The 6 BGP routing policy【Huawei S Series Switches Routing Policy】(2)

Latest reply: Apr 7, 2018 15:11:15 4520 2 1 2

This post is about the 6 BGP routing policy. Please have a read below for more details.


1      BGP Routing Policy (2)

Routing policy described in the previous sections is implemented based on route prefix. BGP is mainly used in large backbone networks, where the number of route prefixes is large. If routing policy is still implemented based on route prefix, the workload will be heavy. The AS_Path filter and Community filter can be used to address this issue.

1.1  AS_Path Filter

1.1.1            AS_Path Filter and Regular Expression

AS_Path Filter

The AS_Path attribute is an ordered list of all the ASs that a route passes through from the source to the destination. In Figure 6-1, the AS_Path attribute of a BGP route can be considered as a string containing spaces and can be used to match routes through a regular expression.

AS_Path attributes of BGP routes

20170322111439916001.png

 

A regular expression is a string that describes a characteristic and verifies whether another string conforms to this characteristic. The AS_Path filter of BGP defines an AS_Path regular expression to match the AS_Path attributes of BGP routes for filter BGP routing information.

For example, ip as-path-filter 1 permit 495 defines an AS_Path filter 1 and a regular expression 45, indicating that any string containing 495 can be matched.

The core of an AS_Path filter is a regular expression. The content of a regular expression is complex. The following describes the AS_Path filter.

Composition of an AS_Path regular expression

An AS_Path filter uses a regular expression to define matching rules. A regular expression consists of the following parts:

l   Metacharacter: defines matching rules.

l   General character: defines matching objects.

Table 6-1 lists metacharacters supported by a BGP AS_Path regular expression.

Metacharacters supported by a BGP AS_Path regular expression

Metacharacter

Description

Instance

.

Matches any single character except "\n", including spaces.

.* matches any AS_Path or route. If you have configured multiple ip as-path-filter clauses in deny mode, define an ip as-path-filter as-path-filter-name permit.* clause to permit other routes.

*

Matches AS_Path with zero or multiple of the previous character.

For details, see the instance of the above-mentioned metacharacter.

+

Matches AS_Path with one or multiple of the previous character.

65+ matches AS_Paths that start with 6 and include one 5 or consecutive 5s.

    The following strings conform to this characteristic: 65, 655, 6559, 65259, and 65529.

    The following strings do not conform to this characteristic: 56, 556, 5669, 55269, and 56259.

|

Matches either expression it separates.

100|65002|65003 matches 100, 65002, or 65003.

^

Matches the strings that start with the following string.

^65 matches AS_Paths that start with 65.

    The following strings conform to this characteristic: 65, 651, 6501, and 65001.

    The following strings conform to this characteristic: 165, 1650, 6650, and 60065.

$

Matches the strings that end with the preceding string.

65$ matches AS_Paths that end with 65.

    The following strings conform to this characteristic: 65, 165, 1065, 10065, and 60065.

    The following strings do not conform to this characteristic: 651, 1650, 6650, 60650, and 65001.

^$ matches an empty string, that is, empty AS_Path. It is often used to match locally originated routes.

(xyz)

Defines a subexpression, which can be null.

100(200)+ matches 100200 and 100200200.

[xyz]

Matches any character within the brackets.

[896] matches AS_Paths that contain any character of 8, 9, and 6.

[^xyz]

Matches a single character that is not contained within the brackets.

[^896] matches AS_Paths that do not contain 8, 9, or 6.

[a-z]

Matches any character within the specified range.

[2-4] matches 2, 3, and 4, and [0-9] matches 0 to 9.

[^a-z]

Matches any character out of the specified range.

[^2-4] matches all characters except 2, 3, and 4, and [^0-9] matches all characters except 0 to 9.

_

Matches a sign, including a comma, left brace, right brace, left bracket, right bracket, and space. It can also function as a start or end character, like ^ and $, at the beginning or end of an expression.

    ^65001_ matches AS_Paths that start with 65001 followed by a sign. Specifically, ^65001_ matches AS_Paths with 65001 as the leftmost AS number (the number of the last AS through which a route passes) or the routes sent by peers in AS 65001.

    _65001_ matches AS_Paths containing 65001 or routes that pass through AS 65001.

