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Understanding a 4-Byte Capable Router AS Path Through a 2-Byte Capable Domain

This section describes what happens when a router that supports 4-byte AS numbers sends the AS path statement to a router that only supports 2-byte AS numbers if the first router is configured with an AS number outside the 2-byte AS number range.

In Figure 1 Router 1 supports 4-byte AS numbers. Router 1 is configured to use a 4-byte AS number, AS 1000000000. Router 2 supports 2-byte AS numbers. Router 2 is configured with a 2-byte AS number, AS 65056.

Figure 1: 4-Byte Capable Router AS Path to a 2-Byte Capable Router

• Router 2 does not accept 4-byte AS numbers in the AS_PATH attribute. You can verify this using the show bgp neighbor command on Router 1.
  user@Router1# show bgp neighbor 192.168.1.9 | match "AS"

  Peer: 192.168.1.9+179 AS 65056 Local: 192.168.1.2+64053 AS 65080
    Peer does not support 4 byte AS extension
  

Figure 2 shows four routers running EBGP. Router 1, Router 2, and Router 4 support 4-byte AS numbers. Router 3 does not support 4-byte AS numbers.

Figure 2: EBGP 4-Byte AS Path Through a 2-Byte AS Domain

• Router 1 sends the 4-byte AS number, AS 1000000000, in the AS_PATH attribute to Router 2.
• Router 2 knows that Router 3 does not support 4-byte AS numbers.
• Router 2 sends the AS_TRANS number, AS 23456, in the AS_PATH attribute in place of the 4-byte AS number to Router 3.
• Router 2 sends the 4-byte AS number, AS 1000000000 in the AS4_PATH attribute to Router 3.
• Because the AS4_PATH attribute is transitive, Router 3 sends both the AS_PATH attribute and the AS4_PATH attribute to Router 4.
• When Router 4 receives the AS_PATH and AS4_PATH attributes, it merges the path statements to create an accurate AS path.

You can display the AS path using the show route command on Router 3. In the following example, notice that the AS number 23456 appears in the AS path and that the AS4_PATH attribute is Unrecognized. Because the AS4_PATH attribute is a transitive attribute, it is forwarded to the next router.

AS path: 65000 23456 I Unrecognized Attributes: 13 bytes

You can display the route details using the show route command on Router 4. In the following example, notice that as the AS path transitions Router 3, as shown in the AS2 (2-byte AS) path, the AS number is displayed as AS_TRANS. This means that Router 3 sees the AS number as 23456. In the AS4 (4-byte AS) path the AS number is displayed as 1000000000. In the merged AS path the correct AS path numbers are displayed for AS 65056, AS 65000, and AS 1000000000.

...
AS path: AS2 PA[3]:65056 65000 AS_TRANS

AS path: AS4 PA[2]:65056 1000000000

AS path: Merged[3]:65056 65000 1000000000 I

Figure 3 shows 4 routers running IBGP. Router 1, Router 2, and Router 4 support 4-byte AS numbers. Router 3 does not support 4-byte AS numbers.

Figure 3: IBGP 4-Byte AS Path Through a 2-Byte AS Domain

• Router 1 sends the 4-byte AS number, AS 1000000000, in the AS_PATH attribute to Router 2.
• Router 2 knows that Router 3 does not support 4-byte AS numbers.
• Router 2 sends the AS_TRANS number, AS 23456, in the AS_PATH attribute in place of the 4-byte AS number to Router 3.
• Router 3 sends both the AS_PATH attribute and the AS4_PATH attribute to Router 4.
• When Router 4 receives the AS_PATH and AS4_PATH attributes, it merges the path statements to create an accurate AS path.

You can display the route details using the show route command on Router 2. In the following example, notice that the AS path is displayed as 1000000000.

...
AS path: 1000000000

You can display the route details using the show route command on Router 3. In the following example, notice that the AS path is displayed as 65000 23456.

...
AS path: 65000 23456 I

You can display the route details using the show route command on Router 4. In the following example, notice that the merged AS path is displayed as 65000 1000000000.

...
AS path: 65000 1000000000 I