Connectivity Guide

4-Byte AS numbers
OS10 supports 4-byte AS number congurations by default. The 4-byte support is advertised as a new BGP capability - 4-BYTE-AS, in
the OPEN message. A BGP speaker that advertises 4-Byte-AS capability to a peer, and receives the same from that peer must encode AS
numbers as 4-octet entities in all messages.
If the AS number of the peer is dierent, the 4-byte speaker brings up the neighbor session using a reserved 2-byte ASN, 23456 called
AS_TRANS. The AS_TRANS is used to interop between a 2-byte and 4-byte AS number.
Where the 2-byte format is 1 to 65535, the 4-byte format is 1 to 4294967295. You can enter AS numbers using the traditional format.
AS number migration
You can transparently change the AS number of an entire BGP network. Changing the AS number ensures that the routes propagate
throughout the network while migration is in progress. When migrating one AS to another and combining multiple AS, an EBGP network
may lose its routing to an IBGP if the AS number changes.
Migration is dicult as all IBGP and EBGP peers of the migrating network must be updated to maintain network reachability. Local-AS
allows the BGP speaker to operate as if it belongs to a virtual AS network besides its physical AS network.
Disable the local-as command after migration. Failure to disable the local-as command after migration causes the local-as
command to replace the original AS number of the system. You must recongure the system with a new AS number.
Router A, Router B, and Router C belong to AS 100, 200, and 300, respectively. Router A acquired Router B — Router B has Router C as
its client. When Router B is migrating to Router A, it must maintain the connection with Router C without immediately updating Router C’s
conguration. Local-AS allows Router B to appear as if it still belongs to Router B’s old network, AS 200, to communicate with Router C.
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