A. Router RAR1 will accept only route /19 from its BGP neighbor. B. Router RAR1 will send only route /19 to its BGP neighbor.

Transcription

1 Refer to the exhibit. Which statement is true? hostname RAR1! <output omitted>! router bgp 100 neighbor remote-as 200 neighbor distribute-list 101 in! access-list 101 permit ip Select the best response. A. Router RAR1 will accept only route /19 from its BGP neighbor. B. Router RAR1 will send only route /19 to its BGP neighbor. C. Only traffic with a destination from /19 will be permitted. D. Only traffic going to /19 will be permitted. Answer: A Wildcard mask for /19 is We can see that there is placed distributed list inbound used to filter routes from BGP neighbor. Distribute list is linked with extended ACL No. 101, which statement permits network /19 and denies all other networks, that comes from implicit deny rule at the end of any access list. More on: Link

2 What are the two reasons for the appearance of as the next hop for a network in the show ip bgp command output? (Choose two.) A. The network was originated via redistribution of an interior gateway protocol into BGP. B. The network was defined by a static route. C. The network was originated via a network or aggregate command. D. The network was learned via EBGP. E. The network was learned via IBGP. Answer: A, C - An entry of indicates that the router has some non-bgp routes to this network. - Displaying routes originated by the local router is tougher. You could use the fact that the local routes have the weight set to This would work if you don t play with BGP weights in network statements. If you ve changed the weights, you should filter the routes based on the BGP nexthop: locally originated routes have the next-hop and all other routes should have a non-zero BGP next-hop. More on: Link1; Link

3 Refer to the exhibit. Which two statements are true about the partial configuration that is provided. (Choose two.) router bgp 100 neighbor internal peer-group neighbor internal remote-as 100 neighbor internal update-source loopback 0 neighbor internal route-map set-med out neighbor internal filter-list 1 out neighbor internal filter-list 2 in neighbor peer-group internal neighbor peer-group internal neighbor peer-group internal neighbor filter-list 3 in A. All the configured neighbors are in autonomous system 100. B. The peer group shortens the IBGP configuration. C. The peer group shortens the EBGP configuration. D. Only the outgoing filters are applied to BGP updates. E. Three AS-path filters are applied to each BGP neighbor. Answer: A, B Peer groups save processing time in generating the updates for all IBGP neighbors ( , and ) and make the router configuration easier to read and manage. Commands: neighbor internal peer-group neighbor internal remote-as 100 neighbor internal update-source loopback 0 neighbor internal route-map set-med out neighbor internal filter-list 1 out neighbor internal filter-list 2 in are linked with internal peer group, that is linked with all internal BGP routers in AS 100 ( , and ). Changes are replicated through the all neighbors that are part of the peer group

4 Refer to the exhibit. Which two statements are correct? (Choose two.) Router#show ip bgp BGP table version is 5 local router ID is Status codes: s suppressed, d damped, h history, * valid, < best, i - internal Origin codes: i - IGP, e - EGP,? - incomplete Network Next Hop Metric LocPrf Weight Path *> ? * ? *> ? * ? *> ? *> / ? A. All six routes will be installed in the routing table. B. Two routes will be installed in the routing table. C. Four routes will be installed in the routing table. D. All the routes were redistributed into BGP from an IGP. E. All the routes were originated by BGP with the network command. Answer: C, D In the output of the show ip bgp command: Sign > - indicates the best path for a route, selected by BGP. This route is offered to the IP routing table. Sign? - signifies that original BGP process can t absolutely verify this network s availability because it is redistributed from the IGP into BGP process

