Exam 350-501 topic 1 question 273 discussion

The point of this route map should be to select only 198.18.15.0/25 prefix outbound to neighbor EDGE-1 and set no-export. The longer prefix /25 will be preferred by ASBR-1 and will send to destination via router EDGE-1 The other destination 198.18.15.128/25 contained in 198.18.15.0/24, will default to EDGE-2 There are no correct answers here. A's logic is correct. accomplishes the desired goal. B logic is correct but has unnecessary prepend C Incorrecct logic does nothing to accomplish the goal But A, B, C are all wrong because they set "no-advertise" on the desired route. D logic is correct . D is the only answer with the correct community "no-export" But D is wrong because it sets route-map as "in" I select A because it has the least number of problems. Answer A

A. configure terminal route-policy ASN65001-SPECIFIC-OUT if destination in (198.18.15.0/25) then set community (no-export, peeras:65001) endif if destination in (198.18.15.0/24) then prepend as-path 65001 3 endif drop end-policy ! router bgp 65001 neighbor 100.65.0.1 address-family ipv4 unicast route-policy ASN65001-SPECIFIC-OUT out exit-address-family exit Note: "no-export" satisfy the requirement of question not going beyond 64611 D: the policy is "in" direction, wrong, though it hs "no-export"

There are 4 types of standard communities: Internet: Match all communities local-as:keep tagged prefixes in the local as no -advertise: prevent tagged prefixes from being advertised to any iBGP peer no-export: prevent tagged prefixes from being announced to eBGP peers

Answer A First, which community? Community no-export is what we need to advertise no further than ASN 64611 - WE NEED THIS ONE Community no-advertise will not advertise it through to ASBR-1 Community internal does not exist Leaves choices of A or D Notice the direction the policy is applied, we need the policy to be applied for advertisments OUT: A has "route-policy ASN65001-SPECIFIC-OUT out" D has "route-policy ASN65001-SPECIFIC-OUT in"

It's C " destination in (198.18.15.128/25) then prepend as-path 65001 3 " to apply under "router bgp 65001 neighbor 100.65.0.1" to meet this point: Routing to IP destinations in the 198.18.15.0/25 block must be preferred via the EDGE-1 PE.

no-export is the key-word. As path prepend for /24 ... what is the requirement ?

Answer is A. A - No-export can be sent and allows advertising to iBGP neighbors in the peering AS without it exporting to another AS. Also advertises the /24. B - internal is not a well-known community. C - no-advertise would not allow it to be advertised internally in the neighboring AS. D - Does not advertise /24.

i think D is correct, as 198.18.15.0/25 should be prefered via EDGE1 whille 198.18.15.128/25 should only use EDGE 1 when link b/w EDGE2 to INT-R1 is down

no-export The "no export" BGP community tells BGP to advertise a prefix only to iBGP neighbors.

I think the right answer is C. Community should be no-advertise.