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BGP Route Selection Rules

BGP Route Selection Rules

Created: Oct 10, 2019 02:34:34Latest reply: Oct 11, 2019 17:44:31 1392 2 0 0 0
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Hi all,


These are the BGP route selection, does anyone have a good Mnemonic for memorizing these?


pDiscards routes with unreachable next hops.
pPrefers the route with the largest PrefVal value. The PrefVal attribute is a Huawei proprietary attribute and is valid only on the device where it is configured.
pPrefers the route with the highest Local_Pref.
pPrefers the following routes in descending order of priority: manually aggregated route, automatically aggregated route, route imported using the network command, route imported using the import command, and route learned from peers.
pPrefers the route with the shortest AS_Path.
pPrefers the route with the Origin attribute IGP, EGP, and Incomplete in sequence.
pPrefers the route with the smallest MED if routes are received from the same AS.
pPrefers an EBGP route to an IBGP route.
pPrefers the route with the smallest IGP metric within an AS.
pPrefers the route with the shortest Cluster_List.
pPrefers the route with the smallest Originator_ID.
pPrefers the route advertised by the router with the smallest Router_ID.

pPrefers the route learned from the neighbor with the lowest IP address.

Best regard!


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Popeye_Wang
Admin Created Oct 10, 2019 02:50:18

Hello@rob9711,

I think it's a good idea to memorize by abbreviated alias.BGP Route Selection Rules-3078363-1

OPPAAAOMTCCRA


bgp


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Popeye_Wang
Popeye_Wang Admin Created Oct 10, 2019 02:50:18

Hello@rob9711,

I think it's a good idea to memorize by abbreviated alias.BGP Route Selection Rules-3078363-1

OPPAAAOMTCCRA


bgp


If you have any problems, please post them in our Community. We are happy to solve them for you!

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This article contains more resources

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wissal
wissal MVE Created Oct 11, 2019 17:44:31

Hello,

Please be informed that we don't need to memorize all these

When multiple routes are available to the same destination, BGP selects one optimal route based on BGP route selection rules and adds it to the IP routing table for traffic forwarding.

Figure 10-18 shows how the optimal route is selected.
Figure 10-18  BGP route selection process
imgDownload?uuid=ca8f0e4ee30c44dd9878177

BGP selects routes by comparing route attributes in a fixed order. When a route attribute is a sufficient condition for determining the optimal route, BGP does not compare the other attributes; If BGP fails to select the optimal route after comparing all route attributes, the route that was first received is selected as the optimal route.Table 10-5 lists the abbreviated alias, route selection rules, and remarks of each matching item. Table 10-5 shows that the route priority is directly proportional to the PreVal or Local_Pref value and inversely proportional to the rest of the attribute values or lengths. In addition, the first column can be summarized as a character string (OPPAAA OMTCC RA), which helps memorize the matching sequence.

Table 10-5  BGP route selection process

Abbreviated Alias

Matching Item

Route Selection Rules

Remarks

O

Origin AS

Valid > NotFound > Invalid

BGP origin AS validation states are applied to route selection in a scenario where the device is connected to an RPKI server.

P

PrefVal

The route with the largest PreVal value is preferred.

The default value is 0.

PrefVal is Huawei-specific and valid only on the device where it is configured.

P

Local_Pref

The route with the largest Local_Pref value is preferred.

The default value is 100.

To modify the default Local_Pref value of BGP routes, run the default local-preference command.

A

NOTE:
A is the initial of the character string (ASNIL).

Route type

A > S > N > I > L, in which:
  • A: indicates that routes are summarized using the aggregate command.
  • S: indicates that routes are summarized using the summary automatic command.
  • N: indicates that routes are imported using the network command.
  • I: indicates that routes are imported using the import-route command.
  • L: indicates that routes are learned from BGP peers.

-

A

Accumulated Interior Gateway Protocol (AIGP)

The route with the smallest AIGP value is preferred.

The route with AIGP to the route without AIGP is preferred.

-

A

AS_Path

The route with the shortest AS_Path length is preferred.

If the bestroute as-path-ignore command is configured, BGP does not compare the AS_Path attribute.

O

Origin

IGP > EGP > Incomplete

-

M

Multi Exit Discriminator (MED)

The route with the smallest MED value is preferred.

The default value is 0.

If the bestroute med-none-as-maximum command is configured, BGP considers the largest MED value (4294967295) as the MED of the route that does not carry an MED.

For details about MED usage, see MED.

T

Peer type

EBGP > IBGP

-

C

IGP metric

The route with the smallest IGP cost is preferred.

If the bestroute igp-metric-ignore command is configured, BGP does not compare the IGP cost.

C

Cluster_List

The route with the shortest Cluster_List length is preferred.

By default, Cluster_List takes precedence over Originator_ID during BGP route selection. To enable Originator_ID to take precedence over Cluster_List during BGP route selection, run the bestroute routerid-prior-clusterlist command.

R

Router ID

The route with the smallest router ID is preferred.

If routes carry the Originator_ID, the originator ID is substituted for the router ID during route selection. The route with the smallest Originator_ID is preferred.

A

Peer IP address

The route learned from the peer with the smallest IP address is preferred.

-

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