| Exam 400-101 | Question id=1014 | Layer 3 Technologies |
You administer the network shown above. You issue the show running-config command on RouterA and receive the following partial output:
router bgp 100
no synchronization
bgp log-neighbor-changes
network 192.168.0.0
network 192.168.1.0
neighbor 10.1.1.2 remote-as 200
neighbor 10.1.1.2 send-community
neighbor 10.1.1.2 route-map map1 out
no auto-summary
!
access-list 1 permit 192.168.0.0 0.0.255.255
!
route-map map1 permit 10
math ip address 1
set community no-advertise
!
route-map map1 permit 20
Which of the following routers will receive the route to 192.168.0.0/24?
| A. |
only RouterB | |
| B. |
only RouterB and RouterC | |
| C. |
only RouterB and RouterD | |
| D. |
RouterB, RouterC, and RouterD | |
| E. |
None of the routers will receive the route. |
Only RouterB will receive the route to 192.168.0.0/24. The neighbor 10.1.1.2 remote-as 200 command specifies that RouterB, which is in autonomous system (AS) 200, is an external Border Gateway Protocol (eBGP) neighbor of RouterA. The neighbor 10.1.1.2 sendcommunity command configures RouterA to send community attribute settings to RouterB. The neighbor 10.1.1.2 routemap map1 out command applies route map map1 to modify outgoing routes from RouterA. Route map map1 will not affect which routes are advertised from RouterA to RouterB; it will only apply the noadvertise community attribute to routes that match access list 1.
Routes that do not match access list 1 are advertised without the attribute. Because route map map1 is configured to apply to only the routes that pass access list 1, the noadvertise community attribute will affect only the route to 192.168.0.0/24.
The neighbor 10.1.1.2 send-community command configures RouterA to send community attribute settings to RouterB. The community attribute is an optional, transitive Border Gateway Protocol (BGP) attribute that is not required to be supported by all BGP implementations. Additionally, BGP implementations that do not support the community attribute are not required to pass the attribute to other routers. By default, Cisco routers do not pass community attributes to BGP neighbors.
The community attribute can be modified in a route map by issuing the set community command with one of the following four keywords:
-no-advertise -prevents advertisements to any BGP peer
-no-export-prevents advertisements to eBGP peers
-local-as -prevents advertising outside the AS, or in confederation scenarios, outside the subAS - internet-advertises the route to any router The set community no-advertise command configures the BGP community attribute to inform neighbor routers to not advertise routes to any BGP peer. Because the community attribute in this scenario applies only to the 192.168.0.0/24 route, RouterB will advertise the route to 192.168.1.0/24 but not the route to 192.168.0.0/24.
The community attribute does not modify how RouterA advertises the routes? it modifies how neighbor routers advertise the routes received from RouterA.