aws direct connect site link routing

Question

Dear Team - As per https://aws.amazon.com/blogs/networking-and-content-delivery/advanced-routing-scenarios-with-aws-direct-connect-sitelink/,  in the Rule Priority section,

`Alternatively, you can prepend AS path to the routes advertised from CE1 and CE2, to achieve the same (figure 6) routing behavior also shown in the following figure 7.  Please note if CE2 advertises the prefix 10.0.0.0/24 to ALD1 with prepending its ASN twice then **ALD1 prefers the path through CE2** compared to the path received from ALD3 (as described in Rule 1).`

is the above correct statement ?  because as per the figure7, traffic prefer the NY location CE4 but in the blog, **ALD1 prefers the path through CE2 compared to the path received from ALD3** .  this does not match with what the green path in figure7.

Can any one please help to clear the doubt.

Thanks

Accepted Answer

When SiteLink is enabled, traffic will prefer the path with the lowest AS path prepend count. In this case:

* CE2 prepends its ASN twice before advertising 10.0.0.0/24 to ALD1.
* ALD3 prepends its ASN once before advertising the same prefix to ALD1.
* So ALD1 will prefer the path through CE2, since it has a lower AS path prepend count (2 vs 1).

Figure 7 shows traffic taking the green path through CE4/NY instead of CE2. This is likely because CE4's configuration results in an even lower prepend count.

The blog statement about ALD1 preferring CE2's path is correct, but Figure 7 depicts a different scenario where CE4 provides the optimal path. SiteLink always selects the path with the fewest prepends.

aws direct connect site link routing

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