Evolution of Routing Techniques


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A beginners guide into routing, its elements and basic working principles. An introduction to most widely used Unicast routing protocols and their working principles.

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Evolution of Routing Techniques

  1. 1. A Presentation by Tusharadri Sarkar Evolution of Routing Techniques December 7, 2009Tusharadri Sarkar1
  2. 2. Delivery, Forwarding, Routing IP and Mask Routing Table Unicast Routing Protocols AGENDA December 7, 2009Tusharadri Sarkar2
  3. 3. What is routing? Why is routing required? At which layer is routing done? How does a router work? Some Basic Questions December 7, 2009Tusharadri Sarkar3
  4. 4. What is routing? Routing is the process of selecting a path in a network along which the packets shall be sent to a destination Routing consists of A Router A set of routing protocols A routing information base (RIB) One or more routing algorithms December 7, 2009Tusharadri Sarkar4
  5. 5. Why is routing required?  For practical limitation of physical connections  For efficiently managing the network traffic  For efficient usage of network resources  For catering to different types of services  For congestion control December 7, 2009Tusharadri Sarkar5
  6. 6. At which layer is routing done?  Generally routing is done at network layer  Multilayer layer routing and Cross layer routing is also prevalent nowadays  Firewalls are often integrated with routers December 7, 2009Tusharadri Sarkar6
  7. 7. Router, Switch and Hub The basic difference is varying intelligence December 7, 2009Tusharadri Sarkar7
  8. 8. How does a router work? December 7, 2009Tusharadri Sarkar8
  9. 9. Delivery The network layer supervises the handling of packets by the underlying physical network Every packet undergoes at least one “Direct Delivery” and one or more “Indirect Delivery” December 7, 2009Tusharadri Sarkar9
  10. 10. Direct and Indirect Delivery To rest of Network Source SourceDestination Destination Direct Indirect Direct Direct December 7, 2009Tusharadri Sarkar10
  11. 11. Logical addressing: IP and MASK Internet Protocol address is a logical and global addressing scheme It uniquely defines the connection of a device/network to the Internet IPv4: 32 bit addressing scheme Address space: 232 = 4294967296 Notations: Dotted Decimal: Notations: Binary: 01110101 1001010 00011101 00000010 December 7, 2009Tusharadri Sarkar11
  12. 12. Logical addressing: IP and MASK  Classful addressing 1st 2nd 3rd 4th 0 10 110 1110 1111 1st 2nd 3rd 4th 0-127 128-191 192-223 224-239 240-255 Classes Class A Class B Class C Class D Class E Class No. of Blocks Block Size Application A 128 16,777,216 Unicast B 16,384 65,536 Unicast C 2,097,152 256 Unicast D 1 268,435,456 Multicast E 1 268,435,456 Reserved December 7, 2009Tusharadri Sarkar12
  13. 13. Logical addressing: IP and MASK Mask: A 32 bit number made of n contiguous 1s followed by (32-n) contiguous 0s (n<32) Default masks for Classful addressing: Given an IP and its mask, one can calculate: First Address Last Address Range of Addresses Class Binary Dotted Decimal CIDR A 11111111 00000000 00000000 00000000 /8 B 11111111 11111111 00000000 00000000 /16 C 11111111 11111111 11111111 00000000 /24 December 7, 2009Tusharadri Sarkar13
  14. 14. Logical addressing: IP and MASK Classless addressing: No more classes but a block of addresses are assigned, provided the following restrictions are strictly followed The addresses in the block must be contiguous The number of addresses must be a power of 2 The first address must be evenly divisible by the total number of addresses allocated Mask is a better way to define a block An example: Given an IP address What are the first, last and the total number of addresses assigned? December 7, 2009Tusharadri Sarkar14
  15. 15. Logical addressing: IP and MASK Binary equivalent of mask /28:  11111111 11111111 11111111 11110000 ( Binary equivalent of the address:  11001101 00010000 00100101 00100111 ( First address: Set the right most 4 bits to 0:  11001101 00010000 00100101 0010000 ( Last address: Set the right most 4 bits to 1:  11001101 00010000 00100101 00101111 ( Number of addresses: 232-n = 24 =16 So, in general a address in classless addressing is mentioned as: x.y.z.t/n December 7, 2009Tusharadri Sarkar15