5 Which statement is true about how BGP would select the best path? A. If the command bgp always-compare-med has been given then the router will prefer the route with the highest MED. B. The router will prefer the route with the lower MED. C. The router will prefer the shortest autonomous system path. D. To influence one route to be preferred its default local preference value will be changed via the use of the command bgp default local-preference 50. Answer: C - The local preference is set to 100 by default. Higher values are preferred. Command bgp default local-preference value router configuration command changes the default local preference to the value specified. The value can be set to a number between 0 and The following process summarizes how BGP chooses the best route on a Cisco router: Step 1. Prefer the route with the highest weight. (Recall that the weight is Cisco proprietary and is local to the router only.) Step 2. If multiple routes have the same weight, prefer the route with the highest local preference. (Recall that the local preference is used within an autonomous system.) Step 3. If multiple routes have the same local preference, prefer the route that was originated by the local router. (A locally originated route has a next hop of in the BGP table.) Step 4. If none of the routes were originated by the local router, prefer the route with the shortest ASpath. Step 5. If the AS-path length is the same, prefer the lowest-origin code (IGP < EGP < incomplete). Step 6. If all origin codes are the same, prefer the path with the lowest MED. (Recall that the MED is exchanged between autonomous systems.) The MED comparison is done only if the neighboring autonomous system is the same for all routes considered, unless the bgp always-compare-med router configuration command is enabled. Note: The most recent Internet Engineering Task Force (IETF) decision about BGP MED assigns a value of infinity to a missing MED, making a route lacking the MED variable the least preferred. The default behavior of BGP routers running Cisco IOS Software is to treat routes without the MED attribute as having a MED of 0, making a route lacking the MED variable the most preferred. To configure the router

6 to conform to the IETF standard, use the bgp bestpath med missing-asworst router configuration command. Step 7. If the routes have the same MED, prefer external paths (EBGP) over internal paths (IBGP). Step 8. If synchronization is disabled (the default) and only internal paths remain, prefer the path through the closest IGP neighbor. This means that the router prefers the shortest internal path within the autonomous system to reach the destination (the shortest path to the BGP next hop). Step 9. For EBGP paths, select the oldest route, to minimize the effect of routes going up and down (flapping). Step 10. Prefer the route with the lowest neighbor BGP router ID value. Step 11. If the BGP router IDs are the same, prefer the route with the lowest neighbor IP address. We can see that B is correct, but is Step 6. Statement in C is in Step 4, so it will be used first for choosing best route. More on: Link

7 Refer to the exhibit. On the basis of the information in the exhibit, which two statements are true? (Choose two.) BGP table version is 12, local router ID is Status codes: s suppressed, d damped, h history, * valid, < best, i - internal Origin codes: i - IGP, e - EGP,? - incomplete Network Next Hop Metric LocPrf Weight Path * i *> i *> / i *> / i *> i A. The output was generated by entering the show ip bgp command on the ISP router. B. The output was generated by entering the show ip bgp command on the SanJose1 router. C. The serial 0/0/1 interface on the ISP router has been configured with the set metric 50 command. D. The serial 0/0/1 interface on the ISP router has been configured with the set metric 75 command. E. When traffic is sent from the ISP to autonomous system 64512, the traffic will be forwarded to SanJose1 because of the lower MED value of SanJose

