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Route summary

ArnoldJSimpson
Beginner
Beginner

Some providers may advertise a specific route from within the summary route, along with this summary route. Why the provider may choose to advertise in this manner?

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JimWicks
Beginner
Beginner

Routing will always prefer the longest prefix match, so fo example if an organization has a /24 prefix they want to make reachable over two peering links, then they would advertize that prefix to both peers, but they could also advertize a /25 covering the lower half of their address range only towards Peer-A if (for some made up reason) they wanted inbound traffic to that sub-set of their address-range to prefer that link to Peer-A................if the link to Peer-A failed then all traffic to the /24 range would use the remaining available link as it is still advertizing the full /24 range.

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balaji.bandi
VIP Community Legend VIP Community Legend
VIP Community Legend

As per i know when you like to do Traffic Engineering over different Router, may require leaking some routes, so it can be done based on the TE.  The advertisement of a subset of more specific routes in addition to performing the summary. Leaking routes can be done at the process by explicitly stating the prefixes to suppress, or on a neighbor level by indicating which prefixes should not be suppressed.

 

 

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JimWicks
Beginner
Beginner

Routing will always prefer the longest prefix match, so fo example if an organization has a /24 prefix they want to make reachable over two peering links, then they would advertize that prefix to both peers, but they could also advertize a /25 covering the lower half of their address range only towards Peer-A if (for some made up reason) they wanted inbound traffic to that sub-set of their address-range to prefer that link to Peer-A................if the link to Peer-A failed then all traffic to the /24 range would use the remaining available link as it is still advertizing the full /24 range.

Generally for the reason, and example, @JimWicks describes.

I.e. for some reason, you want to route a subnet prefix of summarized prefix differently.

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