From: Ivan Walker (ivan@itpro.co.nz)
Date: Mon Jan 12 2009 - 08:53:10 ARST
Hi Praveen,
local-as : the AS that the router is masquerading as is prepended to
incoming routes and prepended to outgoing routes following the configured
AS
no-prepend: the AS that the router is masquerading as is no longer
prepended to incoming routes
replace-as: the AS that the router is configured with as is no longer
prepended to outgoing routes - only the AS that the router is
masquerading as is prepended
dual-as: allow the neighbor to use the configured as or the AS that the
router is masquerading as
the following should help illustrate
R1 Initial Config:
==================
interface Loopback0
ip address 1.1.1.1 255.255.255.0
interface FastEthernet0/0
ip address 12.12.12.1 255.255.255.0
router bgp 111
bgp router-id 1.1.1.1
no bgp default ipv4-unicast
bgp log-neighbor-changes
neighbor 12.12.12.2 remote-as 222
neighbor 12.12.12.2 version 4
address-family ipv4
neighbor 12.12.12.2 activate
no auto-summary
no synchronization
network 1.1.1.0 mask 255.255.255.0
exit-address-family
R2 Initial Config:
===================
interface Loopback0
ip address 2.2.2.2 255.255.255.0
interface FastEthernet0/0
ip address 12.12.12.2 255.255.255.0
router bgp 222
bgp router-id 2.2.2.2
no bgp default ipv4-unicast
bgp log-neighbor-changes
neighbor 12.12.12.1 remote-as 111
neighbor 12.12.12.1 version 4
address-family ipv4
neighbor 12.12.12.1 activate
no auto-summary
no synchronization
network 2.2.2.0 mask 255.255.255.0
exit-address-family
R1#show ip bgp 2.2.2.0
BGP routing table entry for 2.2.2.0/24, version 9
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
222
12.12.12.2 from 12.12.12.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, best
R2#show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
111
12.12.12.1 from 12.12.12.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
++++++++++++++++++++++local-as+++++++++++++++++++++++++++++++
R1 to masquerade as AS666
R1(config-router)# neighbor 12.12.12.2 local-as 666
R2 updated to peer with AS666
R2(config-router)# neighbor 12.12.12.1 remote-as 666
R1#show ip bgp 2.2.2.0
BGP routing table entry for 2.2.2.0/24, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
666 222
12.12.12.2 from 12.12.12.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, best
R2#show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24, version 17
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
666 111
12.12.12.1 from 12.12.12.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
++++++++++++++++++++++no-prepend+++++++++++++++++++++++++++++++
R1(config-router)# neighbor 12.12.12.2 local-as 666 no-prepend
R1#show ip bgp 2.2.2.0
BGP routing table entry for 2.2.2.0/24, version 5
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
222
12.12.12.2 from 12.12.12.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, best
R2#show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24, version 19
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
666 111
12.12.12.1 from 12.12.12.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
++++++++++++++++++++++replace-as+++++++++++++++++++++++++++++++
R1(config-router)# neighbor 12.12.12.2 local-as 666 no-prepend replace-as
R1#show ip bgp 2.2.2.0
BGP routing table entry for 2.2.2.0/24, version 7
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
222
12.12.12.2 from 12.12.12.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, best
R2#show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24, version 21
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
666
12.12.12.1 from 12.12.12.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
++++++++++++++++++++++dual-as+++++++++++++++++++++++++++++++
R2(config-router)#neighbor 12.12.12.1 remote-as 111
[R2] %BGP-3-NOTIFICATION: sent to neighbor 12.12.12.1 2/2 (peer in wrong
AS) 2 bytes 029A
[R2] FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 002D 0104 029A 00B4 0101 0101
1002 0601 0400 0100 0102 0280 0002 0202 00
R1(config-router)# neighbor 12.12.12.2 local-as 666 no-prepend replace-as
dual-as
[R2] %BGP-5-ADJCHANGE: neighbor 12.12.12.1 Up
R1#show ip bgp 2.2.2.0
BGP routing table entry for 2.2.2.0/24, version 9
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
222
12.12.12.2 from 12.12.12.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, best
R2#show ip bgp 1.1.1.0
BGP routing table entry for 1.1.1.0/24, version 23
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
111
12.12.12.1 from 12.12.12.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
Ivan
> Hi GS,
>
> Need some help in BGP local-AS parameter?
>
> I tried it in my lab without any problem but not able to understand the
> difference between No-prepend, Replace-as and dual-as parameter.
>
> Thanks
>
>
> Blogs and organic groups at http://www.ccie.net
>
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