    _65001$ matches AS_Paths that end with a sign followed by 65001. Specifically, _65001$ matches AS_Paths with 65001 as the rightmost AS number (the number of the first AS through which a route passes) or the routes that are originated in AS 65001.

\

Defines an escape character.

It changes the metacharacters following it to ordinary characters.

 

1.1.2            AS_Path Filter Applications

The AS_Path filter defines only a filtering tool, which takes effect only after being invoked somewhere. Two methods are available in BGP to invoke the AS_Path filter: One is to invoke it using the peer command, and the other is to invoke it using a route-policy.

Method 1: Invoke the AS_Path filter using the peer command.

#

ip as-path-filter s1 permit ^100$

#

bgp 65100

  peer 10.1.1.2 as-path-filter s1 import

#

In method 1, an AS_Path filter s1 is defined and associated with a regular expression ^100$. This AS_Path filter can match routes carrying the AS_Path attribute 100 (this attribute contains only the AS number 100) and is applied in the peer command. In this situation, only the routes that match AS_Path filter s1 can be advertised to the BGP peer 10.1.1.2.

Method 2: Invoke the AS_Path filter using a route-policy.

#

ip as-path-filter s1 permit ^100$

#

route-policy huawei permit node 10

 if-match as-path-filter s1

 apply local-preference 100

#

bgp 65100

  peer 10.1.1.2 route-policy huawei import

#

In method 2, a defined AS_Path filter s1 is invoked in the if-match command in a route-policy, and then the LP path attribute is set using the apply command and applied in the peer command (in the import direction) in BGP configuration mode. In this situation, among the BGP routes received from the BGP peer 10.1.1.2, all the routes that match the AS_Path filter s1 have their LP path attributes set to 100.

1.1.3            AS_Path Filter Examples

The preceding sections describe the matching rules and applications of an AS_Path filter. The following provides an example for using an AS_Path filter in BGP to filter routes.

In Figure 6-2, an EBGP peer relationship is established between LSW1 and LSW2, between LSW1 and LSW3, between LSW2 and LSW3, between LSW2 and LSW4, between LSW3 and LSW4, and between LSW4 and LSW5. Each device uses the network command to advertise the IP addresses of LoopBack0 into BGP and filter these routes.

Using an AS_Path filter to filter BGP routes

图X 使用as-path-filter对BGP路由进行过滤.png

 

When no AS_Path filter is used, the original BGP routing table of LSW1 is as follows:

[LSW1] display  bgp routing-table 

 

 BGP Local router ID is 10.1.1.1

 Status codes: * - valid, > - best, d - damped,

               h - history,  i - internal, s - suppressed, S - Stale

               Origin : i - IGP, e - EGP, ? - incomplete

 

 

 Total Number of Routes: 9

      Network        NextHop        MED    LocPrf  PrefVal Path/Ogn

 

 *>   1.1.1.9/32     0.0.0.0         0                 0       i

 *>   2.2.2.9/32     10.1.1.2        0                 0       65200i

 *                     10.1.2.2                          0       65300 65200i

 *>   3.3.3.9/32     10.1.2.2        0                 0       65300i

 *                     10.1.1.2                          0       65200 65300i

 *>   4.4.4.9/32     10.1.1.2                          0       65200 65400i

 *                     10.1.2.2                          0       65300 65400i

 *>   5.5.5.9/32     10.1.1.2                          0       65200 65400 65500i

 *                     10.1.2.2                          0        65300 65400 65500i

Case 1: Define an AS_Path filter s1 to receive only the routes originated in AS65500.

[LSW1] ip as-path-filter s1 permit _65500$   //Define an AS_Path filter s1.

[LSW1] bgp 65100

[LSW1-bgp] ipv4-family unicast

[LSW1-bgp-af-ipv4] peer 10.1.1.2 as-path-filter s1 import //Invoke the AS_Path filter s1 using the peer command.

