OSPF with Multiple ABR & ASBR

Darden, Patrick S. darden at armc.org
Fri Nov 14 11:27:56 CST 2008


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
 






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