4-Byte Autonomous System Number provides us with 4.3billion unique ASN’s , going far beyond the original 2-Byte range of 65536.

So what happens when you have a router that doesn’t yet support 4-Byte ASN? I’ll show you what to do.

You wouldn’t necessarily be configuring all 3 routers in the real world.

First see the simple topology below

4-Byte ASN

4-Byte ASN

R3 in the middle is still using a 2-Byte ASN and the IOS doesn’t support 4-Byte ASN. So for this exact purpose IANA assigned a placeholder, AS23456 which is used by the older routers to communicate with new 4-Byte AS numbers.

This is achieved by using a transitive BGP attribute  NEW_AS_PATH and NEW_AGGREGATOR, much like the ATOMIC_AGGREGATOR used when summarising similar routes. This stores like a small note of the actual 4-Byte ASN which is then used by the 4-Byte capable router. Something you might want to note is the AS_PATH length is also maintained, I will demonstrate this.

This particular method is known as AS-PLAIN

So on R2 and R3 setup BGP, the only difference is on R3 instead of using the actual AS number use the placeholder AS23456.

R2#sh run | sec router bgp
router bgp 132678
 bgp log-neighbor-changes
 network mask
 neighbor remote-as 500
no auto-summary
R3#sh run | sec router bgp
router bgp 500
 no synchronization
 bgp log-neighbor-changes
 neighbor remote-as 23456
 neighbor remote-as 23456
 no auto-summary

Now on R1 I’ve used AS_PATH Prepend to demonstrate the fact that the AS_PATH length doesn’t change, On my route map I prepended the AS path 4 times

R1#sh run | sec router bgp
router bgp 131456
 bgp log-neighbor-changes
 neighbor remote-as 500
 neighbor route-map prepend-AS out
no auto-summary

Now lets confirm the configurations above, using the commands SH IP BGP” from R3. We will expect to see the placeholder ASN23456

   Network          Next Hop            Metric LocPrf Weight Path
*>              0             0 23456 i
*>              0             0 23456 23456 23456 23456 23456 i

Now  let’s do the same from R2 , we should now expect to see the 4-Byte ASN in the AS_PATH.

     Network          Next Hop            Metric LocPrf Weight Path
 *>                  0         32768 i
 *>                            0 500 131456 131456 131456 131456 131456 i

To see exactly what is sent between the router’s I’ve used Wireshark to capture the packets, you can see the message sent from R3, AS_PATH and NEW_AS_PATH

Wireshark Capture

Wireshark Capture


If you need to work out the ASDOT version of an ASPLAIN 4-Byte ASN use the examples below.

4-Byte ASN-194534
1. 194534 / 65535 = 2 (integer)
2. 194534 – ( 65535 * 2) = 63464
3. 63464 – 2 (integer) = 63462
4  ASDOT = 2.63462

Here is a different example this time with an even longer ASN

4-Byte ASN-2394951
1. 2394951 / 65535 = 36 (integer)
2. 2394951 -(65535*36) = 2394951-2359260 = 35691
3. 35691-36 (integer) = 35655
4. ASDOT =36.35655