1 / 26

Network Layer Lecture 15 Imran Ahmed University of Management & Technology

Network Layer Lecture 15 Imran Ahmed University of Management & Technology. Agenda. Introduction & Network layer functions Routing principles Hierarchical routing The Internet protocol (IP) Routing in the Internet. OSPF. It’s a Link-State protocol Used for Intra-AS routing

gyan
Télécharger la présentation

Network Layer Lecture 15 Imran Ahmed University of Management & Technology

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Network Layer Lecture 15 Imran Ahmed University of Management & Technology

  2. Agenda • Introduction & Network layer functions • Routing principles • Hierarchical routing • The Internet protocol (IP) • Routing in the Internet

  3. OSPF • It’s a Link-State protocol • Used for Intra-AS routing • Publicly available protocol • Classless routing protocol • Administrative distance = 110 • OSPF v2 is defined in RFC 2328

  4. OSPF: How It Works • "Hello" packets sent periodically on all OSPF-enabled interfaces. • Adjacencies (virtual point-to-point links) formed between some neighbors. • Once an adjacency is established, trade information with your neighbor. • Topology information is packaged in a "link state announcement“. • Announcements are sent ONCE, and only updated if there's a change (or every 30 minutes).

  5. OSPF: How It Works • Each router sends Link State Announcements (LSAs) over all adjacencies. • LSAs describe router's links, interfaces and state • Each router receives LSAs, adds them into its database, and passes the information along to its neighbors. • Each router builds identical link-state database. • Runs SPF algorithm on the database to build SPF tree. • Forwarding table built from SPF tree.

  6. OSPF: How It Works • When change occurs: • All routers run SPF algorithm. • Install output into forwarding table.

  7. HELLO Packets • Broadcast* HELLO on network segment. • Receive ACK. • Repeat periodically: • Default: HELLO sent every 10 seconds. • Default: if no HELLO heard for 40 seconds, link is assumed to be dead. • Now establish adjacencies. * Actually uses Multicast addresses (224.0.0.9, 224.0.0.10) so that non-OSPF devices can ignore the packets

  8. The HELLO Packet • Router priority • Hello interval • Router dead interval • Network mask • List of neighbors HELLO HELLO HELLO These must match

  9. Neighbors • Bi-directional communication. • Result of OSPF hello packets. • Need not exchange routing information.

  10. Who is Adjacent? • "Adjacent" neighbors exchange routing information. • Not all neighbors are adjacent. • On a point-to-point link: • everyone • On broadcast medium: • not everyone • why?

  11. Broadcast Medium • Select a neighbor: Designated Router (DR): • It is said to be adjacent to all the others routers and exchanges information among them. • All routers become adjacent to DR. • Exchange routing information with the DR. • DR updates all the other neighbors. • Backup Designated Router (BDR).

  12. LSAs propagate along adjacencies DR BDR

  13. Broadcast Medium • Assigning of DR & BDR are as follows: • Checks the router’s priority (by default, 1). • Router with highest priority will be assigned DR. • Router with lowest priority will be assigned BDR. • If priority same then decision will be made on the basis of router id. • Router id – the highest IP address on router’s ports.

  14. Some Other Features of OSPF • Authentication (optional). • Equal-cost multi-path: • more than one "best" path - share traffic. • Proper classless support (CIDR). • Multiple areas: • For very large networks (>150 routers). • Aggregate routes across area boundaries. • Keep route flaps within an area. • Proper use of areas reduce bandwidth and CPU utilization. • Backbone is Area 0.

  15. Hierarchical Structure of OSPF • Two-level hierarchy: local area, backbone. • Link-state advertisements only in area • each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. • A hierarchical structure of OSPF network defines four types of OSPF routers: • Internal routers – these routers are in non-backbone areas and perform only intra-AS routing. • Area border routers (ABR) – these routers belong to both an area and the backbone. It “summarizes” distances to nets in own area, advertise to other Area Border routers. • Backbone routers (non-border routers) – these routers perform routing within the backbone but themselves are not area border routers. • Boundary routers – a boundary router exchanges routing information with routers belonging to other AS. This router might, for example, use BGP to perform inter-AS routing.

  16. Hierarchical OSPF

  17. Internet inter-AS routing: BGP • BGP (Border Gateway Protocol):The de facto standard • BGP provides each AS a means to: • Obtain subnet reachability information from neighboring ASs. • Propagate the reachability information to all routers internal to the AS. • Determine “good” routes to subnets based on reachability information and policy. • Allows a subnet to advertise its existence to rest of the Internet: “I am here”

  18. Inter-AS routing in the Internet: BGP

  19. 3a 3b 2a AS3 AS2 1a 2c AS1 2b eBGP session 3c 1b 1d 1c iBGP session BGP basics • Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions • Note that BGP sessions do not correspond to physical links. • When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix. • AS2 can aggregate prefixes in its advertisement

  20. 3a 3b 2a AS3 AS2 1a 2c AS1 2b eBGP session 3c 1b 1d 1c iBGP session Distributing reachability info • With eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. • 1c can then use iBGP do distribute this new prefix reach info to all routers in AS1 • 1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP session • When router learns about a new prefix, it creates an entry for the prefix in its forwarding table.

  21. Path attributes & BGP routes • When advertising a prefix, advert includes BGP attributes. • prefix + attributes = “route” • Two important attributes: • AS-PATH: contains the ASs through which the advert for the prefix passed: AS 67 AS 17 • NEXT-HOP: Indicates the specific internal-AS router to next-hop AS. (There may be multiple links from current AS to next-hop-AS.) • When gateway router receives route advert, uses import policy to accept/decline.

  22. BGP route selection • Router may learn about more than 1 route to some prefix. Router must select route. • Elimination rules: • Local preference value attribute: policy decision • Shortest AS-PATH • Closest NEXT-HOP router: hot potato routing • Additional criteria

  23. BGP messages • BGP messages exchanged using TCP. • BGP messages: • OPEN: opens TCP connection to peer and authenticates sender • UPDATE: advertises new path (or withdraws old) • KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous msg; also used to close connection

  24. BGP routing policy • A,B,C are provider networks • X,W,Y are customer (of provider networks) • X is dual-homed: attached to two networks • X does not want to route from B via X to C • .. so X will not advertise to B a route to C

  25. BGP routing policy (2) • A advertises to B the path AW • B advertises to X the path BAW • Should B advertise to C the path BAW? • No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers • B wants to force C to route to w via A • B wants to route only to/from its customers!

  26. Why different Intra- and Inter-AS routing ? Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • Intra-AS: single admin, so no policy decisions needed Scale: • hierarchical routing saves table size, reduced update traffic Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance

More Related