1. BGP Routing Policies

2. Foreword
BGP can be used to combine many policy tools, use BGP path attributes such as AS_PATH, COMMUNITY and so on as well as filter routes received from or sent to peers. This section will introduce many of the policy mechanisms within BGP to support manipulation of all traffic.
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3. Objectives Upon completion of this section, you will be able to:
Understand the BGP route selection process
Understand BGP filters
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4. Contents Review of the BGP Route Selection Process BGP path selection
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5. Review of the BGP Route Selection Process
If the route’s next hop is unreachable, ignore it
Select the route with highest Preferred-Value
Select the route with highest local preference
Choose aggregate routes over non aggregate routes
Local manual aggregation routing priority takes precedence over local automatic aggregation routing
Network command presides over import-route command in local route import
Select the route with the shortest AS_PATH.
BGP compares the origin attribute of the route and selects the route with the lowest origin attribute code: IGP is lower than EGP, which is lower than Incomplete.
Select the route with the lowest MED value.
1. Select the route with highest Preferred-Value
2. Select the route with highest local preference
3. Prefer the aggregated route to the detailed routes
1. Prefer the route with the shortest AS_PATH
2. BGP compares the origin attribute of the route and selects the route with the lowest origin attribute code: IGP is lower than EGP, which is lower than Incomplete.
3. Select the route with the lowest MED value.
4. Prefer EBGP routes over IBGP routes
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6. Review of the BGP Route Selection Process (cont.)
Prefer the route with the shortest path to the BGP NEXT_HOP. This is the route with the lowest IGP metric to the next-hop router.
When all the above are identical, they are equivalent route and can be used for load balancing
Note: AS_PATH must be the same
The last 3 route selection rules can be ignored when load balancing is implemented
Selects the route with the shortest cluster-list
Prefer the route with smaller Originator ID. Select the route with smaller router ID if Originator ID is unavailable.
Prefer the route with smaller peer IP address.
8. Prefer the route with the shortest path to the BGP NEXT_HOP. This is the route with the lowest IGP metric to the next-hop router.
When all the above are identical, they are equivalent route and can be used for load balancing.
Note: AS_PATH must be the same
The last 3 route selection rules can be ignored when load balancing is implemented
9. Selects the route with the shortest cluster-list
10. Prefer the route with smaller Originator ID. Select the route with smaller router ID if Originator ID is unavailable.
11. Prefer the route with smaller peer IP address.
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7. Contents Review of the BGP Route Selection Process BGP path selection
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8. BGP Route Selection Parameters
The important parameters that affect the BGP route selection
Preferred Value
Local-Preference
AS-Path
Origin
MED
EBGP/IBGP
IGP Cost
Cluster-list
Communities
Base on the BGP route selection process and the common BGP path attributes, 9 important BGP parameters that affect the BGP route selection are as follow:
•Preferred Value
•LOCAL_PREF
•AS_PATH
•ORIGIN
•MED
•Is the neighbor type EBGP or IBGP?
•Internal cost of IGP
•Cluster-list
•COMMUNITY
The parameters specified above can directly affect the BGP route selection.
Among them, the most common used parameters are LOCAL_PREF, AS_PATH and MED. The function and the configuration of these parameters will be discussed in detail in the following slides.
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9. BGP Local-Preference
default local-preference command is used to configure the BGP default local preference, higher values are preferred.
[Router-bgp] default local-preference preference
By default, the local preference value is 100.
Configuring different local preference values will affect the BGP route selection. When multiple routes to reach a particular network exist on a BGP router, the route with higher local preference will be selected.
Local preference is used only within an AS between IBGP peers, it will not be advertized to other AS`.
LOCAL_PREF attribute is used within an AS between IBGP peers to determine the best path for the data traffic to exit an AS. LOCAL_PREF attributes become very useful when there are multiple routes to leave the AS and reach an outside
network.
default local-preference command is used to configure the BGP default local preference, higher values are preferred.
The range of the LOCAL_PREF value on Huawei equipments are from 0 to Its default value is 100
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10. Configure the Default Value of the Local-Preference
[RTB]bgp 200
[RTB-bgp]default local-preference 2000
AS 200
Local-Pref:2000
AS 100
RTB
/32
RTA
As shown in the diagram above, router D will receive 2 different update information to reach the network /32 from RTB and RTC respectively.
By default, RTD will perform the BGP route selection. The selected best route will be responsible to send the data to the destination /32. Both RTB and RTC change their default local preference value by using the “default localpreference”
command . The local preference of RTB has been changed to 2000 while the local preference of RTC has been changed to As a result, RTD receives 2 routes that carry different local preference value toward /32.
