QID:RTE227

[angelo8888]

View the exhibit. Currently the two eBGP links between AS100 and AS200 have an average inbound load of 65% and 20% respectively. After further investigation, traffic to 10.10.1.16/28 accounts for 45%, and traffic to 10.10.1.32/28 and to 10.10.1.48/28 each account for 20% of the inbound load. The BGP attributes are currently set at their default values in both autonomous systems.

If you want to influence how AS200 sends traffic to AS100, which eBGP configurations would you configure in AS100 to influence AS200 to use the eBGP links more evenly? (Select two.)



Your Answer:    route-map as_50 permit 10
                          match ip address 50
                          set metric 150
                          access-list 50 permit 10.10.1.32 0.0.0.240
                          neighbor 192.168.30.2 route-map as_50 out

Correct Answer: neighbor 192.168.20.2 route-map as_50 out
                          route-map as_50 permit 10
                          match ip address 50
                          set metric 150
                          access-list 50 permit 10.10.1.32 0.0.0.240



Explanation:
We want to reduce the incoming traffic on the 65% route and increase the traffic on the 20% route, by using mechanism that instructs EBGP neighbor in AS 200 to prefer more the path between routers F and B.
The MED is used to decide how to enter an AS. It is used when multiple pathways exist between two autonomous systems, and one AS is trying to influence the incoming path from the other AS. A lower MED value is preferred; the default set to 0 for Cisco routers. So what we want to do is raise the MED on a subnet on the 65% route in AS 100, that when advertised to EBGP neighbor in AS 200, it prefers the 20% route.
Since the 10.10.1.48 favors the 20% from router B route due to route cost,  and 10.10.1.16 favors the 65% route from router A due to the same reasoning, the subnet we should try to manipulate is the 10.10.1.32 because it is traveling the 65% path (20% +45%=65%). So lets raise the MED on the 65% path from router A to 150 so the lower MED on the 20% path from router B is preferred.
Step 1: Define the neighbor that you want to raise the MED to and the route-map you will use to do this:
Neighbor 192.168.30.2 route-map AS_50 out
Step 2: Write the Route map to raise Med to 150 and define the traffic associated with this route-map via ACL:
route-map as_50 permit 10
match IP address 50
set metric 150
access-list 50 permit 10.10.1.32 0.0.0.240
Now you have raised the med of a 20% traffic across the Router A route to the AS200. This traffic will now switch to use router B route as the preferred route due to lower MED. This in turn raises the traffic on router B to 40% (from 20%) and lowers the traffic on router A to 45% (from 65%).


Based on the explanation, shouldn't my answer be correct? Also the access-list entry wildcard mask should be 0.0.0.7 and not 0.0.0.240

[sdunn96]

prolly should be 0.0.0.15 for the access list.

Also BGP uses MED/Metric to advertise to an external partner what is the best way into the AS, when multiple routes are available.
So by (looking at the diagram for the question) setting the MED on router A to 150, the MED on router B I think should be 100 (or 0 could be the default)....
Which for BGP the lower the MED, the better the path.

So they are wanting to have the traffic bound for 10.10.1.32/28, which consumes 20% BW, to be sent over the lesser used link that runs between Routers B and F
Which will then spread the usage to 45% and 40% over the two links.

hope that helps.

[sdunn96]

https://learningnetwork.cisco.com/docs/DOC-2480

Default value for BGP MED is 0, so setting the MED to 150 on the other path will make the path using MED 0 more desirable.

[angelo8888]

i know but it does not explain why my answer was tagged wrong when it was clear in their explanation that  the neighbor ip for the route-map that I used is correct and their answer is wrong but their "correct answer" has the incorrect neighbor ip

plus, as you said, the ACL wildcard should be 0.0.0.15 and not 0.0.0.240

[sdunn96]

The reason your answer is wrong is because the traffic is already going across the Rtr A -> Rtr E link
They are wanting the traffic for the 10.10.1.32 network to choose the B -> F Link.

By using the neighbor statement::
neighbor 192.168.30.2 route-map as_50 out
You are just enforcing what is already in place, and that is that traffic will go through the A -> E link
Because you are setting the metric across the B -> F link with a metric of 150
While A -> E will be 0, and thus the lower metric of the two links.
And BGP prefers the lower MED over the higher MED.

Does that help??

Keep in mind that there is already (in your answer's case) the neighbor statement
RtrB# neighbor 192.168.30.2 remote-as 200
has been issued, so you are working from Rtr B, when you should be on Rtr A

neighbor 192.168.30.2 route-map as_50 out  /* This will apply a policy to a neighbor that has been setup already (going across B -> F link)
route-map as_50 permit 10  /*  Create a route map call as_50
match ip address 50  /*  Match IP address that are found in access list '50'
set metric 150  /* If an IP address match has occurred, then set its metric/MED to 150
access-list 50 permit 10.10.1.32 0.0.0.240  /* standard access list is created, named '50'