  16. 16. Network Address  When a organization is allocated a block of addresses, normally (not always) the first address is treated as the network address  It is not assigned to any device, it defines the organization itself to the rest of the world REST of the WORLD Network Address: All packets with receiver address to are routed to x.y.z.t/nx.y.x.t/n … … December 7, 2009Tusharadri Sarkar16
  17. 17. Routing Table A host or a router maintains a ‘routing table’ with an entry for each specific destination The table can be STATIC or DYNAMIC Static Routing Table: Contains information entered manually by the administrator at the time of creation Cannot be modified automatically when there is any change in the Internet Dynamic Routing Table: Capable of updating the table with the help of routing protocols and algorithms automatically Only option for managing any large network of today December 7, 2009Tusharadri Sarkar17
  18. 18. Routing Table Mask: Defines the mask applied to that entry Network Address: Defines the network address to which the packet is finally delivered. In host specific routing, this is the destination host address Next Hop Address: Defines the address of the hop for the packet Interface: Shows the name of the interfaces Mask Network Interface Next-hop address Interface Flags Reference Count Use … … … … … … … December 7, 2009Tusharadri Sarkar18
  19. 19. Routing Table FLAGS: Defines up to five flags  U (Up): Router is up and running  G (Gateway): Destination is in another network  H (Host-specific): Network address is host-specific address. Otherwise the network address is the destination address  D (Added by redirection): Routing info is added to host routing table by redirection message from ICMP  M (Modified by redirection): Routing info for destination is modified to host routing table by redirection message from ICMP Reference Count: Defines number of users at this route at the moment Use: Defines number of packets transmitted through the router for a destination December 7, 2009Tusharadri Sarkar19
  20. 20. A quick look at a system routing table December 7, 2009Tusharadri Sarkar20
  21. 21. Network Configuration of a System form the Routing Table A UNIX server gives the following result with netstat and ifconfig command  $ netstat –nr  Kernel IP routing table  $ ifconfig eth0  Eth0 Link encap:Ethernet Hwaddr 00:B0:D0:DF:09:5D  Inet addr: Bcast: Mask: What is the network configuration of the server? Destination Gateway Mask Flags Iface U eth0 U lo UG eth0 December 7, 2009Tusharadri Sarkar21
  22. 22. Network Configuration from the Routing Table Rest of the Internet eth0 00:B0:D0:DF:09:5D Default Router December 7, 2009Tusharadri Sarkar22
  23. 23. Forwarding It means placing the packet in its route to its destination Requires a host or a router to have a routing table When the host has a packet to send or the router has received a packet, it looks up this routing table to determine route to the final destination Routing techniques caters to optimizing this table as maintain a full-fledged look-up table is impossible to maintain December 7, 2009Tusharadri Sarkar23
  24. 24. Forwarding Techniques Next-hop method Vs Route Method N1 N2 N3 R1 R2 Host A Host B Routing tables based on routing Destination Route Host B R1, R2, host B Destination Route Host B R2, host B Destination Route Host B Host B Routing tables based on Next-hop Destination Route Host B R1 Destination Route Host B R2 Destination Route Host B Host B For A For R1 For R2 December 7, 2009Tusharadri Sarkar24
  25. 25. December 7, 2009 Forwarding Techniques Network-Specific method Vs Host Specific method System DCBA N2N1 Routing table for host S based on host-specific method Destination Next Hop A R1 B R1 C R1 D R1 Routing table for host S based on network-specific method Destination Next Hop N2 R1 R1 Tusharadri Sarkar25
  26. 26. Forwarding Techniques Default Method: Using a default router N1 N2 R1 R2 Host A Rest of the Internet Default Router Destination Next Hop N2 R2 Any other R1 Routing table for host A December 7, 2009Tusharadri Sarkar26
