Davids 2008 33 bgp detection of link or host failure

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Unformatted text preview: ar end does not respond to the keep-alive messages, the sender will eventually remove the AS from its routing table. ITM440/540 Carol Davids © 2008 34 Border Gateway Protocol • BGP is now in its fourth version. • It enables routing between autonomous systems. • The method it uses is called Path Vector Routing. ITM440/540 Carol Davids © 2008 35 Neighbors • Two autonomous boundary routers connected to the same network are neighbors. • Remember that Autonomous Systems are collections of networks under a common administrative authority. • The illustration on the next page shows Autonomous System Boundary Routers and their relations to networks and neighbors. ITM440/540 Carol Davids © 2008 36 R1 and R2 are neighbors because they are both connected to the same network - N3 R2 N4 N3 N1 N8 AS2 N5 AS1 N2 R1 R2 and R3 are neighbors because they are both connected to the same network - N4 R3 R3 and R4 are neighbors because they are both connected to the same network - N6 N6 N8 AS3 N7 R4 AS4 N9 ITM440/540 Carol Davids © 2008 37 Router operation using Path Vector method When a router receives a Path Vector message it: 1. Verifies that the path being advertised in in accordance with its own policy 2. Updates its routing table if appropriate 3. Modifies the message and forwards it to its neighbors ITM440/540 Carol Davids © 2008 38 Loop Avoidance • A router using BGP prevents loops by checking to see if its own AS is in the path list to the destination. • If it is, then the router ignores the BGP message. ITM440/540 Carol Davids © 2008 39 Path is a list of Attributes • The path is a list of attributes. • Each attribute describes some aspect of the advertised path. • There are two kinds of attributes - Well-known and Optional. • Well-known attributes are recognized by all routers running BGP. • Optional attributes need not be recognized by all BGP routers. ITM440/540 Carol Davids © 2008 40 Examples of well-known Attributes • NEXT_HOP is the well-known attribute that identifies the next-hop router ITM440/540 Carol Davids © 2008 41 Examples of well-known Attributes • AS_PATH is the ordered list of AS’es that need to be traversed to reach the destination network ITM440/540 Carol Davids © 2008 42 Examples of well-known Attributes • ORIGIN defines the source routing protocol that provided the routing information contained in this message • It could be RIP or OSPF for example. ITM440/540 Carol Davids © 2008 43 BGP Message Types • There are 4 types of BGP message: • Open • Update • Keep-alive • Notification. ITM440/540 Carol Davids © 2008 44 Open • Open message is sent to create a neighborhood relationship with another router. • The response to an Open message is a Keepalive message ITM440/540 Carol Davids © 2008 45 Update • Update message is used to withdraw previously-advertised paths and to advertise new paths. • A single update message can be used to withdraw several paths. • A single update message can only advertise one new path. ITM440/540 Carol Davids © 2008 46 Keep-alive • Keep-alive messages are exchanged by neighbors to assure each other that they are up and running. • A timer is set after the receipt of a keep-alive message from Router N and a new BGP message must be received from Router N before that timer expires . • Failing this, the receiving router will start some kind of remedial action. ITM440/540 Carol Davids © 2008 47 Notification • Notification messages are sent to report errors or to close the connection between the two routers. ITM440/540 Carol Davids © 2008 48 The End ITM440/540 Carol Davids © 2008 49...
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