[LSW1-bgp-af-ipv4] peer 10.1.2.2 as-path-filter s1 import

After the preceding configurations are complete, the BGP routing table is as follows:

[LSW1] display bgp routing-table 

 

 BGP Local router ID is 10.1.1.1

 Status codes: * - valid, > - best, d - damped,

               h - history,  i - internal, s - suppressed, S - Stale

               Origin : i - IGP, e - EGP, ? - incomplete

 

 

 Total Number of Routes: 3

      Network        NextHop       MED    LocPrf    PrefVal Path/Ogn

 

 *>   1.1.1.9/32     0.0.0.0        0                 0        i

 *>   5.5.5.9/32     10.1.1.2                          0       65200 65400 65500i

 *                     10.1.2.2                          0        65300 65400 65500i

The preceding command output shows that the routes originated in AS65500 are permitted and other routes are denied.

Case 2: Define an AS_Path filter s2 to deny the routes originated in AS65500 and allow receiving other routes.

[LSW1] ip as-path-filter s2 deny _65500$

[LSW1] ip as-path-filter s2 permit .*

[LSW1] bgp 65100

[LSW1-bgp] ipv4-family unicast

[LSW1-bgp-af-ipv4] peer 10.1.1.2 as-path-filter s2 import

[LSW1-bgp-af-ipv4] peer 10.1.2.2 as-path-filter s2 import

After the preceding configurations are complete, the BGP routing table is as follows:

[LSW1]display bgp routing-table 

 

 BGP Local router ID is 10.1.1.1

 Status codes: * - valid, > - best, d - damped,

               h - history,  i - internal, s - suppressed, S - Stale

               Origin : i - IGP, e - EGP, ? - incomplete

 

 

 Total Number of Routes: 7

      Network        NextHop        MED    LocPrf   PrefVal  Path/Ogn

 

 *>   1.1.1.9/32     0.0.0.0         0                 0       i

 *>   2.2.2.9/32     10.1.1.2        0                 0       65200i

 *                     10.1.2.2                          0       65300 65200i

 *>   3.3.3.9/32     10.1.2.2        0                 0       65300i

 *                     10.1.1.2                          0       65200 65300i

 *>   4.4.4.9/32     10.1.1.2                          0       65200 65400i

 *                     10.1.2.2                          0       65300 65400i

The preceding command output shows that the routes originated in AS65500 are denied and other routes are permitted.

Case 3: Define an AS_Path filter s3 to deny the routes that pass through AS65400.

[LSW1] ip as-path-filter s3 deny _65400_

[LSW1] ip as-path-filter s3 permit .*

[LSW1] bgp 65100

[LSW1-bgp] ipv4-family unicast

[LSW1-bgp-af-ipv4] peer 10.1.1.2 as-path-filter s3 import

[LSW1-bgp-af-ipv4] peer 10.1.2.2 as-path-filter s3 import

After the preceding configurations are complete, the BGP routing table is as follows:

[LSW1] display bgp routing-table 

 

 BGP Local router ID is 10.1.1.1

 Status codes: * - valid, > - best, d - damped,

               h - history,  i - internal, s - suppressed, S - Stale

               Origin : i - IGP, e - EGP, ? - incomplete

 

 

 Total Number of Routes: 5

      Network        NextHop        MED    LocPrf   PrefVal  Path/Ogn

 

 *>   1.1.1.9/32     0.0.0.0         0                 0       i

 *>   2.2.2.9/32     10.1.1.2        0                 0       65200i

 *                     10.1.2.2                          0       65300 65200i

 *>   3.3.3.9/32     10.1.2.2        0                 0       65300i

 *                     10.1.1.2                          0       65200 65300i

The preceding command output shows that the routes that pass through AS65400 are denied and other routes are permitted.

Case 4: Define an AS_Path filter s4 to deny the routes that pass through AS65400. AS65400 is not the first or last AS that routes pass through.

[LSW1] ip as-path-filter s4 deny ._65400_.

[LSW1] ip as-path-filter s4 permit .*

[LSW1] bgp 65100

[LSW1-bgp] ipv4-family unicast

[LSW1-bgp-af-ipv4] peer 10.1.1.2 as-path-filter s4 import

[LSW1-bgp-af-ipv4] peer 10.1.2.2 as-path-filter s4 import

After the preceding configurations are complete, the BGP routing table is as follows:

[LSW1] display bgp routing-table 

 

 BGP Local router ID is 10.1.1.1

 Status codes: * - valid, > - best, d - damped,

               h - history,  i - internal, s - suppressed, S - Stale

               Origin : i - IGP, e - EGP, ? - incomplete

 

