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Configuring IP Services

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Disabling BayRS Local Preference Calculation and Route Selection

A BGP speaker calculates a local preference value for each route that it receives

from an external peer, and passes this value as the local_pref attribute in routing

updates that it announces to its internal BGP (IBGP) peers. A BGP speaker that

receives a routing update from an IBGP peer uses the local_pref attribute in its

best-route selection process.

By default, BGP calculates a local-preference value by using the algorithm

described in “BGP-4 Local Preference Calculation” on page 8-8. The best-route

selection process is described in “Best-Route Selection” on page 8-9.

Different implementations of BGP have different ways of arriving at a local

preference value. However, within an AS, all BGP speakers should use the same

method to determine local preference and to select the best routes. If you are

configuring BayRS BGP in a network that also includes routers from other

vendors, you can use the BCC or Site Manager to disable BayRS local-preference

calculation and route selection. Instead, BGP assigns a value of 100 to externally

received routes and uses the following formula for best-route selection:

1. If the next hop is inaccessible, do not consider it.

2. Consider larger BGP administrative weights first.

3. If the routers have the same weight, consider the route with the highest local

preference.

4. If the routes have the same local preference, prefer the route that the local

router originated.

5. If no route was locally originated, prefer the shorter AS path.

6. If the routes have the same AS path length, prefer the lowest origin code (IGP

is preferred over EGP, which is preferred over incomplete.).

7. If the origin codes are the same and all paths come from the AS, prefer the

path with the lowest multiexit discriminator (MED) metric. Treat a missing

MED metric as 0.

8. If the MED metrics are the same, prefer external paths over internal paths.

9. If IGP synchronization is disabled and only internal paths remain, prefer the

path through the closest neighbor.

10. Prefer the route with the lowest IP address value for the BGP router ID.

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