8 F. When traffic is sent from the ISP to autonomous system 64512, the traffic will be forwarded to SanJose2 because of the higher MED value of SanJose2. Answer: A, E For Easier understanding of BGP route-selection process we will use following table: Step CONDITION No. 1. if {Max_weight (route1; route2) else {Max_weight (route1; route2) = FALSE} 2. if {Max_ local preference (route1; route2) else{ Max_ local preference (route1; route2) = FALSE} 3. if{just_one_route_is_ Originated_by_the_local_ router Note: in BGP table this route has next-hop ACTION on BGP attribute values then{highest weight} =>USE ROUTE with Max_weight SAME HIGHEST weights => go to 2. then{highest local preference} =>USE ROUTE with Max_ local preference SAME HIGHEST local preference => go to 3. then{use ROUTE that is Originated_by_the_local_ router} route1 to route2 to network network / / 0=0 => go to 2. 0=0 => go to 2. Note: External route - no local preference / / => go to 3. => go to 3. Note: For route1 next-hop is Note: External route- no local preference Note: For route1 nexthop is else{ }=FALSE => go to 4. => go to 4. => go to if{just_one_route_got_shortest_as_path then{use ROUTE with Shortest_AS_path } Note: route1 got 1 hop path (64512 i) Note: route2 got 1 hop path (64512 i) else{ }=FALSE => go to 5. => go to 5. => go to if{just_one_route_got_mark: then{use ROUTE with Note:not applicable Note: not applicable IGP<EGP<incomplete highest mark} Note: in BGP table: i-igp, e-egp,?- incomplete else{ }=FALSE => go to 6. => go to 6. => go to if{just_one_route_got: lowest MED then{use ROUTE with lowest MED} ( lowest!!! ) USE THIS ROUTE!!! Note: in BGP table value for MED is metric. else{ }=FALSE => go to if{just_one_route_got_ebgp_path: over_ other_ routes_with_ibgp_path(s) then{use ROUTE with EBGP_path} Note: You have to decide from the network topology if route is EBGP path (to external BGP AS) or internal (into internal BGP AS). else{ }=FALSE => go to if{sychronization_is_disabled_and internal_paths_remain: then{use ROUTE within AS with shortest path to the destination} Note: You have to decide from the

9 network topology which is shortest internal paath (how many hops to the destination router). else{ }=FALSE => go to if{just_oldest_route_from_ebgp_paths: then{use oldest EBGP ROUTE} Note: To minimize routes going up and down (flapping). else{ }=FALSE => go to if{just_one_route_with_lowest_ neighbor_bgp_router_id: then{use ROUTE that got next-hop router with lowest neighbor BGP router ID} Note: Use show ip bgp neighbors command to check next hop router ID on local router. else{ }=FALSE => go to if{just_one_route_with_lowest_ neighbor_ip_address: then{use ROUTE that got lowest neighbor IP address} Note: Use present show ip bgp or show ip bgp neighbors command to check neighborip address. else{ }=FALSE => go to 10. Information related with the process of BGP route-selection given in the table above is taken from presented BGP table. BGP table version is 12, local router ID is Status codes: s suppressed, d damped, h history, * valid, < best, i - internal Origin codes: i - IGP, e - EGP,? - incomplete Network Next Hop Metric LocPrf Weight Path * i *> i *> / i *> / i *> i As we can see from results in BGP route-selection process table, choosen route is route through next-hop IP address (IP address of SanJose1 router)

10 Refer to the exhibit. On the basis of the configuration that is provided, how would the BGP updates that come from router R1 be replicated inside autonomous system 65200? A. All BGP updates that are received on router R2 will be sent to routers R3 and R4. Routers R3 and R4 will then forward those BGP updates to router R5. B. All BGP updates that are received on router R2 will not be sent to routers R3 and R4. C. All BGP updates that are received on router R2 will be sent directly to router R5. D. None of the BGP updates that are received on router R2 will ever be received by router R5. Answer: D Synchronization can be off because all routers within the transit path in AS are running IBGP. The network in our task illustrates IBGP update behavior in a partially meshed neighbor environment

11 Router R2 receives an EBGP update from Router R1. Router R2 has two IBGP neighbors, router R3 and R4, but does not have an IBGP neighbor relationship with router R5. Therefore, router R3 and R4 learn about any networks that were added or withdrawn behind router R2. Even if router R3 and R4 have IBGP neighbor sessions with router R5, they assume that the autonomous system is fully meshed for IBGP and do not replicate the update and send it to router R5. Sending the IBGP update to router R5 is router R2 s responsibility because it is the router with firsthand knowledge of the networks in and beyond autonomous system So, router R5 does not learn of any networks through router R2 and does not use router R2 to reach any networks in autonomous system or other autonomous systems behind autonomous system Graphical representation is shown below