 

 Total Number of Routes: 7

      Network        NextHop        MED    LocPrf   PrefVal  Path/Ogn

 

 *>   1.1.1.9/32     0.0.0.0         0                 0       i

 *>   2.2.2.9/32     10.1.1.2        0                 0       65200i

 *                     10.1.2.2                          0       65300 65200i

 *>   3.3.3.9/32     10.1.2.2        0                 0       65300i

 *                     10.1.1.2                          0       65200 65300i

 *>   4.4.4.9/32     10.1.1.2                          0       65200 65400i

 *                     10.1.2.2                          0       65300 65400i

The preceding command output shows that the routes carrying the AS_Path attribute in which AS65400 is neither the first nor last AS number are denied and other routes are permitted.

Case 5: Define an AS_Path filter s5 to match locally originated routes and deny routes from other ASs.

[LSW1] ip as-path-filter s5 permit ^$

[LSW1] bgp 65100

[LSW1-bgp] ipv4-family unicast

[LSW1-bgp-af-ipv4] peer 10.1.1.2 as-path-filter s5 import

[LSW1-bgp-af-ipv4] peer 10.1.2.2 as-path-filter s5 import

After the preceding configurations are complete, the BGP routing table is as follows:

[LSW1] display bgp routing-table 

 

 BGP Local router ID is 10.1.1.1

 Status codes: * - valid, > - best, d - damped,

               h - history,  i - internal, s - suppressed, S - Stale

               Origin : i - IGP, e - EGP, ? - incomplete

 

 

 Total Number of Routes: 1

      Network        NextHop        MED    LocPrf    PrefVal Path/Ogn

 

 *>   1.1.1.9/32     0.0.0.0         0                 0             i

The preceding command output shows that only locally originated routes with empty AS_Path are permitted and other routes are denied.

1.2  Community Filter

1.2.1            Community Attribute

Community Attribute Functions

The Community attribute is a way to group BGP routes in a certain community and apply routing decisions according to these communities.

l   The Community attribute is a set of 4-byte values, in the format AA:NN. As defined in RFC1997, the first two bytes indicate an AS number, and the last two bytes indicate a community identifier used for management.

l   The Community attribute is a BGP route tag that simplifies the implementation of routing policy. A specific Community attribute can be assigned to some routes. Based on this Community attribute, these routes can be managed and routing policy can be performed on these routes.

Community attribute application scenario

20170322111441195003.png

 

In Figure 6-3, a large number of routes in AS100 are imported into BGP, and these routes are used for voice calls and video surveillance. These routes are advertised to AS200 through BGP. Devices in AS200 need to carry out different policies on the routes used for voice calls and video surveillance. How to match these routes? You can use ACLs or IP prefix lists to match the routes one by one. This method, however, has a heavy workload and is inefficient because a large number of route prefixes exist.

You can use the Community attribute to address this issue. When these routes are imported into AS100, respective Community attributes are added to the routes to differentiate the routes used for voice calls and video surveillance. That is, the Community attribute 100:1 is added to the routes used for voice calls, and the Community attribute 100:2 is added to the routes used for video surveillance. These Community attributes are transmitted to AS200 along with the routes. In AS200, to carry out different policies for the routes used for voice calls and video surveillance, match the routes based on their respective Community attributes. For example, you can match all the routes used for voice calls based on the Community attribute 100:1.

BGP Well-Known Community Attributes

BGP defines some well-known Community attributes. Table 6-2lists these Community attributes.

BGP well-known Community attributes

Community Attribute

Description

internet

By default, all routes belong to the Internet community. A route with this attribute can be advertised to all BGP peers.

no-advertise

A route with this attribute cannot be advertised to any other BGP peers.

no-export

A route with this attribute cannot be advertised outside the local AS. If a confederation is defined, the route with this attribute cannot be advertised to the ASs outside the confederation but can be advertised to other sub-ASs in the confederation.

no-export-subconfed

A route with this attribute cannot be advertised outside the local AS or advertised to other sub-ASs in the confederation.

 

1.2.2            Setting the Community Attribute for Routes

To use the Community filter, ensure that the Community attribute is carried in routes. The following describes how to set the Community attribute for routes.

