Examination. IP routning på Internet och andra sammansatta nät, DD2491 IP routing in the Internet and other complex networks, DD2491

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1 Examination IP routning på Internet och andra sammansatta nät, DD2491 IP routing in the Internet and other complex networks, DD2491 Date: December :00 18:00 1. No help material is allowed - You are not allowed to use dictionaries, books, or calculators! 2. You may answer questions in English or Swedish. 3. Questions are not sorted according to level of difficulty 4. Max is 32 points. 16 points is required to pass. 5. Course responsible is Olof Hagsand, phone

2 1. Terminology (5p) Briefly explain the following concepts related to inter-domain routing and BGP: a) Next-hop self b) Provider-independent address block c) Transit peering d) Multi-homed non-transit AS e) AS-path prepending f) BGP Path attribute g) Multi-hop peering h) BGP multi-path i) Conditional advertisment j) Prefix hijacking 2. BGP attributes 1 (2p) / / / R1 R2 R3 R4 R5 R / / Regard the figure where three small networks (1, 2 and 3) run an internal gateway protocol and exchange routes using BGP and run IBGP internally. The address assignment is shown in the figure. Defaults settings are used in BGP and the IGPs. Briefly answer the following questions regarding the BGP announcements (UPDATEs) received by router R1. You may have to motivate your answer. a) What is the value of NEXT-HOP for /30? b) What is the value of AS-PATH for /30? c) What is the value of AS-PATH for /30? d) What is the value of the origin attribute for /30? 3. BGP attributes 2 (2p) A good example of the use of optional transitive attributes is the transition mechanism currently taking place in the Internet for four-byte AS numbers. Describe how this transition works. In particular, how are optional transitive attributes in BGP used? 4. Path-vector protocol (2p) The underlying route distribution algorithm in BGP is path-vector. a) Describe how path-vector extends distance-vector and what advantages it has over distance-vector. (1p)

3 b) The path-vector algorithm in BGP is limited with respect to internal BGP. What limitation is this, and what is the consequence for IBGP peering? (1p) 5. Transport (2p) BGP uses TCP as underlying protocol for peerings between speakers. Other examples include OSPF that uses IP, RIP uses UDP and IS-IS uses link-layer protocols. What would be the consequences in BGP operation if instead of TCP, BGP would use the following protocols as peering protocol? What are the effects of protocol complexity, peering architecture, etc. a) A link-layer protocol (eg Ethernet) (1p) b) UDP or IP directly. (1p) 6. Load balancing (3p) There are several ways to perform load-balancing in BGP. Outline the differences between load-balancing (with respect to BGP peering) using: Multi-path BGP (1p) Loopback peering (1p) Link-level aggregation groups (LAG) (1p)

4 Site A A1 A2 A4 A3 Site B Site C B1 B4 C1 C4 B2 B3 C2 C3 7. Route reflection (5p) The network in the figure represents a national operator network running a single autonomous system (AS 14) consisting of the three sites A, B, and C. All routers within each site (e.g., A1-A4) are in one location and are connected full mesh within the site. The three sites (A, B, C) are interconnected with long-range single-mode fiber, and the one-way latency between a pair of sites is around 50ms. It is expensive for the operator to transmit data over these connections. The operator communicates using ebgp with other AS:s at each site as shown in the figure (eg B1 and B2). The external peerings represent both transit, customer and peering traffic. a) If the operator uses IBGP full mesh within the whole network, how many IBGP peerings exist in total? How many peerings are there over each long-range interconnection (eg how many TCP connections between site A and B)?(1p) b) Design a route reflector configuration for the network. The design has the following requirements: (1) avoid uneccesary duplications of peering traffic long-range connections; (2) redundancy in the sense that no single route reflector or backbone link should be a singlepoint of failure for transit traffic. Describe your design and draw a figure. Motivate your decisions. (4p)

5 8. AS confederations (3p) Regard again the figure in the previous exercise. Suppose the operator instead uses AS confederations to scale IBGP, with one sub-as per site with numbers 65001, 65002, and 65003, respectively. a) Describe how such a configuration would work. How are the sub-as numbers assigned and which BGP peerings will exist in the network? (1p) b) How is BGP extended (which attributes) in AS confederations? How is loop avoidance made using these attributes? (2p) 9. Protecting peering relationships (3p) An attacker may try to bring BGP peerings down and thus cause instability in an IP network. An attacker can also try to insert information into an existing connection between two BGP speakers. Describe the following techniques for protecting BGP peering: How do they work, which are their limitations? a) The generalized TTL security mechanism. (1p) b) TCP MD5 authentication. (1p) c) Route filtering. (1p) 10.L3VPN (5p) An operator runs L3VPN in its network using BGP/MPLS. In the main routing instance on one of the border routers, the following BGP routing state can be observed: Prefix Extended communities (RT) :1:: / : :1:: / : :1:: / : :1:: / : :2:: / : :2:: / : :2:: / : :2:: / :200 a) What is the purpose of a route distinguisher? Which are the route distinguishers in the routing table above? (1p) A router has a local VRF with the following local IPv4 prefixes in its IPv4 routing table: / / /24 b) Propose route target import and export rules for the VRF that would result in this situation. (3p) c) Using the network above, suppose a 'hub-and-spoke' L3VPN variant is requested. How can this be designed? Would the address assignment have to change? (1p)

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