neighbor CE as-override
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RFC 1771 9.3 Route Selection Criteria- If the local AS appears in the AS path of the new route being considered,then that new route cannot be viewed as better than any other route. If such a route were ever used, a routing loop would result. |
New AS path update procedures have been implemented to reuse the same AS number on
VPN B sites.
R6(config)#router bgp 56 R6(config-router)#add ipv4 vrf B R6(config-router-af)#neigh 192.168.46.4 as-override R6(config-router-af)#^Z |
R4 is now receiving that information from R6 that comes as a VPNV4 Route from R5 that is talking BGP with R2 that is announcing that network into this domain
R4#show ip bgp summ BGP router identifier 4.4.4.4, local AS number 24 BGP table version is 9, main routing table version 9 2 network entries using 240 bytes of memory 2 path entries using 104 bytes of memory 3/2 BGP path/bestpath attribute entries using 372 bytes of memory 1 BGP AS-PATH entries using 24 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 Bitfield cache entries: current 1 (at peak 2) using 32 bytes of memory BGP using 772 total bytes of memory BGP activity 2/0 prefixes, 5/3 paths, scan interval 60 secs Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 192.168.46.6 4 56 88 83 9 0 0 01:10:06 1 R4#show ip bgp BGP table version is 9, local router ID is 4.4.4.4 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 *> 2.2.2.2/32 192.168.46.6 0 56 56 i *> 4.4.4.4/32 0.0.0.0 0 32768 i |
See that the AS Path is now replaced with the Carrier AS #, The same must be configured in R5 to R2
R2#deb ip bgp upda BGP updates debugging is on for address family: IPv4 Unicast R2#clear ip bgp * in *May 11 12:32:26.687: BGP(0): 192.168.25.5 rcv UPDATE w/ attr: nexthop 192.168.25.5
origin i,originator 0.0.0.0, path 56 24, community , extended community
BGP(0): 192.168.25.5 rcv UPDATE about 4.4.4.4/32 --
DENIED due to:AS-PATH contains our own AS;
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ok, we can override this rule, instead of sending the Customer AS#, we replace the Customer AS # with ISPF AS# in order to respect RFC 1771, 9.3 rule, using the neighbor as-override command, at the of each PE Router, that is talking BGP via the Customer via BGP VRF Address Family.
R5(config)#router bgp 56 R5(config-router)#address-family ipv4 vrf B R5(config-router-af)#neigh 192.168.25.2 as-override R5(config-router-af)#^Z R5# *May 11 12:28:55.154: %BGP-5-ADJCHANGE: neighbor 192.168.25.2 vpn vrf B Down AS-override change! This WARNING is for ISPs: the BGP Relationship is reestablished.. you know what i mean (BW, CPU, Routes being advertised here and there…. etc. |
Now on R2 we are getting this:
*May 11 12:33:19.175: BGP(0): 192.168.25.5 send UPDATE (format) 2.2.2.2/32, next 192.168.25.2,metric 0, path Local *May 11 12:33:19.183: BGP(0): 192.168.25.5 rcvd UPDATE w/ attr: nexthop 192.168.25.5, origin i,path 56 56 *May 11 12:33:19.183: BGP(0): 192.168.25.5 rcvd 4.4.4.4/32 *May 11 12:33:19.183: BGP(0): Revise route installing 1 of 1 routes for 4.4.4.4/32 -> 192.168.25.5(main) to main IP table R2#show ip bgp BGP table version is 13, local router ID is 2.2.2.2 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 *> 2.2.2.2/32 0.0.0.0 0 32768 i *> 4.4.4.4/32 192.168.25.5 0 56 56 i |
The neigh as-override command, allows the provider to replace customer AS with its own AS
and prepend his AS number to the AS Path. lets do a some test of as path prepending on R4
R4(config)#route-map PREPEND R4(config-route-map)#set as-path prepend 45 22 24 R4(config-route-map)#router bgp 24 R4(config-router)#neigh 192.168.46.6 route-map PREPEND out R4(config-router)#^Z |
Lets see what R6 (the PE router ) is receiving..The route in the PE (Ingress)
R6#clear ip bgp vpnv4 unicast 24 in R6# R6#show ip bgp vpnv4 vrf B neighbors 192.168.46.4 routes | b Network Network Next Hop Metric LocPrf Weight Path Route Distinguisher: 6.6.6.6:46 (default for vrf B) *> 4.4.4.4/32 192.168.46.4 0 0 24 45 22 24 i Total number of prefixes 1 |
The route send as VPNV4 route to R5, and annouced to R2 via BGP (watch the AS Sequence String)
R5#show ip bgp vpnv4 all neighbors 192.168.25.2 advertised-routes BGP table version is 60, local router ID is 5.5.5.5 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: 5.5.5.5:25 (default for vrf B) *>i4.4.4.4/32 6.6.6.6 0 100 0 24 45 22 24 i Total number of prefixes 1 |
The route @ R2
R2#show ip bgp BGP table version is 13, local router ID is 2.2.2.2 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 *> 2.2.2.2/32 0.0.0.0 0 32768 i *> 4.4.4.4/32 192.168.25.5 0 56 56 45 22 56 i |
From this test, we can conclude, that BGP loop detection prevents Customers from
reusing their AS.
The neighbor as-overide command replaces the customer AS number, with the ISP AS number.
Saludos, Victor Cappuccio.-