One of the routes carries the local preference 2000 with the IP prefix of RTB as the next hop IP address. Another route carries the local preference 1000 with the IP prefix of RTC as the next hop IP address. In this circumstance (all other parameters adopt the default value), the route with RTB as next hop IP address will be selected as the best route which in charge of the data transmission to reach /32. This is because the highest local preference is preferred.
Local-Pref:1000
RTD
[RTC]bgp 200
[RTC-bgp]default local-preference 1000
RTC
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11. Configuring Local-Preference via Policy
AS 200
NLRI /24
Local-Pref:2000
AS 100
RTB
RTD
/24
/24
RTA
NLRI /24
Local-Pref:2000
LOCAL_PREF is an important parameter that affect the BGP route selection within an AS. BGP can work together with some policy tools to implement load balancing in some complex network.
As shown in the diagram above, RTD can reach /24 and in AS100 via 2 different routes. We can set the next hop IP address as RTB for the data to reach /24 and set the next hop IP address as RTC for the data to reach /24. These can be achieved by configuring the local
preference via policy.
RTC
RTD can reach AS100 via 2 different routes.
The next hop is RTB for the traffic to reach /24.
The next hop is RTC for the traffic to reach /24.
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12. Policy Configuration on RTB#
acl number 2000
rule 5 permit source
bgp 200
peer as-number 100
peer as-number 200
ipv4-family unicast
undo synchronization
peer enable
peer route-policy test1 import
route-policy test1 permit node 10
if-match acl 2000
apply local-preference 2000
route-policy test1 permit node 20
apply local-preference 1000
Explanation of the configuration: #
acl number 2000
rule 5 permit source
Specify the range of IP addresses that match the ACL 2000.
bgp 200
peer as-number 100
peer as-number 200
ipv4-family unicast
undo synchronization
peer enable
peer route-policy test1 import
Apply route policy test1 for the route information received from peer
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13. Policy Configuration on RTC#
acl number 2000
rule 5 permit source
bgp 200
peer as-number 100
peer as-number 200
ipv4-family unicast
undo synchronization
peer enable
peer route-policy test1 import
route-policy test1 permit node 10
if-match acl 2000
apply local-preference 2000
route-policy test1 permit node 20
apply local-preference 1000
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14. BGP MED default med command is used to configure the default MED value
[Router-bgp] default med med
By default, the MED value is 0.
Configuring different MED values will affect the BGP route selection.
The lowest MED value is preferred. The MED value is considered as metric or cost. We prefer the route with lowest cost.
MED attributes that sent to an EBGP peer, will only be seen within that AS. It is not passed beyond the receiving AS.
MED attribute is used to determine the entry point to an AS. It is used when multiple paths exist between 2 AS’ and one AS is trying to influence the entry point from another AS. MED attribute is considered as metric or cost. Its value is in the range of 0 to The lowest MED value is preferred. By default, the
MED value is 0.
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15. Configuring MED via Policy
Prefix/Mask Med / /
AS 100
AS 200
RTA
RTB
RTE
RTF
/24
/24
Prefix/Mask Med / /
The MED value is configured in AS100 via policy to influence the BGP route selection in AS200. The configuration of MED value realize the incoming traffic control when multiple paths exist.
There are 2 networks in AS100: /24 and /24. The routing updates of these 2 networks is sent to AS200 via RTA and RTC. On RTA, the MED value is configured as 1000 for the data to reach /24 and the
MED value is configured as 2000 for the data to reach /24. On RTC, the MED value is configured as 2000 for the data to reach /24 and the MED value is configured as 1000 for the data to reach /24. After the configuration, the traffic of RTF in AS200 can reach via RTC and the data traffic of RTF in AS200 can reach via RTA. In AS100, the load balancing is implemented base on the incoming traffic.
RTC
RTD
The MED value is configured in AS100 via policy to influence the BGP route selection in AS200. The configuration of MED value realize the incoming traffic control when multiple paths exist.
