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300-415 · Question #169

Refer to the exhibit showing a GUI configuration snippet with 'Address Family' set to On and 'ipv4-unicast', 'Route Policy In' and 'Route Policy Out' set to On, and 'Policy Name' as 'eBGP_Community_Po

The correct answer is C. policy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 no-export default-action accept lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24. To prevent specific server networks from being advertised outside the AS while applying a community tag, a route policy should match those networks and set the BGP 'no-export' community.

Policies

Question

Refer to the exhibit showing a GUI configuration snippet with 'Address Family' set to On and 'ipv4-unicast', 'Route Policy In' and 'Route Policy Out' set to On, and 'Policy Name' as 'eBGP_Community_Policy'. The engineering must assign community tags to 3 of its 74 critical server networks so as soon as possible as they are advertised to BGP peers. These server networks must not be advertised outside AS. Which configuration fulfills this requirement?

Options

  • Apolicy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 no-advertise default-action accept lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24
  • Bpolicy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 local-as default-action reject lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24
  • Cpolicy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 no-export default-action accept lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24
  • Dpolicy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 no-advertise default-action reject lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24

How the community answered

(31 responses)
  • A
    16% (5)
  • B
    3% (1)
  • C
    74% (23)
  • D
    6% (2)

Why each option

To prevent specific server networks from being advertised outside the AS while applying a community tag, a route policy should match those networks and set the BGP 'no-export' community.

Apolicy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 no-advertise default-action accept lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24

Using 'no-advertise' prevents the routes from being advertised to any BGP peer, including internal ones, which is a stricter restriction than 'not advertised outside AS'.

Bpolicy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 local-as default-action reject lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24

The 'local-as' community is typically used within BGP confederations, and the 'default-action reject' would prevent all other server networks from being advertised.

Cpolicy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 no-export default-action accept lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24Correct

The 'set community 999:65000 no-export' command correctly applies the desired community tag and ensures that the specified server networks are not advertised to external BGP peers, fulfilling the 'must not be advertised outside AS' requirement. The 'default-action accept' allows other routes not matching the prefix list to be processed normally.

Dpolicy route-policy eBGP_Community_Policy sequence 1 match address Community_Prefix action accept set community 999:65000 no-advertise default-action reject lists prefix-list Community_Prefix ip-prefix 20.20.20.0/24 ip-prefix 21.21.21.0/24 ip-prefix 22.22.22.0/24

Similar to A, 'no-advertise' is too restrictive, preventing advertisement even within the AS, and 'default-action reject' would block other server networks.

Concept tested: BGP communities for route advertisement control

Source: https://www.cisco.com/c/en/us/td/docs/routers/sdwan/configuration/routing/ios-xe-17/routing-book-sdwan/bgp-config.html

Topics

#BGP Communities#Route Policy#Prefix Lists#BGP Advertisement Control

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