[109288] in North American Network Operators' Group
RE: OSPF with Multiple ABR & ASBR
daemon@ATHENA.MIT.EDU (Darden, Patrick S.)
Fri Nov 14 12:28:19 2008
Date: Fri, 14 Nov 2008 12:27:56 -0500
In-Reply-To: <d0fea3580811140752x12c4079an12728e4303f67949@mail.gmail.com>
From: "Darden, Patrick S." <darden@armc.org>
To: "devang patel" <devangnp@gmail.com>
Cc: nanog@nanog.org
Errors-To: nanog-bounces@nanog.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.
=20
So, first step is weight your chosen paths equally, and make sure they =
are preferred over other possible paths.
=20
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.
=20
Depending on your router, you turn ip route cache on for endpoint =
balancing, and turn it off to enable per packet balancing.
=20
Cisco has something called CEF which I have never used, which supposedly =
enhances OSPF load balancing--uses special algorithms to speed it up.=20
=20
--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.=20
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=20
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
=20