  27. 27. Forwarding Process  In classless addressing, at least 4 columns are required  The routing table is searched based on the network address and mask Mask Network Address Next-hop Address Interface … … … … … … … … … … … … Extract Destination Address Search Table Forwarding Module To ARP Next –hop address and interface no. December 7, 2009Tusharadri Sarkar27
  28. 28. Managing Routing Table in Classless Addressing  Address aggregation: Blocks of addresses of different interface and mask are aggregated into one single block in routing table  Several levels of aggregation are possible Org 1 Org 2 Org 3 Org 4 m0 m1 m2 m3 m4 m0 m1 R1 R2 December 7, 2009Tusharadri Sarkar28
  29. 29. Managing Routing Table in Classless Addressing Address aggregation: Routing tables for router R1 and router R2 For R2, any packet with destination addresses to are sent to interface m0 regardless of any of the organizations Mask NA NHA Iface /26 … m0 /26 … m1 /26 … m2 /26 … m3 /0 Default m4 Mask NA NHA Iface /24 … m0 /0 Default m1 Routing table for R1 Routing table for R2 December 7, 2009Tusharadri Sarkar29
  30. 30. Longest Mask Matching  What happens if Org. 4 is not geographically close to the other 3 Orgs?  Can we still use Address Aggregation and assign the block to Org. 4? R2 R1 R3 Org 1 Org 2 Org 3 Org 4 m0 m1 m2 m3 m0 m2 m1 m0m1 m2 December 7, 2009Tusharadri Sarkar30
  31. 31. Longest Mask Matching  Answer: YES  Reason: LONGEST MASK MATCHING  The “Routing Table” is sorted from the longest mask to the shortest mask Mask NA NHA Iface /26 … m0 /26 … m1 /26 … m2 /0 Default m3 Mask NA NHA Iface /26 … m0 /0 Default m2 Mask NA NHA Iface /26 … m1 /24 … m0 /0 Default m2 Routing table for R1 Routing table for R2 Routing table for R3 December 7, 2009Tusharadri Sarkar31
  32. 32. Hierarchical Routing Hierarchical routing can greatly minimize the size of the routing tables For example, a regional ISP is granted a 16,384 (214 ) addresses starting from It is divided in to 4 sub-blocks each of size 4096 for 3 local ISPs. For them the mask is /20 1st local ISP divides its assigns sub-blocks into 8 smaller blocks for small ISPs. For them the mask becomes /23 Each small ISPs divides them into 128 sub-blocks for households. For them the mask becomes /30, and so on… December 7, 2009Tusharadri Sarkar32
  33. 33. Hierarchical Routing  The logical representation is displayed here Total 16,384 Total 4096 Total 4096 Total 4096 Total 4096 512 512 ISP 1 ISP 2.1 ISP 3.1 ISP 3.8 ISP 2.2 ISP 2.3 Total 4 Large Orgs. Total 16 Small Orgs. 128 Each 128 Each December 7, 2009Tusharadri Sarkar33
  34. 34. Geographical Routing The same concept of hierarchical routing can be extended in geographical routing To decrease the size of the routing tables further, segregation is done in geographical level as well For example, the entire address space is divided into few large blocks One block is assigned to North America, one to Asia, one to Africa, one to Europe and so on… So, for all the routers of the ISPs outside Europe, every router will have one and only entry for all the addresses assigned to Europe December 7, 2009Tusharadri Sarkar34
  35. 35. Unicast Routing Protocols Routing protocols are needed to maintain and update dynamic routing tables A routing protocols is a combination of set of rules (algorithms) and procedures Unicast routing protocols applies where each incoming packet has to be delivered to one and only one destination Router decides the next hope of a packet in a ‘Autonomous System’ based on ‘Optimization’ 3 most popular and basic Unicast Routing Protocols are: RIP (Distance Vector routing), OSPF (Path Vector routing) and BGP (Link State routing) December 7, 2009Tusharadri Sarkar35
  36. 36. Autonomous Systems An ‘Autonomous System’ or ‘AS’ is group of networks and routers under the authority of a single administration Routing inside AS : Intra-domain routing Routing between AS : Inter-domain routing AS1 AS4AS3 AS2 December 7, 2009Tusharadri Sarkar36
  37. 37. Routing Protocols Scope December 7, 2009Tusharadri Sarkar37
  38. 38. Optimization The router must always choose the optimum path between two networks for the packets There is a cost associated with each packet for passing through a network, called ‘Metric’ The metric is different depending on the routing protocols. For example:  In RIP, the hop count is used as the metric  In OSPF, the administrator can assign a cost for network based on the type of service required  In BGP, the administrator can set the cost based on the policy of the network December 7, 2009Tusharadri Sarkar38
