IP routning inom enkla datornät, 2D1490 IP routing in simple networks, 2D1490. Datum: 23 Maj :00 13:00

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1 Examination IP routning inom enkla datornät, 2D1490 IP routing in simple networks, 2D1490 NADA/KTH Datum: 23 Maj :00 13:00 Inga hjälpmedel är tillåtna - Ni får ej använda böcker eller miniräknare! Svara på svenska eller engelska. Frågorna är ej sorterade efter svårighetsgrad. Det är maximalt 40 poäng. För godkänt krävs 20 poäng. Alla frågor är på engelska. Kursansvarig är Olof Hagsand, KTH CSC, tel:

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3 1. Routers and routing (5p) Answer briefly: 1. What is the difference between a host and a router? (1p) 2. What is equal-cost multipath? (1p) 3. What is route re-distribution? (1p) 4. What is the role of a control-plane of a router? (1p) 5. What is the difference between a static and an aggregated route? (1p)

4 2. Spanning-Tree (5p) Study the network below. It consists of four learning bridges connected by links. All bridges run the spanning-tree protocol. The number on each learning bridge is its bridge-id. The number on the ports are the port numbers. There is a shared Ethernet between bridges 222 and 444. Note that the administrator has (for some obscure reason) assigned costs on each link equal to the port-numbers! Using the notation in the bottom of the figure, draw the spanning tree as it will be after the spanning-tree protocol has converged. Either make your own drawing, or draw on this paper (but be sure to submit it). Motivate your choices root port designated blocked/listening/forwarding

5 3. Distance-vector (5p) Suppose there are three routers A, B, and C attached to the same subnetwork /27, and a network /27 attached to both routers A and B as shown in the Figure below. All routers run RIP-2 with split horizon and poison reverse. All metrics are in hops. After RIP has converged, which RIP update messages (for /27) are sent over the /27 network? That is, how is /27 announced (including nexthop and costs) by each router? Also, which route (to /27) will C install in its routing table? Motivate your answer..3 C /27 A /27 B

6 4. Link-state routing (10p) Regard the network below consisting of routers and Ethernet links. The routers all run OSPF (Open Shortest Path First) and have router-ids A-D according to the figure. There are three areas: The backbone area and areas 1 and 2. There are also four external networks ( /24, /24, /24 and /24) which are redistributed into OSPF in area 0 according to the figure. A B / / Area / /30 C D.2 Area / / /24 Area 0 Specify the LSAs (Link-State Advertisment) in the following cases as they appear in router D. Specify for all LSAs, LS type, LS ID and advertizing router. For router LSAs, specify also link type and link id for each associated link. For network LSAs specify netmask and connected router IDs. For summary and external LSAs, specify network mask. a) Regular areas. Specify the LSAs in router D (from area 2's perspective) if area 1 and 2 are regular areas. b) Stub areas. Specify the LSAs in router D if area 1 and 2 are stub areas. (If it is more convenient, specify only the changes from (a)) c) Not-so-stubby areas (NSSAs). Specify the LSAs in router D if area 1 and 2 are NSSAs and if the external routes imported by D are imported into area 2 instead of area 0. That is, /24 and /24 are redistributed into OSPF by router D into area 2.

7 5. Multicast (5p) Source-specific Multicast (SSM) is a new variant of multicast where the source is explicitly given. In contrast, the default multicast model uses anysource multicasting (ASM). Which changes need to be made in the networking infrastructure to support SSM? You should cover which protocols are affected and how. Also describe advantages and disadvantages with SSM with respect to ASM.

8 6. MPLS (5p) The figure below illustrates a network with MPLS-enabled routers. The routers are placed in Swedish towns. The numbers by the router ports are interface names. All link metrics are one. An operator wishes to establish traffic engineered LSP:s (Labelled switched paths) between the routers using MPLS(Multi-protocol label switching), RSVP (Resource reservation Protocol) and IS-IS(Intermediate-System to Intermediate System). For example, one LSP should go from Kristianstad to Västerås via Jönköping (using RSVP loose source routing). Another LSP should go from Kristianstad to Borlänge via Halmstad, Göteborg, Jönköping, Västerås in that order (using RSVP strict source routing). Your assignment is to show the end-result after RSVP has established these two LSPs, by filling in the MPLS forwarding tables of involved routers. The forwarding tables should be filled in using MPLS label operations, labels and interfaces. You should use pen-ultimate popping. You do not need to specify FECs for the ingress. Borlänge Uppsala Göteborg Jönköping Västerås Linköping Stockholm Halmstad Kristianstad

9 7. Load-balancing (5p) Load balancing can be achieved on many levels in networks today. For the following three network scenarios, describe for each case if and how the traffic balancing is made and compare the technologies. You should cover how the decision is made to load-balance, and which mechanisms there are for deciding which packet should follow which link or path, and which drawbacks there may be. You should also make clear the differences between control-plane and forwarding-plane: which actions/functionality is made by the control-plane, which functionality/action is made by the forwarding plane? The three cases are as follows: 1) Two learning bridges running STP connected by two links where the connected ports of the links belong to the same LAG (Link-aggregate group). 2) Two learning bridges running STP connected by two separate links (no LAG). 3) Two routers running OSPF connected by two equal cost paths (not necessarily directly connected).