Discussion

The AS path line lists all AS paths from the AS_PATH attribute, which shows the
ASs through which the announcement for the prefix has traveled. The first AS in the
path is the most recent AS, and the last AS is the originating AS. All routes in this
output have come directly from the peer AS, so there is just one AS number in the
path, AS 65500.
Following the path is the information from the BGP ORIGIN attribute, which indicates
how BGP learned the prefix. The I here means that the prefix was learned from
an IGP. Routes learned from an EGP peer would have an E, and those learned some
other way would show as INCOMPLETE. The final line for each BGP route entry shows
the next hop toward the destination and the interface the router will use to reach
that next hop. Of the three BGP routes, the one to 192.168.16.1/32 is active and is
marked with an asterisk.
Checking the BGP connection status on the peer, you now see the BGP routes:
aviva@RouterD> show bgp summary
Groups: 1 Peers: 1 Down peers: 0
Table Tot Paths Act Paths Suppressed History Damp State Pending
inet.0 4 1 0 0 0 0
Peer AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#A
ctive/Received/Damped...
10.0.31.2 65500 721 718 0 0 5:57:50 1/4/0
0/0/0
The Table section of the output shows that the inet.0 routing table has four BGP
routes, which matches what we saw in the routing-table entries above. In the Peer
section, the State column indicates that four BGP routes have been received from
peer 10.0.31.2 and that one is active. Again, this corresponds with the routing-table
entries.
Another way to find out about BGP routes is to look at what the routing table has
received from BGP:
aviva@RouterD> show route receive-protocol bgp 10.0.31.2
inet.0: 11 destinations, 15 routes (11 active, 0 holddown, 0 hidden)
Prefix Nexthop MED Lclpref AS path
0.0.0.0/0 10.0.31.2 65500 I
10.0.31.0/24 10.0.31.2 65500 I
172.19.121.0/24 10.0.31.2 65500 I
* 192.168.16.1/32 10.0.31.2 65500 I
Again, you see the four routes learned from BGP but in a format that is much easier
to scan. For each route, you also see the value of four BGP attributes: NEXT_HOP,
MED, LOCAL_PREF, and AS_PATH. The active route is marked with an asterisk.
Use the following command to see which routes the router has advertised:
aviva@RouterF> show route advertising-protocol bgp 10.0.31.1
inet.0: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden)
Prefix Nexthop MED Lclpref AS path
* 0.0.0.0/0 Self I
430 | Chapter 13: BGP
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