  39. 39. Distance Vector Routing In DVR, the least cost route between any two nodes is the route with minimum distance Each node maintains a vector (table) of minimum distance to every node known There are three steps involved:  Initialization: At the beginning, each node knows the distance to its immediate neighbors  Sharing: Periodically or in triggered time, the nodes share their vectors with other nodes  Updating: Based on the shared info, nodes updates their vectors about path to indirectly connected nodes December 7, 2009Tusharadri Sarkar39
  40. 40. DVR: Initialization To Cost Next A 0 _ B 5 _ C 2 _ D 3 _ E ∞ A C B ED 5 3 2 4 34 To Cost Next A 3 _ B ∞ C ∞ D 0 _ E ∞ To Cost Next A 5 _ B 0 _ C 4 _ D ∞ E 3 _ To Cost Next A ∞ B 3 _ C 4 _ D ∞ E 0 _ To Cost Next A 2 _ B 4 _ C 0 _ D ∞ E 4 _ Table of “A” Table of “B” Table of “C” Table of “D” Table of “E” December 7, 2009Tusharadri Sarkar40
  41. 41. DVR: Sharing and Updating Each node will share its routing table on periodic basis or triggered condition Full routing table needed not be shared. In our scenario, only column 1 and column 2 will be shared. Next Hop Address (column 3) will be calculated based on that Receiving a partial table from its neighbor, a node calculates a temporary updated table Then each row of the old and new table are compared based on the next node entry (col. 3) If next node entry is different, the row with smaller cost is chosen. If there is a tie, old entry is kept If next node entry is same, the new entry is chosen December 7, 2009Tusharadri Sarkar41
  42. 42. DVR: Updating Table for “A” To Cost Next A 0 _ B 5 _ C 2 _ D 3 _ E ∞ To Cost A 2 B 4 C 0 D ∞ E 4 To Cost Next A 4 C B 6 C C 2 C D ∞ C E 6 C To Cost Next A 0 _ B 5 _ C 2 _ D 3 _ E 6 C Old Table of “A”Table Received from “C” Modified Table of “A” New Table of “A” Compare December 7, 2009Tusharadri Sarkar42
  43. 43. DVR: The Finalized Tables To Cost Next A 0 _ B 5 _ C 2 _ D 3 _ E 6 C A C B ED 5 3 2 4 3 4 To Cost Next A 3 _ B 8 A C 5 A D 0 _ E 9 A To Cost Next A 5 _ B 0 _ C 4 _ D 8 A E 3 _ To Cost Next A 6 C B 3 _ C 4 _ D 9 C E 0 _ To Cost Next A 2 _ B 4 _ C 0 _ D 5 A E 4 _ Table of “A” Table of “B” Table of “C” Table of “D” Table of “E” December 7, 2009Tusharadri Sarkar43
  44. 44. DVR: Two Node Loop Instability X A B XX X X A AA A B B B B 2 4 4 4 4 4 . . . X 2 _ X ∞ _ X 10 B X 10 B X ∞ _ X 6 A X 14 A X 6 A X 6 A X ∞ _ Before Failure After Failure After A receives update from B After B receives update from A Finally December 7, 2009Tusharadri Sarkar44
  45. 45. DVR: Preventing Instability Defining ‘INFINITY’: Infinity should be defined as a smaller number say, 100. In RIP ‘Infinity’ is often defined as 16. So, the network can’t have more than 15 hops anywhere. Split Horizon: Each node sends only part of its table through each interface. In our case, B would not advertize its part of the table to A which contains information about X (i.e. the route of X is through A, so A already knows). Split Horizon & Poison Reverse: While sharing its table with A, B will add a tag to the route information of X (i.e. “I know this route comes from you. Please do not use this value”). December 7, 2009Tusharadri Sarkar45
  46. 46. Routing Information Protocol Routing Information Protocol (RIP) is an implementation of ‘Distance Vector Algorithm’ with the following considerations: 1. In an autonomous system, we are dealing with routers and networks (links). Only routers have routing tables, networks not 2. The destination in a routing table is a network always 3. The metric used by RIP is the no of hops needed to reach the destination 4. Infinity is defined as 16 5. The next-node column defines the address of the router to which packet is to be sent December 7, 2009Tusharadri Sarkar46
  47. 47. Link State Routing Domain Topology: Here, each node in the domain has an entire topology of the domain Link State: For each node, the number of other links and nodes, their connectivity type, cost (metric) and the condition of the links (Up or Down) constitutes link state Shortest Path Tree: Based on the link states, a node can use Dijkstra’s Algorithm to create a ‘Shortest Path Tree’ which can used as the routing table There are four sets of operations required Creation of Link State Packets (LSPs) Flooding of LSPs Formation of shortest path tree Calculation of routing based on the tree December 7, 2009Tusharadri Sarkar47
