Re: AD Intersite replication
- From: "Jmnts" <Jmnts@xxxxxxxxxxx>
- Date: Thu, 20 Apr 2006 01:14:13 +0100
Hi
Please let me contribute for more confusion.... :)
I keep wondering about the following. Please help me clear my doubts.
Suppose I have a single win 2003 domain spanned acrosss two IP sites. There
are two Physical WAN links between these sites, one a High bandwidth &
other a Low bandwidth line.
It doesn't matter in this case because you only have to sites.
(This must be done at Router level - You see in router level you can say
something like this: to this IP Address destination always use the High
bandwidth, if the High bandwidth isn't available then use for the same
IPAddress the Low bandwidth line as an alternative) Are you with me so far?
When I have to create site links using AD Sites & services I create two
logical links in AD & set the cost. I wish to know how does this logical
link map to the Physical WAN links.
It doesn't. (The DC uses the default gateway to go from one place to
another).
So why Site links are important?
Simple:
For example (I'll take the Jorge's example in scenario 2 (Again site links
are only relevant when you have more than 2 sites))
1st thing to understand is: By default, all site links are transitive,
which simply means that if sites A and B are linked and sites B and C are
linked, then site A and site C are transitively linked.
2nd Thing is: Lower costs = to faster and reliable links.
Now:
LocA and LocB are connected by a WAN link WanA-B (10 MB)
LocA and LocC are connected by a WAN link WanA-C (10 MB)
LocB and LocC are connected by a WAN link WanB-C (64KB)
In AD you create site link A-to-B for WANA-B and assign costs 40
In AD you create site link A-to-C for WANA-C and assign costs 40
In AD you create site link B-to-C for WANB-C and assign costs 100
Replication between LocA and LocB will take place over site link A-to-B (and
thus WanA-B)
Replication between LocA and LocC will take place over site link A-to-C (and
thus WanA-C)
Replication between LocB and LocC will take place over site link A-to-B and
site link A-to-C (and thus WanA-B and WanA-C)... Why the cummulative cost of
site link A-to-B and site link A-to-C (40+40=80) is lower than the cost of
site link A-to-C (100)
Can't you see what is happening in link B-to-C for WANB-C?
WANB-C wouldn't be used unless, WanA-B and WanA-C fails.
Now I think that your confusion starts here.
To this scenario we could have for example:
LocA - 192.168.0.0/24 <-> 01Router 10Mb <-> LocB - 192.168.1.0/24
LocA - 192.168.0.0/24 <-> 02Router 10Mb <-> LocC - 192.168.2.0/24
LocB - 192.168.1.0/24 <-> 03Router 64Kb <-> LocC - 192.168.2.0/24
LocA tries to replicate with LocB what would happen?
Simple: uses the 01Router -> LocB.
LocA tries to replicate with LocC what happen?
Simple: uses the 02Router -> LocB.
LocC tries to replicate with LocB what would happen?
Simple: uses the 02Router -> LocA -> 01Router -> then LocB.
Why doesn't use the 03Router (Only one Hop using 03Router)?
Simple: Because you "told" by configuring the cost that is more fast to
reach and replicate to LocB using 02Router and 01Router, than going directly
to LocB using 03Router. And in fact you should take less time to replicate
using this two routers than 03 Router. Right?
So how LocC knows that must use 02Router and 03Router to reach LocB?
It doesn't know. The only information that the Site Links Provide is: To
reach faster to the LocB "we" must go through IPSubnet 192.168.0.0/24 (wich
is LocA), and this is ONE of the important things to understand why is
important to link ipsubnets to sites.
How does he know that it's possible to reach LocB going through LocA?
You already told him that the site link is transitive (remember? That's why
transitive links are important, and that's why you must set up the routers
according with the information that you're providing in Sites Links, when
you "say" that site links are transitive, you must be sure that routers can
reach to the Subnets refered).
I may wish to achive any one of the following.
1] always try to repliate using low bandwidth line first, if unavailable
then use the High bandwidth line or
2] always try to replicate using High bandwidth line first, if unavailable
then use low bandwidth line
LocC Only uses the 03Router if 02Router Fails or 01Router Fails.
What is exactly the cost of the link? Is it used to point to the available
high bandwidth WAN link? or is it used to map a logical link to a physical
kink or what?
Already answered.
FOR MORE INFORMATION:
In Active Directory, site structure mirrors the location of user
communities.
Site structure corresponds to the physical environment and is maintained
separately from the logical environment, which is represented by the domain
structure. Because sites are independent of the domain structure, a single
domain can include a single site or multiple sites, and a single site can
include multiple domains.
Sites have two main roles: to facilitate authentication and the replication
of data between sites.
