Allied Telesis AT-8824 Guía de instalación descargar pdf (Pagina 46)

46 BGP Enhancements Release Note

Software Version 2.7.5

C613-10454-00 REV A

BGP Enhancements

Software Version 2.7.5 includes the following enhancements for BGP:

■ Changes to Algorithm for Determining the Best Route

■ Automatic Summarising: Advertising as Few Routes as Possible

■ Importing and Advertising the Default Route

This section describes each enhancement, then the new and modified

commands in Command Reference Updates.

Changes to Algorithm for Determining the Best Route

When multiple routes to a destination exist, BGP now uses the rules in the

following table to determine which route is the best one. If a rule results in

selection of a single route, the router or switch uses that route. If multiple

routes still match, the router or switch goes to the next rule.

Rule For this... the router or switch chooses the route that...

1 local_preference has the highest local preference. How the router or

switch determines the local preference depends on the

source of the route:

• For routes that the router or switch learned via an

EBGP session, or for routes it learned from sources

such as an IGP or static configuration, the router or

switch calculates the value of the preference itself.

• For routes that the router or switch learned from an

IBGP peer, the router or switch uses the preference

supplied by the peer—the update message for that

route contains a local_preference attribute indicating

the degree of preference.

2 route type came into the BGP routing table from a preferred source.

The order of preference is:

a. routes imported into the BGP routing table from the

router or switch’s RIB, using BGP import or network

entries

b. routes learned from an IBGP or confederation peer

c. routes learned from an EBGP peer

d. routes learned through a BGP aggregate entry

3 AS_path has the shortest AS path.

4 origin has the preferred origin. The order of preference is:

a. IGP

b. EGP

c. INCOMPLETE

5 Multi_Exit_Discriminator

value

has the lowest MED value. This rule applies if the local

system is configured to take into account the value of the

Multi_Exit_Discriminator (MED), and if the multiple

routes are learned from the same neighbouring AS.

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Allied Telesis AT-8824 Guía de instalación Pagina 46

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