  48. 48. Link State Routing: A C B ED 5 3 2 4 State of Links for “A”  Initial Condition: D A B E 5 2 3 3 2 4 4 5 4 3 34 3 State of Links for “D” State of Links for “B” State of Links for “E” State of Links for “C” December 7, 2009Tusharadri Sarkar48
  49. 49. Link State Routing:  Dijkstra’s Algorithm: Formation of Shortest Path Tree START STOP Tentative list is empty? Set root to local node and move it to tentative list Among nodes in tentative list, move the ones with shortest path to permanent list Add each unprocessed neighbor of last moved node to tentative list if it is not there already. If neighbor is in tentative list with larger cumulative cost, replace with new one YES NO December 7, 2009Tusharadri Sarkar49
  50. 50. Link State Routing: A B 0 1. Set root to A and move A to tentative list  Creation of Shortest Path Tree for node A: A Permanent List: Empty Tentative List: A(0) Root December 7, 2009Tusharadri Sarkar50
  51. 51. Link State Routing: A B 0 2. Move A to permanent List. Add B, C, D to tentative list  Creation of Shortest Path Tree for node A: A Permanent List: A(0) Tentative List: B(5), C(2), D(3) Root 5B 2 C 3 D December 7, 2009Tusharadri Sarkar51
  52. 52. Link State Routing: A B 0 3. Move C to permanent List. Add E tentative list  Creation of Shortest Path Tree for node A: A Permanent List: A(0), C(2) Tentative List: B(5), D(3), E(6) Root 5B 2 C 3 D 6E December 7, 2009Tusharadri Sarkar52
  53. 53. Link State Routing: A B 0 4. Move D to permanent List.  Creation of Shortest Path Tree for node A: A Permanent List: A(0), C(2), D(3) Tentative List: B(5), E(6) Root 5B 2 C 3 D 6E December 7, 2009Tusharadri Sarkar53
  54. 54. Link State Routing: A B 0 5. Move B to permanent List.  Creation of Shortest Path Tree for node A: A Permanent List: A(0), B(5), C(2), D(3) Tentative List: E(6) Root 5B 2 C 3 D 6E December 7, 2009Tusharadri Sarkar54
  55. 55. Link State Routing: A B 0 6. Move E to permanent List.  Creation of Shortest Path Tree for node A: A Permanent List: A(0), B(5), C(2), D(3), E(6) Tentative List: Empty Root 5B 2 C 3 D 6E December 7, 2009Tusharadri Sarkar55
  56. 56. Link State Routing:  Calculation of Routing Table from Shortest Path Tree Node Cost Next A 0 _ B 5 _ C 2 _ D 3 _ E 6 C Routing table for node A  We can see that the routing table of A as deduced by Link State Routing is the same as Distance Vector Routing  In real scenario, the routing table is determined by the cost assigned to each node by the administrator December 7, 2009Tusharadri Sarkar56
  57. 57. Open Shortest Path First (OSPF) OSPF is based on Link State Routing Protocol Area: A collection of networks, hosts and routers all contained within an autonomous system Area Border Routers: Summarizes all the information about an area and shares it across Backbone: A special area among all areas in an AS which all other areas must be connected to. The backbone always has area code ‘0’ Backbone Routers: Routers in a backbone. A backbone router can also be area border router Virtual Link: If the connection between an area and backbone is broken the administrator can create an alternate connection between routers December 7, 2009Tusharadri Sarkar57
  58. 58. OSPF: Implementation net net net net net net net net net net Area 1 Area 2 Area 0 (Backbone) ABRABR BR BR AS BR Autonomous System (AS) December 7, 2009Tusharadri Sarkar58
  59. 59. Path Vector Routing Why DVR and LSR are not suitable for inter-domain routing?  Reason: Scalability DVR becomes instable and intractable for a large number of hops (even more than 16) LSR needs a huge amount of resource to calculate its shortest paths. It also causes heavy traffic in the network because of flooding of LSP How path vector routing eliminates them? Well, it is simply derived from DVR, but does not assign hop count as the metric/cost... December 7, 2009Tusharadri Sarkar59