Setting the Community attribute for routes

图X设置路由前缀的Community属性.png

 

Requirement Description

In Figure 6-4, LSW1 in AS100 advertises two routes 10.1.1.0/24 and 10.1.2.0/24 through BGP. In the BGP process of LSW1, use an export route-policy to modify the Community attribute of the route 10.1.1.0/24 to 100:1 so that downstream devices can carry out policies on the route based on this Community attribute.

Configuration

The key configuration of R1 is as follows:

#

ip ip-prefix huawei index 10 permit 10.1.1.0 24  //Define an IP prefix list to match the target route.

#

route-policy RP permit node 10     //Define a route-policy to set the Community attribute for the target route.

 if-match ip-prefix huawei

 apply community 100:1

route-policy RP permit node 20     //Permit the remaining routes.

#

bgp 100

 router-id 1.1.1.1

 peer 192.168.12.2 as-number 200

 #

 ipv4-family unicast

  undo synchronization

  network 10.1.1.0 255.255.255.0    //Advertise the route 10.1.1.0/24.

  network 10.1.2.0 255.255.255.0    //Advertise the route 10.1.2.0/24.

  peer 192.168.12.2 enable

  peer 192.168.12.2 route-policy RP export //Apply the route-policy in the export direction of the specified BGP peer.

  peer 192.168.12.2 advertise-community     //Advertise the Community attribute to the specified BGP peer.

#

The key configuration of R2 is as follows:

#

bgp 200

 router-id 2.2.2.2

 peer 192.168.12.1 as-number 100

 peer 192.168.23.2 as-number 300

 #

 ipv4-family unicast

  undo synchronization

  peer 192.168.12.1 enable

  peer 192.168.23.2 enable

  peer 192.168.23.2 advertise-community   //Advertise the Community attribute to the specified BGP peer.

#

By default, the Community attribute is not updated to BGP peers along with the route prefix. Therefore, the peer advertise-community command needs to be configured on LSW1 and LSW2 to advertise this attribute to BGP peers.

Verification

After the preceding configurations are complete, check the BGP route 10.1.1.0 in the BGP routing table of LSW3.

<LSW3> display bgp routing-table 10.1.1.0

 

 BGP local router ID : 3.3.3.3

 Local AS number : 300

 Paths:   1 available, 1 best, 1 select

 BGP routing table entry information of 10.1.1.0/24:

 From: 192.168.23.1 (2.2.2.2)

 Route Duration: 01h08m07s 

 Direct Out-interface: Vlanif23

 Original nexthop: 192.168.23.1

 Qos information : 0x0

 Community:<100:1>     //This Community attribute is set and transmitted to downstream devices using a route-policy on LSW1.

 AS-path 200 100, origin igp, pref-val 0, valid, external, best, select, active,

pre 255

 Not advertised to any peer yet

The preceding command output shows that after the Community attribute is set for the route, this route carries a Community attribute when reaching LSW3, which then can carry out some policies on the route based on the Community attribute.

More methods are available to set the Community attribute for BGP routes. Table 6-3 lists common methods to set this attribute.

Example for setting the Community attribute for BGP routes

Command

Description

apply community 100

Changes the community name to 100.

apply community 100 150

Changes the community name to 100 or 150. That is, BGP routes belong to two communities.

apply community 100 150 additive

Adds two Community attributes 100 and 150. That, BGP routes belong to three Communities.

apply community none

Deletes the Community attribute of BGP routes.

 

1.2.3            Using a Community Filter to Match BGP Routes

The previous section describes how to set the Community attribute for BGP routes. After an upstream router sets this attribute, it is transmitted along with routes so that downstream devices can carry out policies based on this attribute. BGP provides a tool, Community filter, to match BGP routes based on the Community attribute. The following describes how to use a Community filter to match BGP routes.

Using a Community filter to match BGP routes

图X 使用community-filter匹配BGP路由.png

 

Requirement Description

In Figure 6-5, LSW1 in AS100 advertises three BGP routes. The Community attribute is set for the routes and transmitted along with the routes to LSW3 in AS300. Before policies are carried out, check the BGP routing table of LSW3.

