RE: OSPF with Multiple ABR & ASBR

["Darden, Patrick S." <darden () armc org>]

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 () gmail com]
Sent: Friday, November 14, 2008 10:52 AM
To: Darden, Patrick S.
Cc: nanog () 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 () 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