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16. Policy Configuration on RTA#
bgp 100
peer as-number 200
peer as-number 100
peer as-number 100
ipv4-family unicast
undo synchronization
peer enable
peer route-policy test1 export
peer enable
peer enable
route-policy test1 permit node 10
if-match ip-prefix 1
apply cost 2000
route-policy test1 permit node 20
apply cost 1000
ip ip-prefix 1 index 10 permit greater-equal 24 less-equal 24
Explanation of the configuration: #
bgp 100
peer as-number 200
peer as-number 100
peer as-number 100
ipv4-family unicast
undo synchronization
peer enable
peer route-policy test1 export \\apply rout e
policy test1 for the route sent to peer
peer enable
peer enable
route-policy test1 permit node 10
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17. Policy Configuration on RTC#
bgp 100
peer as-number 200
peer as-number 100
peer as-number 100
ipv4-family unicast
undo synchronization
peer enable
peer route-policy test1 export
peer enable
peer enable
route-policy test1 permit node 10
if-match ip-prefix 1
apply cost 2000
route-policy test1 permit node 20
apply cost 1000
ip ip-prefix 1 index 10 permit greater-equal 24 less-equal 24
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18. AS-PATH Filter
We can define multiple filtering policies (permit or deny) under the same as-path-filter number. The operation “OR” is applied during the matching process. This indicates that at least one occurrence is required to make the test true. No occurrence indicates that the test failed
AS_PATH Filter uses the regular expression method to filter the AS_PATH attribute information.
We can define multiple filtering policies (permit or deny) under the same as-pathfilter number. The operation “OR” is applied during the matching process. This indicates that at least one occurrence is required to make the test true. No occurrence indicates that the test failed AS_PATH Filter uses the regular expression method to filter the AS_PATH attribute information.
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19. Regular Expressions
Regular expressions is a type of BGP filtering method.
Regular expressions are a formula used to match the string according to certain rules. The decision process is done (permit or deny) on the AS_PATH attribute of the BGP route, based on the matched string. In fact, we can regard the regular expression as an ACL for the AS_PATH.
Multiple permit or deny filters can be defined using regular expressions. The operation “OR” is applied for the matching process.
Regular expression is a formula used to match the string according to certain rules. Next, the decision process is done (permit or deny) on the AS_PATH attribute of the BGP route base on the matched string.
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20. Regular Expressions Characters Explanation ^
Match the beginning of a string. For example “^200” indicates match only if the first value of the AS_PATH is 200. $
Match the end of a string. For example “200$” indicates match only if the last value of the AS_PATH is 200. .
Match any single character, including white space. +
Matches one or more sequences of the pattern. _
Match any delimiters. For example, comma, bracket, white space and so on. *
Match zero or more sequences of the pattern.
( )
To group smaller regular expressions into larger regular expressions, ”()” is often used together with “|”. |
It is an operator that means “or”.
[ ]
Enable you to specify a range of single characters., ”[]” is often used together with “-”. -
Separates the end points of a range.
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21. ip as-path-filter 1 permit ^12.*74$
String Matching (1)
^ (Caret) matches the beginning of a line
^ regular expression
$ (Dollar) matches the end of a line.
$ regular expression
ip as-path-filter 1 permit ^12.*74$
Match the AS number that starts with sequence12 and ends with sequence 74
“^” represents start of a line. The regular expression ^12 can match sequence 12 of the first AS number 123 in AS_PATH ( ). In other words, the AS number of the routing information must be start with sequence 12.
“$” represents the end of a line. The regular expression 74$ can match sequence 74 of the last AS number 2374 in AS_PATH ( ). In other words, the AS number of the routing information must be end with sequence 74.
“^” is used at the front of a group of matching symbols.
Example
AS_PATH ( )
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22. ip as-path-filter 1 permit 23|43
String Matching (2)
| (Bar) is used to specify an OR operation.
Regular expression 1| Regular expression 2
ip as-path-filter 1 permit 23|43
Match AS23 or AS43
Regular expression 23|43 matches AS23 or AS43.
AS shown in the example above, the regular expression 23|43 matches AS_PATH ( ) for 3 times. Regular expression 23|43 can match some characters of the whole AS number. For example, it matches 43 of the AS743. In addition, it matches the 23 of the AS123 and AS2374.
Example
AS_PATH ( )
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23. ip as-path-filter 1 permit [1-3][47]
String Matching (3)
[ ] (Brackets) enable us to specify a range of single characters., ”[]” is often used together with “-”.
- Hyphen
[regular expression 1-regular expression 2]
ip as-path-filter 1 permit [1-3][47]
Match AS number 14, 17, 24, 27, 34 or 37
Regular expression [1-3] matches the character “1”, “2” or “3”. Regular expression [47] matches the character “4” or “7”. When used together, regular expression [1- 3][47] matches the AS number of 14, 17, 24, 27, 34 or 37.
