CCN Activity Session for Section 4.2 – BGP
For this sheet, we’ll be thinking about the Border Gateway Protocol (BGP).
Question
Fill in the blank: RIP advisements typically announce the number of hops to various destinations. BP updates on the other hand, announce the _______ to the various destinations.
Sequence of AS on the routes
Question
How does BGP use the NEXT-HOP and AS-PATH attributes?
Routers use the AS-PATH attribute to detect and prevent looping advertisements; they also use it in choosing among multiple paths to the same prefix. The NEXT-HOP attribute indicates the IP address of the first router along an advertised path (outside of the AS receiving the advertisement) to a given prefix. When configuring its forwarding table, a router uses the NEXT-HOP attribute.
Question
Describe how the administrator of an upper-tier ISP can implement policy when configuring BGP.
The answer to this question is selective route advertisement. Consider a tier-1 ISP, B, which may not to carry transit traffic between two other tier-1 ISPs, say A and C, with which B has peering agreements. To implement this policy, ISP B would not advertise to A routes that pass through C; and would not advertise to C routes that pass through A.
Consider the network shown below. ISP B provides national backbone service to regional ASP A. ISP C provides national backbone service to to regional ISP D. Each ISP consists of one AS. B and C peer with each other in two places using BGP. Consider traffic going from A to D. B would prefer to hand over the traffic to C on the West Coast (so C has to carry it cross-country), which C would prefer to receive it via the peering point on the East Coast (so that B would have carried the traffic cross country).
Question
What BP mechanism might C use, so that B would hand over A-to-D traffic at its East Coast peering point?
One way for C to force B to hand over all of B’s traffic to D on the east coast is for C to only advertise its route to D via its east coast peering point with C.