BGP NEXT-HOP: Mock Lab 1 TS - Ticket 4 - Workbook v5

ticket 4 is about bgp:

During the tshoot R6 needs to reach 150.1.5.5 that is in another AS. The output of the bgp table shows that there is a best route, but the next-hop is not in RIB, then my question is why is the reason that the output doesnt say that next-hop is innaccesible on R6.

R6#show ip bgp 150.1.5.5     
BGP routing table entry for 150.1.5.5/32, version 4
Paths: (2 available, best #1, table default)
  Advertised to update-groups:
     3          4        
  Refresh Epoch 2
  3000, (Received from a RR-client)
    192.0.59.5 from 150.1.9.9 (150.1.9.9)
      Origin IGP, metric 0, localpref 100, valid, internal, best
  Refresh Epoch 2
  3000, (Received from a RR-client)
    192.0.105.5 from 150.1.10.10 (150.1.10.10)
      Origin IGP, metric 0, localpref 100, valid, internal

R6#show ip ro 192.0.59.5
% Network not in table
R6#show ip ro 192.0.105.5
% Network not in table

I was surprise that solution for the ticket is use next-hop self, because I didnt see the innaccesible for the route 150.1.5.5.

NEXT_HOP is inaccessible

If the router doesn't know how to reach a route's next hop, a recursive lookup will fail, and the route can't be added to BGP. For example, if a BGP router receives a route for 150.1.5.5 with a NEXT_HOP attribute of 192.0.59.5, but doesn't have an entry in its routing table for a subnet containing 192.0.59.0, the received route for 150.1.5.5 is useless and won't be installed in the routing table.   THE EXPECT OUTPUT

R6#show ip bgp 150.1.5.5

BGP routing table entry for 150.1.5.5/32, version 4
Paths: (2 available, best #1, table default)
  Advertised to update-groups:
     3          4        
  Refresh Epoch 2
  3000, (Received from a RR-client)
    192.0.59.5 (inaccessible) from 150.1.9.9 (150.1.9.9)
      Origin IGP, metric 0, localpref 100, valid, internal, best

Comments

  • the same event I have in Mock Lab 1 TS - Ticket 6 - Workbook v5, I have an ipv6 bgp using neighbor with IPV4 ADDRESS,

    you know that this peer IPV4 create a next-hop ::FFFF:40:40.1.20 which doesnt exit in RIB and the route is not installed in RIB.To avoid this issue the next-hop is changed by a IPV6 that exist in RIB (use a route-map).

    Now we see that next-hop is a IPV6 address which exist in RIB, but now the prefix 20001::1/128 is not installed in RIB because the next-hop is inaccessible.

    R4#show bgp ipv6 un 2001::1/128
    BGP routing table entry for 2001::1/128, version 0
    Paths: (1 available, no best path)
      Not advertised to any peer
      Refresh Epoch 2
      30000 2000
        2001:40:40:1::20 (inaccessible) from 40.40.1.20 (4.2.2.2)
          Origin incomplete, localpref 100, valid, external

    !

    R4#show ipv6 route 2001:40:40:1::20
    Routing entry for 2001:40:40:1::/64
      Known via "connected", distance 0, metric 0, type connected
        directly connected via Ethernet0/1.40

    !

     

    the configuration in R4

    R4#show run int eth0/1.40
    !
    interface Ethernet0/1.40
     encapsulation dot1Q 40
     ip address 40.40.1.1 255.255.255.0
     ipv6 address 2001:40:40:1::1/64
    !
     route-map CORRECT_NH_OUT permit 10
     set ipv6 next-hop 2001:40:40:1::1
     route-map CORRECT_NH_IN permit 10
     set ipv6 next-hop 2001:40:40:1::20
     !

    router bgp 1000
     address-family ipv6
      neighbor 40.40.1.20 activate
      neighbor 40.40.1.20 route-map CORRECT_NH_IN in
      neighbor 40.40.1.20 route-map CORRECT_NH_OUT out
     exit-address-family

     

     

     

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