the good place to ask this question or not!!!

I am looking for feed back on having OSPF multi-area, lets say if you
have
multiple location in nonbackbone areas and those nonbackbone areas are
connected with the one backbone area. For example: OSPF AREA1 has the
connectivity to OSPF AREA0 using two ABR, so what is the optimum way to
achieve the load balancing or load sharing for traffic entering or
leaving
the area, what are the possible way to configure it?

regards
Devang Patel