Exam 350-501 topic 1 question 114 discussion

The best answer is "C", and in the following you can see why? We're asked about manipulating BGP attributes to influence CE1 uses PE1 as a main path reaching to the Internet. To do that, the network administrators on the customer side have two options which are either Weight or Local-Pref; both have to be manipulated on the CE1, so by having that in mind, we can easily cross out options A and D. Between B and C, C seems better option as MED is used once we intend to influence traffic reaching to our ASN, and works when BGP peers belong to the same ASN which seems they are as PE1 and PE2 look like the PE routers of the same ISP.

C is correct Answer: You can amipulate MED for routes you advertise out and assign NED to received Routes. Make it higher for the less prefered link and default zero will be on the prefered link.

Can't be A or D for obvious reasons. C might be excluded because by default the MED is 0 or does not exist (if does not exist is it considered as 0 by default). 0 is the most preferred, so a better MED than 0 can't be configured.

This is the only router the customer has access to.

Answer A: the local preference defaults to 100. Higher local preferences are preferred. If you set the local preference on PE2 to a lower value, then PE1 will be preferred by CE1 as the primary exit path. Answer B? A horrible idea. origin can be set with route maps, but thats not the intended purpose. Origin should be left alone. Answer C: incorrect. Setting MED on the inbound routes at PE1 has no effect on incoming traffic. you would have to set lower MED on the outbound prefixes to allow the Provider to prefer PE1... and that is ONLY IF both PE1 and PE2 connect to the same provider. you would also have to set a higher med on PE2 so that PE1 is preferred. -None of the answers are a good solution because asymmetric routing can still occur. but the question asks for a preffered path "TO the internet", so that means outbound traffic. I'll stick with answer A. Thoughts?

Answer B

A- this would work if routers in same AS since LP is passed in iBGP B- this might work C- MED defaults to 0 so changing only CE1-PE1 would not do the work D- Weight is locally significicant, so NO

The issue with C is that, MED defaults to Zero. Thus how can you manipulate on CE-PE1 inbound... you would need to increase med on CE-PE2 inbound for this to work. Seems like B will be the only working solution based on the available info we have.

I just noticed ccie_race went for C "If we consider that PE1 and PE2 belongs to the same AS" but I wonder if the choices changed, it sound like he is talking about the A for local preference. If they are all on the same AS that will work. Otherwise, I think it is option B to modify the Origin

A is correct. Lower LP from PE2 will definatly force traffic from CE1 towards PE1. C is not complete; we have to manipulate MED from both PEs not only from one; default is MED =0; how to beat this if only manipulating MED for routes from PE1?

i confirm C is correct

C seems correct If we consider that PE1 and PE2 belongs to the same AS