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A.2.2 Tagged Interior Gateway Protocol Connected: An Internet Encyclopedia
A.2.2 Tagged Interior Gateway Protocol

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A.2.2 Tagged Interior Gateway Protocol

A.2.2 Tagged Interior Gateway Protocol

Certain IGPs can tag routes exterior to an AS with the identity of their exit points while propagating them within the AS. Each border gateway should use identical tags for announcing exterior routing information (received via BGP) both into the IGP and when propagating this information to other internal peers (peers within the same AS). Tags generated by a border gateway must uniquely identify that particular border gateway--different border gateways must use different tags.

All Border Gateways within a single AS must observe the following two rules:

  1. Information received from an internal peer by a border gateway A declaring a set of destination associated with a given address prefix to be unreachable must immediately be propagated to all of the external peers of A.

  2. Information received from an internal peer by a border gateway A about a set of reachable destinations associated with a given address prefix X cannot be propagated to any of the external peers of A unless/until A has an IGP route to the set of destinations covered by X and both the IGP and the BGP routing information have identical tags.

These rules guarantee that no routing information is announced externally unless the IGP is capable of correctly supporting it. It also avoids some causes of "black holes".

One possible method for tagging BGP and IGP routes within an AS is to use the IP address of the exit border gateway announcing the exterior route into the AS. In this case the "gateway" field in the BGP UPDATE message is used as the tag.

An alternate method for tagging BGP and IGP routes is to have BGP and the IGP agree on a router ID. In this case, the router ID is available to all BGP (version 3 or higher) speakers. Since this ID is already unique it can be used directly as the tag in the IGP.


Next: A.2.3 Encapsulation

Connected: An Internet Encyclopedia
A.2.2 Tagged Interior Gateway Protocol

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