<LSW3> display bgp routing-table 

 

 BGP Local router ID is 3.3.3.3

 Status codes: * - valid, > - best, d - damped,

               h - history,  i - internal, s - suppressed, S - Stale

               Origin : i - IGP, e - EGP, ? - incomplete

 

 

 Total Number of Routes: 3

      Network            NextHop        MED        LocPrf    PrefVal Path/Ogn

 

 *>   10.1.1.0/24        192.168.23.1                          0      200 100i

 *>   10.1.2.0/24        192.168.23.1                          0      200 100i

 *>   10.1.3.0/24        192.168.23.1                          0      200 100i

The preceding command output shows that the BGP routing table of LSW3 contains three routes. Routing policy needs to be carried out on LSW3 to permit only the routes carrying the Community attribute 100:999.

Configuration

The previous section has described how to set the Community attribute. The following describes how to use a Community filter to filter routes.

The key configuration of LSW3 is as follows:

#

ip community-filter 1 permit 100:999  //Permit the route carrying the Community attribute 100:999.

#

route-policy huawei permit node 10   //Invoke the Community filter using a route-policy.

 if-match community-filter 1

#

bgp 300

 router-id 3.3.3.3

 peer 192.168.23.1 as-number 200

 #

 ipv4-family unicast

  undo synchronization

  peer 192.168.23.1 enable

  peer 192.168.23.1 route-policy huawei import   //Use the route-policy in the import direction of the specified BGP peer.

#

Verification

After the preceding configurations are complete, check the BGP routing table of LSW3.

[LSW3] display bgp routing-table 

 

 BGP Local router ID is 3.3.3.3

 Status codes: * - valid, > - best, d - damped,

               h - history,  i - internal, s - suppressed, S - Stale

               Origin : i - IGP, e - EGP, ? - incomplete

 

 

 Total Number of Routes: 2

      Network            NextHop        MED        LocPrf    PrefVal Path/Ogn

 

 *>   10.1.1.0/24        192.168.23.1                          0      200 100i

 *>   10.1.2.0/24        192.168.23.1                          0      200 100i

The defined Community filter matches the route carrying the Community attribute 100:999. Therefore, the two routes 10.1.1.0/24 and 10.1.2.0/24 are permitted on LSW3, and other BGP routes are denied.

For this scenario, the following provides three common methods to configure the Community filter:

Case 1

ip community-filter 1 permit 100:1 100:999

Only the routes that carry two Community attributes 100:1 and 100:999 can be matched. Therefore, the route 10.1.1.0/24 is matched.

Case 2

ip community-filter 1 permit 100:1

ip community-filter 1 permit 100:2

The routes that carry the Community attribute 100:1 or 100:2 can be matched. Therefore, the routes 10.1.1.0/24 and 10.1.2.0/24 are matched.

Case 3

ip community-filter 1 permit internet

By default, all routes carry the Internet attribute. Therefore, all routes are matched.

That’s all for routing policy. All datacom network engineers want to precisely control the IP routing table to ensure that service traffic can be forwarded based on the planned traffic model. This thread helps you master the use of routing policy to flexibly control routes. Thank you for your attention.

For more details, click the following hyperlink:

1 Routing Policy

Describes various tools used in routing policy and invoking between these tools.

2 Route-Policy

Describes the components, matching rules, and applications of route-policy.

3 IP Prefix List

Describes how to use an IP prefix list and differences between it and ACL.

4 Filter-Policy

Describes filter-policy principles and applications.

5 BGP Routing Policy (1)

Describes applications of IP prefix list, filter-policy, and route-policy in BGP.

6 BGP Routing Policy (2)

Describes applications of AS_Path filter and Community attribute in BGP.

Collection of Chapters 1 Through 6 (Click Here to Download the PDF Document)

Provides the collection of the preceding chapters.

  

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user_2790689
Created Mar 22, 2017 05:40:37 Helpful(0) Helpful(0)

thank you
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wissal
MVE Created Apr 7, 2018 15:11:15 Helpful(0) Helpful(0)

useful document, thanks
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Telecommunications%20engineer%2C%20currently%20senior%20project%20manager%20at%20an%20operator%2C%20partner%20of%20Huawei%2C%20in%20the%20radio%20access%20network%20department%2C%20for%2020%20years%20I%20managed%20several%20types%20of%20projects%2C%20for%20the%20different%20nodes%20of%20the%20network.

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