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  1. 1. BGP · PART 1 Attributes About BGP Name Description Type Path VectorWell-known Mandatory · Must be supported and propagated eBGP AD 20 1 Origin Origin type (IGP, EGP, or unknown) iBGP AD 200 List of autonomous systems which the Standard RFC 4271 2 AS Path advertisement has traversed Protocols IP 3 Next Hop External peer in neighboring AS Transport TCP/179Well-known Discretionary · Must be supported; propagation optional Authentication MD5 Metric for internal neighbors to reach 5 Local Preference external destinations (default 100) Terminology Includes ASes which have been dropped Autonomous System (AS) 6 Atomic Aggregate due to route aggregation A logical domain under the control of aOptional Transitive · Marked as partial if unsupported by neighbor single entity 7 Aggregator ID and AS of summarizing router External BGP (eBGP) BGP adjacencies which span autonomous 8 Community Route tag system boundariesOptional Nontransitive · Deleted if unsupported by neighbor Internal BGP (iBGP) BGP adjacencies formed within a single AS Multiple Exit Metric for external neighbors to reach the 4 Discriminator (MED) local AS (default 0) Synchronization Requirement A route must be known by an IGP before 9 Originator ID The originator of a reflected route it may be advertised to BGP peers 10 Cluster List List of cluster IDs Packet Types 13 Cluster ID Originating cluster Open Update Cisco proprietary, not communicated to -- Weight peers (default 0) Keepalive Notification Path Selection Neighbor States Attribute Description Preference Idle · Neighbor is not responding 1 Weight Administrative preference Highest Active · Attempting to connect Communicated between peers Connect · TCP session established 2 Local Preference Highest within an AS Open Sent · Open message sent 3 Self-originated Prefer paths originated locally True Open Confirm · Response received 4 AS Path Minimize AS hops Shortest Established · Adjacency established Prefer IGP-learned routes over 5 Origin IGP EGP, and EGP over unknown Troubleshooting 6 MED Used externally to enter an AS Lowest show ip bgp [summary] 7 External Prefer eBGP routes over iBGP eBGP show ip bgp neighbors 8 IGP Cost Consider IGP metric Lowest show ip route [bgp] 9 eBGP Peering Favor more stable routes Oldest clear ip bgp * [soft]10 Router ID Tie breaker Lowest debug ip bgp […] Influencing Path Selection Weight neighbor weight 200 Local Preference bgp default local-preference 100 MED default-metric 400 Route Map neighbor route-map Foo Ignore Ignore Cost bgp bestpath as-path ignore bgp bestpath cost-community ignore AS Path Communitiesby Jeremy Stretch v2.1-r1
  2. 2. BGP · PART 2 Configuration Example interface Serial1/0 Router A AS 65100 description Backbone to B ip address F2/0 ! A interface Serial1/1 S1/0 S1/1 description Backbone to C ip address ! interface FastEthernet2/0 description LAN ip address AS 65200 ! S1/0 S1/0 router bgp 65100 F0/0 F0/0 no synchronization network mask network mask B C network F2/0 F2/0 neighbor South peer-group neighbor South remote-as 65200 neighbor peer-group South OSPF neighbor peer-group South no auto-summaryinterface FastEthernet0/0 Router B interface FastEthernet0/0 Router C description Backbone to C description Backbone to B ip address ip address! !interface Serial1/0 interface Serial1/0 description Backbone to A description Backbone to A ip address ip address! !interface FastEthernet2/0 interface FastEthernet2/0 description LAN description LAN ip address ip address! !router ospf 100 router ospf 100 network area 0 network area 0 network area 1 network area 2! !router bgp 65200 router bgp 65200 no synchronization no synchronization redistribute ospf 100 route-map LAN_Subnets redistribute ospf 100 route-map LAN_Subnets neighbor remote-as 65200 neighbor remote-as 65200 neighbor remote-as 65100 neighbor remote-as 65100 no auto-summary no auto-summary! !access-list 10 permit access-list 10 permit! !route-map LAN_Subnets permit 10 route-map LAN_Subnets permit 10 match ip address 10 match ip address 10 set metric 100 set metric 100 Router A Routing Table Router B Routing Table is subnetted, 2 subnets is subnetted, 2 subnetsC is directly connected, S1/1 B [20/0] via is directly connected, S1/0 C is directly connected, S1/0C is directly connected, F2/0 is subnetted, 1 subnetsB [20/100] via C is directly connected, F0/0B [20/100] via B [20/0] via C is directly connected, F2/0 O IA [110/2] via, F0/0by Jeremy Stretch v2.1-r1