Example
AS_PATH ( )
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24. ip as-path-filter 1 permit _34512 170$
String Matching (4)
_ (Underscore) matches any delimiters. For example, comma, bracket, white space and so on.
ip as-path-filter 1 permit _ $
AS34512 and AS170 are directly connected
“.”and “_”are different in that “.”can be used to match any single character, including white space while “_”is used to match any delimiters. Example of the delimiters are comma, left parenthesis, right parenthesis, the beginning of theinput string, the end of the input string, space and so on.
"_ $"among AS-PATH( ) indicates that AS and AS170 are indicates connected ."_" indicates any delimiters. The underscore in thisregular expression represents the white space in between the " ".
Example
AS_PATH ( )
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25. ip as-path-filter 1 permit [1-3].[47]
String Matching (5)
. (Dot) matches any single character, including space.
ip as-path-filter 1 permit [1-3].[47]
AS_PATH ( )
The "."sign in the middle of the regular expression [1-3].[47] can match any single character, including space. So the AS_PATH ( ) matches the regular expression [1-3].[47] 3 times: “1 7”, “237”and “374”.
AS_PATH ( )
AS_PATH ( )
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26. ip as-path-filter 1 permit _621 .* 170$
String Matching (6)
* (Asterisk) matches zero or more sequences of the pattern.
+ (Plus) matches one or more sequences of the pattern.
ip as-path-filter 1 permit _621 .* 170$
Accept the route originating from AS170, and traverses AS621
In the diagram above, the most difficult part is the “.+” sign in the middle of the regular expression. As per discussed earlier, “.” sign in the regular expression matches any single character including white space while “+” sign matches one or more
sequences of the pattern. Therefore, “.+” sign matches multiple single characters including white space or sequence of the pattern. In the example above, the “.+” sign matches “ 743
34512 ” of the AS_PATH ( ).
AS_PATH ( )
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27. Common Used Regular Expressions
Meaning
＝＝＝＝＝＝＝＝＝|| ＝＝＝＝＝＝＝＝＝＝＝＝＝ ^$ .*
_10_
^10$
^10_
^[0-9]+$
Match the routes originated in local AS
Match all routes
Match the routes traversing AS10
Match the routes with only AS10 in AS_PATH
Match the routes received from AS10 directly
Match the routes with only one AS in AS_PATH
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28. AS 200 AS 100 AS 300 AS 400 AS-Path Filter RTB RTC RTA RTD
AS 100
AS 300
RTB
/24
/24
RTC
RTA
RTD
AS 400
Base on the requirement specified in the diagram above, we need to configure the filtering policies on RTC. These filtering policies are implemented to the routes coming from the EBGP peer RTA and EBGP peer RTD.
RTC wants to receive only routing information from AS300.
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29. Configuration on RTC # bgp 400 peer 10.4.4.2 as-number 100
ipv4-family unicast
undo synchronization
peer enable
peer as-path-filter 1 import
peer enable
peer as-path-filter 1 import
ip as-path-filter 1 permit ^300_
Explanation of the configuration: #
bgp 400
peer as-number 100
peer as-number 300
ipv4-family unicast
undo synchronization
peer enable
peer as-path-filter 1 import \\apply as-path-filter to the route coming from the peer RTA
peer enable
peer as-path-filter 1 import \\apply as-path-filter to the route
coming from the peer RTD
ip as-path-filter 1 permit ^300_
\\receive only the routing information from AS300
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30. BGP Community
Expected direction of the traffic flow
AS 200
AS 100
RTA
RTB
1000M
1000M
/24
AS 300
In the example above, ASBR RTA and ASBR RTD in AS100 announce the network /24 to all other nodes in the network.
As a result, both RTB and RTC have 2 different paths to reach AS100. By default, the routers will select the best path to reach AS100. For example, RTC will select the path between RTC and RTD to reach the network /24. From the diagram above, the bandwidth of the link between RTC and RTD is 10M only. In
contrast, the bandwidth of the links between RTA-RTB and RTB-RTC are 1000M.
Therefore, we would like to direct the traffic to reach the network /24 by using the path via RTC-RTB-RTA.
RTD
RTC
10M
Default direction of the traffic flow
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31. Configuration on RTA bgp 100 peer 10.4.4.1 as-number 100#
ipv4-family unicast
undo synchronization
peer enable
peer enable
peer route-policy set_community export
peer advertise-community
route-policy set_community permit node 10
apply community 100:1
Explanation of the configuration:
bgp 100
peer as-number 100
peer as-number 200 #
ipv4-family unicast
undo synchronization
peer enable
peer enable
peer route-policy set community export
\\apply routing policy “set community” to the route advertise to the peer.
peer advertise-community
\\advertise community attributes to a peer or peer group. By default, no community attribute is advertised to any peer or peer group.
