QuistED.net

• BGP filtering can be done on any routers
• Filtering can be done inbound and outbound
• After filtering is applied BGP neighbor must be reset or cleared for the filter to take effect
  • clear ip bgp 20.20.20.20 in/out  preferred
  • clear ip bgp *   (hard reset)
  • clear ip bgp 20.20.20.20 soft in/out

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sh ip bgp

R1#sh ip bgp
BGP table version is 6, local router ID is 10.10.10.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
     0.0.0.0          0.0.0.0                                0 i
 r>  1.1.1.0/30       20.20.20.1               0             0 20 ?
 r>  20.20.20.0/24    20.20.20.1               0             0 20 ?
 *>  30.30.30.0/24    20.20.20.1               0             0 20 ?
 *>i 40.40.40.0/24    1.1.2.2                  0    100      0 i

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Injecting routes into BGP.

• Use the network command.
  • Different than the network command in iGP; it isn’t used to listen/active interfaces.
  • the BGP network command looks for the prefix in the routing table and originates that into the BGP table.
  • If no mask is defined, IOS assumes a classful network.
  • Classful route is added if:
    • the exact route is in the ip routing table
    • Any subset of routes are in the routing table (only with auto-summery)
  • create a null0 route
    •  ip route 30.30.30.0 255.255.255.0 null0

router bgp 20
 bgp log-neighbor-changes
 network 30.30.30.0 mask 255.255.255.0
 neighbor 10.10.10.1 remote-as 10
 neighbor 10.10.10.1 ebgp-multihop 255
 neighbor 10.10.10.1 update-source Loopback1

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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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