Configure Site Link Costs

SiteLink objects can be assigned a cost, which is an administrative setting that gives priority to one link over another, where multiple paths between the same two sites exist. The lowest cost, intuitively enough, has the highest priority. When you assign costs to your links, consider available bandwidth, reliability, latency, and actual costs charged by your ISP based on time or amount of data crossing the line (if applicable). If you leave the default link costs set to 100, or assign costs without proper planning, this may result in large replication streams being directed through unreliable or overutilized WAN links. Table 6-3 shows some of the suggested cost link values.

You can assign site link costs in the inter-site transport link properties, on the same tab where you configure the replication interval (as was shown in earlier Figure 6-17).

Site Link Bridging

Site link cost configuration becomes more complicated as you factor in more sites, so by default, all site links are bridged. If you have Site A, Site B, and Site C, and there is a site link X defined between Site A and Site B, and another site link Y defined between Site B and Site C, Active Directory assumes that there is a virtual link between Site A and Site C, with a combined cost of X + Y.

Site link bridging allows you to maintain a smaller set of site links. You don't have to define all possible links involving all existing sites, but instead use just one link to connect each branch office to the corporate head office. In fact, this is very similar to transitivity in trust relationships. Figure 6-21 shows the benefits of this feature.

If bridging is used, administrators will only have to assign costs to three links; the rest will be taken care of by KCC. Otherwise, you need to define and calculate costs for six links; and if you don't, replication will not be transitive (meaning that we essentially achieve hub-and-spoke configuration here, where branch offices only

TABLE 6-3 |

Link

Cost

Recommended

LAN, backbone

1

Site Link Costs

T1 to backbone

200

(orders of

56 Kbps

500

magnitude)

Branch office

100

International link

5000

Site link bridging

Site link bridging

Site Link Versus Site Link Bridge

Bridged site

Link 3

Bridged site

Link 3

replicate with the head office). Without bridging, the number of site links that must be defined manually increases progressively with the addition of new sites.

If Site A and Site C had a manual site link defined in addition to bridging, the replication path between Site A and Site B would depend on the costs of links between Site A and Site C (Z), and Site C and Site B (Y). If the cumulative cost of (Z + Y) is less than direct connection between Site A and Site B, then replication traffic takes a detour. So instead of replicating directly from Site A to Site B, it will replicate first to Site C, and then to Site B. If you have an infrastructure with many sites and some sites have more than one site link to other sites, ISTG will automatically calculate and choose the least-cost route from any site to any site. If one of the links goes down, traffic will be rerouted using other site links, if possible.

Bridging can be harmful in situation where one or more branch offices can't route or "see" other branch offices, as explained in previous chapters. To turn off default bridging on all site links, you need to turn off the Bridge All Site Links setting in the IP or SMTP container properties, as shown in Figure 6-22, and then define site link bridge groups manually.

After you configure the Site Link Bridge setting manually for individual links, KCC will again be able to use these links to create automatic transitive site links, leaving out those links not included in the group. To create a new site link bridge, right-click the transport container (IP or SMTP), and choose New Site Link Bridge. A screen will prompt you to assign a name for the bridge group and add two or more site links for which bridging should be re-enabled.

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Responses

  • Bell
    What is sitelink and in groups of three students per group,configure a manual site link bridging?
    2 years ago
  • jana
    What is active directory site cost?
    1 year ago
  • FIKRU
    How to set a lower cost on an active directory site?
    1 year ago
  • balbo
    What happens when all site link costs are the same?
    8 months ago
  • niamh
    How to find replication links and cost?
    6 months ago
  • Amanda
    What is cost and interval in ad site and services?
    5 months ago
  • haylom
    How kcc find link cost in active directory?
    3 months ago
  • robert
    How do we choose site and service cost in Active directory?
    1 month ago

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