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32. Configuration on RTD bgp 100 peer 10.4.4.2 as-number 100#
ipv4-family unicast
undo synchronization
peer enable
peer enable
peer route-policy set_community export
peer advertise-community
route-policy set_community permit node 10
apply community 100:2
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33. Configuration on RTC bgp 300 peer 10.2.2.1 as-number 200#
ipv4-family unicast
undo synchronization
peer enable
peer route-policy set_local_pref import
peer advertise-community
peer enable
peer route-policy set_local_pref import
peer advertise-community
route-policy set_local_pref permit node 10
if-match community-filter 1
apply local-preference 200
Route-policy set_local_pref permit node 20
if-match community-filter 2
apply local-preference 50
ip community-filter 1 permit 100:1
ip community-filter 2 permit 100:2
Explanation on the configuration: #
ip community-filter 1 permit 100:1 \\configure the filtering list for community attribute
ip community-filter 2 permit 100:2
ip community-filter basic-comm-filter-num { deny | permit } [ communitynumber| aa:nn ] * &<1-16> [ internet | no-export-subconfed | no-advertise | no-export ]
ip community-filter adv-comm-filter-num { deny | permit } regular-expression In the basic community attribute list, we can specify only the community number or well-known community attributes.
In the advanced community attribute list, we can specify the regular expression that is used for string matching.
Example:
# The basic community list with list number 1
[Quidway] ip community-filter 1 permit internet
# The advanced community attribute list with list number
[Quidway] ip community-filter 100 permit ^10
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34. Display the Community Attribute
[RTC]display bgp routing-table community
Total Number of Routes: 2
BGP Local router ID is
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Network NextHop MED LocPrf PrefVal Community
* / <100:2>
*> <100:1>
display bgp routing-table community [ aa:nn &<1-13> ] [ no-advertise | noexport | no-export-subconfed ][ whole-match ]
community：display the routing information that belong to a specified BGP community.
aa:nn：specify the community number.
no-advertise ： specify the BGP routes that carry no-advertise community attribute. Routes received carrying this value
cannot be advertised at all, to either EBGP or IBGP peers.
no-export：specify the BGP routes that carry no-export community attribute.
Routes received carrying this value cannot be advertised to EBGP peers or, if a confederation is configured, the routes
cannot be advertised outside of the confederation.
no-export-subconfed：specify the BGP routes that carry no-export-subconfed community attribute. All routes received carrying a communities attribute containing this value MUST NOT be advertised to external BGP peers (this includes peers in other members autonomous systems inside a BGP
confederation). whole-match：indicates the exact matching.
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35. Display the Community Attribute (Cont.)
[RTC]display bgp routing-table
BGP local router ID :
Local AS number : 300
Paths: 2 available, 1 best
BGP routing table entry information of /24:
From: ( )
Original nexthop:
Community:<100:1>
AS-path , origin igp, localpref 200, pref-val 0, valid, external, best,
pre 255
Advertised to such 1 peers:
From: ( )
Original nexthop:
Community:<100:2>
AS-path 100, origin igp, MED 0, localpref 50, pref-val 0, valid, external, pre
255
Not advertised to any peer yet
display bgp routing-table [ ipv4-address ] [ { mask | mask-length } [ longerprefixes ] ]
ipv4-address：display the network address in IPv4 format.
mask/mask-length：specify the subnet mask in dotted decimal format/the length of the subnet mask
longer-prefixes：allow for the longer prefix match.
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36. Summary Describe the route policy tools used in BGP.
List out the parameters that affect the BGP route selection.
Describe the difference between “+” and “*” used in regular expressions. Describe also the difference between “.” and “_” .
1. Describe the route policy tools used in BGP.
The route policy tools used in BGP are mainly consist of ACL, IP-PREFIX list, Filter-List, route policy. In addition, the policy tools AS-PATH-FILTER and COMMUNITY-FILTER are solely designed for BGP.
2. List out the parameters that affect the BGP route selection.
The important parameters that affect the BGP route selection are as follow:
Preferred Value
Local-Preference
AS-Path
Origin
MED
EBGP/IBGP
IGP Cost
CLUSTER ID
Communities
Among them, the commonly used parameters are Preferred
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