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• Multiple path attributes to determine the best path for a certain prefix.
• When no path attributes are set, BGP uses the AS_PATH to determine the best route.
  • Shortest AS path wins
• AS_SEQ is the component of the AS_PATH attribute.
  • 10.10.10.0/24 20 10
    • AS-SEQ (in order)
• With summarization:
  • 10.10.0.0/16 20 10 { 1 2 5 4 3 }  
  • Prefix AS-PATH AS-SEQ  { AS-SET } 
    • AS-SET are not in order
• Prevents Routing loops.
• If a BGP router received an update with it’s own AS#, it indicates a loop.
• If a BGP router received an update with it’s own AS#, it drops the route.

BGP message types

New relation between two BGP peers:

• Open
  • type 1
  • Version4 (ipv4), AS#, Hold time, Router-iD, parameters.

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• idle:
  • BGP is down or waiting next retry
• Connect:
  • The BGP is waiting for the TCP connection to be completed
• Active:
  • The TCP Connection failed, Connect-retry timer running, listening for incoming TCP Connections
• Opensent:
  • The TCP connection exists and a BGP Open Message as been send.
  • The matching Open Message has not yet been received
• Openconfirm:
  • Open Message has been send and received
• Established:
  • All neighbor parameters match
  • Relationship works
  • Peers can exchange update messages

• Local router find the outgoing interface for the neighbor
• the IP of the outgoing interface is used as the source IP by default.
• With one link a failure in that link can lose the neighborship.
  •  configure two neighbor commands
  • Use the loopback interfaces as TCP endpoints
• Loopback links between two AS might miss the route to the peer’s loopback
  • fixed with multihop (TTL=255 instead of TTL=1).
    
    • create routes between peers to reach loopback address.
    • configure update-source so the right source-ip is used.

R10#(config)router bgp 10
R10#(config-router)neighbor 20.20.20.20 remote-as 20
R10#(config-router)neighbor 20.20.20.20 ebgp-multihop
R10#(config-router)neighbor 20.20.20.20 update-source loopback0

R20#(config)router bgp 20
R20#(config-router)neighbor 10.10.10.10 remote-as 10
R20#(config-router)neighbor 10.10.10.10 ebgp-multihop
R20#(config-router)neighbor 10.10.10.10 update-source loopback0

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• eBGP ad  20
• iBGP  ad 200
• BGP uses TCP port 179
• Designed as an Exterior gateway Protocol.

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• PBR intercepts packets before regular routing
• PBR overwrites the router’s natural destination
• PBR is tied to route-maps
  • Define the match criteria
  • Define the action for the packets
•  Packet forwarding
  • Outgoing interface
  • IP Next-Hop

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• Can be used by various
  • PBR
  • Static route
  • HSRP
• Feature to monitor destination
  • Cpu creates data to monitor
  • Operations
    • ICMP, RTP, TCP, UDP, DNS, DHCP, HTTP, FTP
  • Different operations can be monitored at the same time
• The receiver can be a router or host
  • Might be configured as a SLA Responder

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• One link within each routing domain
• Working routing config for each routing domain
• Redistribution configuration between routing protocols
  • Mutual redistribution
  • Bi-directional redistribution

Redistribution into EIGRP

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There are four types of stub areas

• stub
• totally-stubby
• not-so-stubby areas (NSSA)
• totally not-so-stubby areas
• All stub areas do not allow Type 5 (external) LSAs (ABR always filters them)
• For the totally stub areas the ABR filters the Type 3 LSAs.

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