Border Gateway Protocol (BGP)

AS_PATH

AS_PATH is the BGP attribute that keeps track of each AS a route advertisement has passed through. AS_PATH is used by confederations and by exterior BGP (EBGP) to help prevent routing loops. A router knows there is a loop if it receives an AS_PATH with that routers AS in it. The figure below shows the route between router A and router B. The AS_PATH from A to B would read 701,702,703 for each AS the route passes through.

As of the start of 2010, the industry upgraded from 2-byte to 4-byte AS_PATHs. This upgrade was due to the imminent exhaustion of 2-byte AS_PATH numbers. FortiOS supports 4-byte AS_PATHs in its BGP implementation.

 

AS_PATH of 701,702, 703 between routers A and B

The BGP commands related to AS_PATH include:

config router bgp

set bestpath-as-path-ignore {enable | disable}

end

 

MULTI_EXIT_DESC (MED)

BGP AS systems can have one or more routers that connect them to other ASes. For ASes with more than one connecting router, the Multi-Exit Discriminator (MED) lists which router is best to use when leaving the AS. The MED is based on attributes such as delay. It is a recommendation only, as some networks may have different priorities.

BGP updates advertise the best path to a destination network. When the FortiGate unit receives a BGP update, the FortiGate unit examines the Multi-Exit Discriminator (MED) attribute of potential routes to determine the best path to a destination network before recording the path in the local FortiGate unit routing table.

FortiGate units have the option to treat any routes without an MED attribute as the worst possible routing choice. This can be useful because a lack of MED information is a lack of routing information which can be suspicious — possibly a hacking attempt or an attack on the network. At best it signifies an unreliable route to select.

The BGP commands related to MED include:

config router bgp

set always-compare-med {enable | disable}

set bestpath-med-confed {enable | disable}

set bestpath-med-missing-as-worst {enable | disable}

set deterministic-med {enable | disable}

config neighbor

set attribute-unchanged [as-path] [med] [next-hop]

end end


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6 thoughts on “Border Gateway Protocol (BGP)

  1. Hi Mike,
    if i configure the following on fortigate1:
    config router bgp
    set as 65000
    set router-id 10.2.2.254
    config neighbor
    edit “10.2.2.253”
    set next-hop-self enable
    set remote-as 65000
    set send-community6 disable
    next

    config redistribute “static”
    set status enable
    end

    fortigate2 should get the default route 0.0.0.0 0.0.0.0 from fortigate1 as it is static ?

    how can i redistribute the default route(fortigate1) to fortigate2 ?

    thanks
    regards

    • There is a really good KB article that explains how to do this. You can find it here

      If you want to redistribute static routes you would enable the following

      config router bgp
      config redistribute static
      set status enable
      end
      end

      An example of the config would be like this

      config router prefix-list
      edit “only_dflt”
      config rule
      edit 1
      set prefix 0.0.0.0 0.0.0.0
      unset ge
      unset le
      next
      end
      next
      end

      config router route-map
      edit “only_default_route”
      config rule
      edit 1
      set match-ip-address “only_dflt”
      next
      end
      next
      end

      config router bgp
      set as 2
      config neighbor
      edit 10.142.0.110
      set remote-as 1
      set route-map-in “only_default_route”
      next
      end

      set router-id 10.142.0.205
      end

      Let me know if this helped answer your question!

      Thanks!

  2. config router bgp
    set as 65041
    set router-id 162.53.156.138
    config neighbor
    edit “10.104.55.1”
    set ebgp-enforce-multihop enable
    set soft-reconfiguration enable
    set remote-as 64699
    set send-community6 disable
    next
    edit “10.104.55.2”
    set ebgp-enforce-multihop enable
    set soft-reconfiguration enable
    set remote-as 64699
    set send-community6 disable
    next

    i am trying to accomplish above but i can see only one neighbour is establish and other is in ACTIVE state…

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