By: TK

TK — Sat, 19 Feb 2011 22:21:30 +0000

Great post.. such a simple concept but no one can explain it like you did. Great job.

By: vcappuccio

vcappuccio — Sun, 10 Aug 2008 03:25:28 +0000

Hi Chien, for your first question: IPv4 Address Family does not support AS-Override (at least on the version that I am running) [sourcecode language='css'] R2(config-if)#do show ver | in IOS Cisco IOS Software, 7200 Software (C7200-P-M), Version 12.2(25)S13, RELEASE SOFTWARE (fc1) [/sourcecode] it only supports AS-Override on the VRF address family, so yes, this feature could be used extensively on MPLS VPN Scenarios, or MultiVRF (VRF Lite) Configurations, also lets just not forget about the allowas-in feature that is available on the IPv4 address family and on the ipv4 vrf address family as well, that could probably help as an alternate configuration (with the combination of those 2 features, anything could be possible). for your second question, I´ve configured the following topology R1 in AS 1 -- R2 in AS 2 -- R3 in AS 1 [sourcecode language='css'] R2(config-if)#do show ip vrf inter Interface IP-Address VRF Protocol FastEthernet0/0 10.1.12.2 TEST up FastEthernet1/0 10.1.23.2 TEST up R2(config-if)# R2(config-router-af)#do show ip bgp vpnv4 all summ BGP router identifier 10.1.23.2, local AS number 2 BGP table version is 3, main routing table version 3 1 network entries using 137 bytes of memory 1 path entries using 68 bytes of memory 2/1 BGP path/bestpath attribute entries using 216 bytes of memory 1 BGP AS-PATH entries using 40 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP using 461 total bytes of memory BGP activity 2/1 prefixes, 2/1 paths, scan interval 15 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.1.12.1 4 1 5 4 3 0 0 00:00:09 1 10.1.23.3 4 1 4 5 3 0 0 00:00:36 0 R2(config-router-af)# (config-router-af)#do show ip bgp vpnv4 all BGP table version is 3, local router ID is 10.1.23.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path Route Distinguisher: 10:1 (default for vrf TEST) *> 1.1.1.1/32 10.1.12.1 0 0 1 23 1 23 1 23 1 23 1 i R2(config-router-af)#neigh 10.1.23.3 as-over 00:29:36: %BGP-5-ADJCHANGE: neighbor 10.1.23.3 vpn vrf TEST Down AS-override change R2(config-router-af)# R2(config-router-af)#do clear ip bgp * in R2(config-router-af)#do show ip bgp vpnv4 all neigh 10.1.23.3 adver R2(config-router-af)#do show ip bgp vpnv4 all summ BGP router identifier 10.1.23.2, local AS number 2 BGP table version is 3, main routing table version 3 1 network entries using 137 bytes of memory 1 path entries using 68 bytes of memory 2/1 BGP path/bestpath attribute entries using 216 bytes of memory 1 BGP AS-PATH entries using 40 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP using 461 total bytes of memory BGP activity 2/1 prefixes, 2/1 paths, scan interval 15 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.1.12.1 4 1 7 6 3 0 0 00:01:10 1 10.1.23.3 4 1 5 6 0 0 0 00:00:21 Active R2(config-router-af)# 00:30:07: %BGP-5-ADJCHANGE: neighbor 10.1.23.3 vpn vrf TEST Up R2(config-router-af)# R2(config-router-af)#do show ip bgp vpnv4 all summ BGP router identifier 10.1.23.2, local AS number 2 BGP table version is 3, main routing table version 3 1 network entries using 137 bytes of memory 1 path entries using 68 bytes of memory 2/1 BGP path/bestpath attribute entries using 216 bytes of memory 1 BGP AS-PATH entries using 40 bytes of memory 0 BGP route-map cache entries using 0 bytes of memory 0 BGP filter-list cache entries using 0 bytes of memory BGP using 461 total bytes of memory BGP activity 2/1 prefixes, 2/1 paths, scan interval 15 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.1.12.1 4 1 7 6 3 0 0 00:01:24 1 10.1.23.3 4 1 9 11 3 0 0 00:00:04 0 R2(config-router-af)# R2(config-router-af)#do show ip bgp vpnv4 all neigh 10.1.23.3 adver BGP table version is 3, local router ID is 10.1.23.2 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path Route Distinguisher: 10:1 (default for vrf TEST) *> 1.1.1.1/32 10.1.12.1 0 0 1 23 1 23 1 23 1 23 1 i R2(config-router-af)# [/sourcecode] All As 1 information is replaced with 2 now [sourcecode language='css'] R3#show ip bgp BGP table version is 2, local router ID is 10.1.23.3 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 1.1.1.1/32 10.1.23.2 0 2 2 23 2 23 2 23 2 23 2 i R3# [/sourcecode] I hope this helps Victor.-

By: Chien-Bon Chou

Chien-Bon Chou — Sun, 10 Aug 2008 01:36:27 +0000

Hi,
I have questions here.
1.)the command as-override only can be used for MPLS-VPN scenario, not public Internet?
2.)only head-end or tail-end asn will be replaced by local asn?
For instance:
If the origin ASN of prefix 6.6.6.6/32 is 100 ,and the as_path on R5 is 24 45 22 24 100. if we configured as-oveeride on R2, whether the AS Path will be change from 24 45 22 24 100 to 56 56 45 22 56 100 or not?
Thank you

Comments on: neighbor CE as-override

By: TK

By: vcappuccio

By: Chien-Bon Chou