Site Links
For intersite replication to occur, you must customize how Active Directory
replicates information by setting up site links. Site links are logical,
transitive connections between two or more sites that mirror the network
links and allow replication to occur.
Site Link Transitivity
By default, all site links are transitive, which simply means that if sites
A and B are linked and sites B and C are linked, then site A and site C are
transitively linked.
Site link transitivity is enabled or disabled by selecting the Bridge All
Site Links check box in the Properties dialog box for either the IP or the
SMTP intersite transport. By default, site link transitivity is enabled for
each transport.
Site Links Bridges
Because site links are transitive by default, it is seldom necessary to
create site link bridges. In other words, if site link transitivity is
enabled, then manually creating a site link bridge is redundant and has no
effect. However, if site link transitivity is disabled, you need to manually
create a site link bridge if a transitive link is required to handle your
organization's replication strategy.
Subnets
Computers on TCP/IP networks are assigned to sites based on their location
in a subnet or a set of subnets. Subnet information is used to find a domain
controller in the same site as the computer that is authenticated during the
logon process, and is used during Active Directory replication to determine
the best routes between domain controllers.
Site Link Transitivity
By default, all site links are transitive, which simply means that if sites
A and B are linked and sites B and C are linked, then site A and site C are
transitively linked.
Site link transitivity is enabled or disabled by selecting the Bridge All
Site Links check box in the Properties dialog box for either the IP or the
SMTP intersite transport. By default, site link transitivity is enabled for
each transport.
If you disable site link transitivity for a transport, all site links for
that transport are affected and none of them are transitive. You must
manually create site link bridges to provide transitive replication. The
following are some reasons why you might want to disable site link
transitivity:
To have total control over replication traffic patterns
To avoid a particular replication path, such as a path that involves a
firewall
If your IP network is not fully routed
To ensure efficient replication and fault tolerance, you must configure site
link cost, replication frequency, and replication availability information
for all site links.
Configuring Site Link Cost
Configure site link cost to indicate the cost of the connection in relation
to the speed of the link. Higher costs are used for slow links, and lower
costs are used for fast links. For example, if you have a high-speed T1 line
and a dial-up network connection in case the T1 line is unavailable,
configure a lower cost for the T1 line and a higher cost for the dial-up
network connection. Active Directory always chooses the connection on a
per-cost basis, so the least expensive connection is used as long as it is
available.
Configuring Site Link Replication Frequency
Configure site link replication frequency for site links by providing an
integer value that tells Active Directory how many minutes it should wait
before using a connection to check for replication updates. The replication
interval must be at least 15 and no more than 10,080 minutes (equal to one
week). A site link must be available for any replication to occur, so if a
site link is scheduled as unavailable when the number of minutes between
replication updates has passed, no replication occurs.
--
Best Regards
Systems Administrator
MCSA + Exchange
"Jorge de Almeida Pinto [MVP]"
<SubstituteThisWithMyFullNameSeparatedByDots@xxxxxxxxx> wrote in message
news:%23wLcYR3YGHA.4944@xxxxxxxxxxxxxxxxxxxxxxx
(1) The WAN link B-C is there and it can be used but not primary. See it
as a last resort thing if A-B or A-C goes down
(2) site link are already auto bridged. Check the properties of the IP
container in sites and services. Depending on your environment you might
wanna disable auto site link bridging (e.g. firewalls or a non-fully
routed network)
No need to create a site link bridge as info replicates from B to A and
from A to C and therefore it will get to C. No need for A to directly
replicate to C or vice versa.
(3) if A-B or A-C is down it might use B-C for replication (also see (1))
depending how long the WAN is down, the configuration of the site links,
etc.
--
Cheers,
(HOPEFULLY THIS INFORMATION HELPS YOU!)
# Jorge de Almeida Pinto # MVP Windows Server - Directory Services
BLOG --> http://blogs.dirteam.com/blogs/jorge/default.aspx
-----------------------------------------------------------------------------
* This posting is provided "AS IS" with no warranties and confers no
rights!
* Always test before implementing!
-----------------------------------------------------------------------------
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"Vicky" <Vicky@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:4545ED1B-AFAD-444D-AE18-27F9E17E930B@xxxxxxxxxxxxxxxx
Dear Jorge,
I desperately need to have a clear understanding of AD Intersite
replication. I have read many articles from various sources & my doubts
are
not yet clear. Please dont feel that I am becoming a pain. It would be a
great help to me, if you could resolve my queries.
As per the scenerio 2 that you have given I have the following questions.
1] why would you at all create a site link between site B & C if AD is
going
to use A to B & A to C link to replicate between B to C
2] Are you tryiing to tell me that we need to create a Site Link Bridge
to
merge site links A to B & A to C so that site B & C could replicate.
