Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Instrukcja Użytkownika Pobierz pdf (Strona 71)

Configuring Global BGP Parameters

308628-15.0 Rev 00

3-27

Best-Route Selection

BGP uses the rules in Table 3-4 as tie-breakers to select the best BGP route. This

table lists the rules in the order in which they are evaluated.

Using the BCC

To configure local preference calculation, go to the BGP prompt (for example,

box; ip; /jointfilesconvert/97454/bgp

) and enter:

local-pref-calculation

<state>

state

is one of the values shown in Table 3-5.

Table 3-4. Best-Route Selection Rules

Default Preference Tiebreaker Rules

(local-pref-calculation enabled)

Calculated Preference Tiebreaker Rules

(local-pref-calculation disabled)

1. Choose the route with the lower route weight.

2. Choose the route with the higher local preference

attribute.

3. Choose the route with the shortest AS path.

4. Choose the route with the smallest ORIGIN

(IGP < EGP < INCOMPLETE).

5. Choose the route with the lower multiexit

discriminator (MED) attribute if both routes

include this optional attribute.

6. Choose the route with the lower interior cost to

the next hop.

7. Choose external BGP over IBGP.

8. Choose the route with the lower BGP identifier.

9. Choose the route with the lower BGP connection

remote address.

10.Choose the route with the lower BGP connection

local address.

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

2. Consider larger BGP administrative weights first.

3. If the routes 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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