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Establishing a Peer Relationship Between a 4-Byte Capable Router and a 2-Byte Capable Router Using a 2-Byte AS Number

This section describes what happens when a router that supports 4-byte AS numbers establishes a peer relationship with a router that only supports 2-byte AS numbers if both routers are configured with AS numbers in the 2-byte AS number range.

In Figure 1, Router 1 is running Junos OS Release 9.2 that supports 4-byte AS numbers. Router 1 is configured to use a 2-byte AS number, AS 12596. Router 2 is running Junos OS Release 8.5 that supports 2-byte AS numbers. Router 2 is configured with a 2-byte AS number, AS 60000.

Figure 1: 4-Byte Capable Router Having a Peer Relationship With a 2-Byte Capable Router Using a 2-Byte AS Number

• The following example shows the relevant portion of the Router 1 configuration.
  user@Router1# show configuration

  …
  autonomous-system 12596;
  …
  local-address 192.168.1.10;
  export static-to-bgp;
  peer-as 60000;
  

• To verify that the AS path of route 1.2.3.4 contains AS 12596, use the show route command on Router 2. The following example shows that the BGP peer session is established in the normal way and that the AS path of route 1.2.3.4 contains AS 12596:
  user@Router2# show route 1.2.3.4

  1.2.3.4/32         *[BGP/170] 00:01:29, localpref 100, from 192.168.1.10
                        AS path: 12596 I
                      > via at-0/1/0.1001
  

• To display the session-establishment messages logged on Router 1, use the show log messages command. The following example shows that Router 1 discovers that Router 2 does not support 4-byte AS numbers:
  user@Router1# show log messages

  Nov  7 09:41:39.443493 bgp_4byte_aspath_add_cap():153 AS4-Peer 192.168.1.9 (External AS 60000)(SEND): 4 byte AS capability added, AS 12596
  Nov  7 09:41:39.443582 bgp_send: sending 67 bytes to 192.168.1.9 (External AS 60000)
  […]
  Nov  7 09:41:39.448055 bgp_4byte_aspath_adjust():1279 AS4-Peer 192.168.1.9 (External AS 60000)(SEND): Adjust BGP update to Old/New BGP speaker format
  Nov  7 09:41:39.448132 bgp_4byte_aspath_adjust():1290 AS4-Peer 192.168.1.9 (External AS 60000)(SEND): Cached information of previous update format is not used
  Nov  7 09:41:39.448162 bgp_generate_2byte_aspath():422 AS4-Peer 192.168.1.9 (External AS 60000)(SEND): Generating 2 byte AS path from 4 byte as-path
  Nov  7 09:41:39.448198 bgp_send: sending 64 bytes to 192.168.1.9 (External AS 60000)