  60. 60. Path Vector Routing Speaker node: In path vector routing, a special node acts on behalf on the entire AS. It summarizes all the information of that AS, creates a routing table and advertizes it to other ASs What is advertized? Not the metrics but the paths in an AS Policy: Every AS will have a well defined policy Paths are decided upon by the speaker nodes by consulting the policies in neighboring ASs Reason: Different ASs will have different policies & priorities associated with them December 7, 2009Tusharadri Sarkar60
  61. 61. Path Vector Routing Initialization: At the beginning each SN knows only about all other nodes inside its AS Sharing: Just as in DVR, the speaker nodes will then share their tables with immediate neighbors periodically or on trigger Updating: On receiving a two column table from neighbor, a speaker node will update its table by adding the nodes not present in its routing table as well as adding its own AS and other ASs that sent the table Loop Prevention Policy Routing Optimum Path December 7, 2009Tusharadri Sarkar61
  62. 62. Path Vector Routing A1 D1 C1 B1 A2 A3 A4 A5 C2 C3 B2 B3 B4 D2 D3 D4 Dest. Path A1 AS1 A2 AS1 A3 AS1 A4 AS1 A5 AS1 Dest. Path C1 AS3 C2 AS3 C3 AS3 Dest. Path B1 AS2 B2 AS2 B3 AS2 B4 AS2 Dest. Path D1 AS4 D2 AS4 D3 AS4 D4 AS4 AS1 AS2 AS3 AS4 TableofA1 TableofD1 TableofC1 TableofB1 December 7, 2009Tusharadri Sarkar62
  63. 63. Path Vector Routing  Stabilized tables of all AS after update: Dest Path A1 … A5 AS1 … AS1 B1 … B4 AS1-AS2 … AS1-AS2 C1 … C3 AS1-AS3 … AS1-AS3 D1 … D4 AS1-AS2-AS4 … AS1-AS2-AS4 Dest Path A1 … A5 AS2-AS1 … AS2-AS1 B1 … B4 AS2 … AS2 C1 … C3 AS2-AS3 … AS2-AS3 D1 … D4 AS2-AS3-AS4 … AS2-AS3-AS4 Dest Path A1 … A5 AS3-AS1 … AS3-AS1 B1 … B4 AS3-AS2 … AS3-AS2 C1 … C3 AS3 … AS3 D1 … D4 AS3-AS4 … AS3-AS4 Dest Path A1 … A5 AS4-AS3-AS1 … AS4-AS3-AS1 B1 … B4 AS4-AS3-AS2 … AS4-AS3-AS2 C1 … C3 AS4-AS3 … AS4-AS3 D1 … D4 AS4 … AS4 Table of A1 Table of B1 Table of C1 Table of D1 December 7, 2009Tusharadri Sarkar63
  64. 64. Path Vector Routing Some important features of updating: Loop Prevention: The instability of DVR is avoided in PVR; upon receiving a message the router checks to see if its AS is in the path Policy Routing: Upon receiving a message a router checks the path with policy. If an AS in the path is against policy it can ignore that Optimum path: Router find the path that fits the organization best. A path from AS4 to AS1 can either be AS4->AS3->AS2->AS1 or AS4->AS3->AS1. Here we will choose the path with less number of ASs involved This is not a general rule. There are complex criteria which are always involved in real scenario December 7, 2009Tusharadri Sarkar64
  65. 65. Border Gateway Protocol (BGP) BGP was introduced in 1989 Some features of BGP: Types of AS  Stub AS: An AS which is connected to another AS. A stub is either a source or a sink  Multihomed AS: An AS which is connected to more than one AS, but it is only a sink or source. Example: A large corporation which is connected to more than one regional or national ASs  Transit AS: A multihomed AS that allows flow of data traffic through it. Example: All national and international ISPs December 7, 2009Tusharadri Sarkar65
  66. 66. Border Gateway Protocol (BGP) Some features of BGP: Path Attributes  Well known attribute: Every BGP router must recognize  Well known mandatory attribute: It must appear in the description of a route; e.g. origin, next-hop  Well known discretionary attribute: It must be recognized but need not be included always in update  Optional Attribute: Need not be recognized by all BGP routers  Optional transitive attribute: It must be passed to the next router by the router that has not implemented it  Optional non-transitive attribute: It must be discarded if the receiving router has not implemented it December 7, 2009Tusharadri Sarkar66
  67. 67. Border Gateway Protocol (BGP) Some features of BGP: BGP Sessions: A BGP session is a connection setup between two BGP routers for the sake of exchanging router information A session in BGP is a connection at the TCP level.  External BGP Session (E-BGP): Takes place when two speaker nodes exchange routing information  Internal BGP Session (I-BGP): Takes place when a speaker node collects information from other nodes in the its own As A1 A2 A3 A4 A5 AS1 C1 C2 C3 AS3 December 7, 2009Tusharadri Sarkar67
  68. 68. Reference December 7, 2009Tusharadri Sarkar Data Communications and Networking –Behrouz A. Forouzen 68
  69. 69. Thank You December 7, 2009Tusharadri Sarkar69