3] also if the WAN link between A to B is down, then how would
replication
between site B & C happens?
"Jorge de Almeida Pinto [MVP]" wrote:
OK.....two simple scenarios
scenario 1
-> Location A
-> Location B
LocA and LocB are connected by a redundant WAN link WanA (10 MB) and
WanB
(64KB)
In AD you create site A and B and you create site link A-to-B and
configure
its properties accordingly.
In AD it is not possible to map/link site link A-to-B to one of the WAN
links. It just wants to replicate between the two locations and it does
not
care which WAN link is used. To determine which WAN to use you should
configure this at ROUTER level where you configure something like always
use
WanA and if WanA is not working use WanB. This can not be configured in
AD.
Configuring two site links with different costs does not help in any way
as
AD will always use the site link with the lowest cost to create its
connection objects accordingly
scenario 2
-> Location A
-> Location B
-> Location C
LocA and LocB are connected by a WAN link WanA-B (10 MB)
LocA and LocC are connected by a WAN link WanA-C (10 MB)
LocB and LocC are connected by a WAN link WanB-C (64KB)
In AD you create site A for LocA
In AD you create site B for LocB
In AD you create site C for LocC
In AD you create site link A-to-B for WANA-B and assign costs 40
In AD you create site link A-to-C for WANA-C and assign costs 40
In AD you create site link B-to-C for WANB-C and assign costs 100
Replication between LocA and LocB will take place over site link A-to-B
(and
thus WanA-B)
Replication between LocA and LocC will take place over site link A-to-C
(and
thus WanA-C)
Replication between LocB and LocC will take place over site link A-to-B
and
site link A-to-C (and thus WanA-B and WanA-C)... Why the cummulative
cost of
site link A-to-B and site link A-to-C (40+40=80) is lower than the cost
of
site link A-to-C (100)
more info on AD replication:
http://technet2.microsoft.com/WindowsServer/en/Library/1f3bb1c1-ba8a-4b4e-9f23-f240566e3d661033.mspx
--
Cheers,
(HOPEFULLY THIS INFORMATION HELPS YOU!)
# Jorge de Almeida Pinto # MVP Windows Server - Directory Services
BLOG --> http://blogs.dirteam.com/blogs/jorge/default.aspx
-----------------------------------------------------------------------------
* This posting is provided "AS IS" with no warranties and confers no
rights!
* Always test before implementing!
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
"Vicky" <Vicky@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:A3519691-E6B0-46F2-A1D5-C1A82223D963@xxxxxxxxxxxxxxxx
Dear Jorge,
My question to you is,
why would we then create two site links between two sites, if AD is
always
going to use the site link with the lowest cost.
I can understand that two AD site links can have different replication
timing set.
When would AD use a site link with a higher cost?
What do you mean by "best path for replication" ? If Site links have
nothing to do with the physical WAN links?
"Jorge de Almeida Pinto [MVP]" wrote:
sites and site links do not have a direct link to a physical location
or
a
WAN line...
So if you have two site links with the same sites, AD will always use
the
sitelink with the lowest cost to setup the connection objects.
AD is not aware of the network and does not care either
site links are used to help AD determine the best path of replication
and
some other things like when to replicate.
in your case you should have ONE site link and the determination of
which
WAN to use should be configured at network level on the router
--
Cheers,
(HOPEFULLY THIS INFORMATION HELPS YOU!)
# Jorge de Almeida Pinto # MVP Windows Server - Directory Services
BLOG --> http://blogs.dirteam.com/blogs/jorge/default.aspx
-----------------------------------------------------------------------------
* This posting is provided "AS IS" with no warranties and confers no
rights!
* Always test before implementing!
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
"Vicky" <Vicky@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:2A3DCDB1-9CBC-47AB-A4F3-A430D747A2FC@xxxxxxxxxxxxxxxx
I keep wondering about the following. Please help me clear my
doubts.
Suppose I have a single win 2003 domain spanned acrosss two IP
sites.
There
are two Physical WAN links between these sites, one a High
bandwidth &
other
a Low bandwidth line.
When I have to create site links using AD Sites & services I create
two
logical links in AD & set the cost.
I wish to know how does this logical link map to the Physical WAN
links.
I may wish to achive any one of the following.
1] always try to repliate using low bandwidth line first, if
unavailable
then use the High bandwidth line or
2] always try to replicate using High bandwidth line first, if
unavailable
then use low bandwidth line
How is the above possible?
What is exactly the cost of the link? Is it used to point to the
available
high bandwidth WAN link? or is it used to map a logical link to a
physical
kink or what?
.
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