Should have added that XR does the loop prevention check first, to quote RFC 4271 BGP-4
"If the AS_PATH attribute of a BGP route contains an AS loop, the BGP route should be excluded from the Phase 2 decision function. AS loop detection is done by scanning the full AS path (as specified in the AS_PATH attribute), and checking that the autonomous system number of the local system does not appear in the AS path. Operations of a BGP speaker that is configured to accept routes with its own autonomous system number in the AS path are outside the scope of this document."
The word "local" suggests the updates should be sent first by a peer & it's down to the local BGP speaker to check the as_path for the same AS as the local system so possibly going out of RFC compliance but either way it won't break anything and is a more intuitively coded in XR me thinks....
-- BR Tony > On 22 Jun 2014, at 22:14, Tony Singh <mothafungla_at_gmail.com> wrote: > > > > Default behaviour as far as I can tell. > It's better right? > Conserve bandwidth :) but an arse to figure out for the first time! > > -- > BR > > Tony > >> On 22 Jun 2014, at 22:07, kumar yes <2014ccie_at_gmail.com> wrote: >> >> Hell All, >> >> In my GNS3 LAB, I have enabled MP BGP to send L3 routes between Customer >> router 1 and customer Router 2 using BGP as PE-CE Protocol. >> >> PEs running AS 100 running IOS-XR >> CEs running AS 80 running IOS >> >> I also configured RPL with PASS statement under EBGP sessions both in and >> out. >> >> THE issue is I donbt see that PEs advertising any routes that received from >> the CE1 to CE2 and vise versa >> >> I know that the routes will be discarded on CE end if it receives prefix >> from same AS (80) but in this case the PEs not advertising the routes to CE >> but when I configure as override under EBGP neighbor on PEs then the PEs >> started adveristing routes to CEs. >> >> I did the same on PEs running IOS and i see PEs adveriting routes to CEs >> (without any additional configuration) and CE discarded the route as >> expected. >> >> -------------------------------------------------------------- >> >> RP/0/0/CPU0:PE1#show bgp vrf CUST1 neighbors 192.168.1.9 routes >> Sun Jun 22 14:02:09.727 UTC >> BGP VRF CUST1, state: Active >> BGP Route Distinguisher: 1111:1111 >> VRF ID: 0x60000002 >> BGP router identifier 33.33.33.33, local AS number 100 >> BGP table state: Active >> Table ID: 0xe0000002 RD version: 105 >> BGP main routing table version 105 >> Status codes: s suppressed, d damped, h history, * valid, > best >> i - internal, r RIB-failure, S stale, N Nexthop-discard >> Origin codes: i - IGP, e - EGP, ? - incomplete >> Network Next Hop Metric LocPrf Weight Path >> Route Distinguisher: 1111:1111 (default for vrf CUST1) >> *> 9.9.9.9/32 192.168.1.9 0 0 80 i >> >> ---------------------------------------------- >> RP/0/0/CPU0:PE1#show bgp vrf CUST1 neighbors 172.168.2.5 advertised-routes >> Sun Jun 22 14:01:44.099 UTC >> >> *RP/0/0/CPU0:PE1#show bgp vrf CUST1 neighbors 172.168.2.5* >> Sun Jun 22 14:05:53.371 UTC >> >> BGP neighbor is 172.168.2.5, vrf CUST1 >> Remote AS 80, local AS 100, external link >> Remote router ID 5.5.5.5 >> BGP state = Established, up for 01:37:05 >> Last read 00:00:12, Last read before reset 00:00:00 >> Hold time is 180, keepalive interval is 60 seconds >> Configured hold time: 180, keepalive: 60, min acceptable hold time: 3 >> Last write 00:00:00, attempted 19, written 19 >> Second last write 00:01:00, attempted 19, written 19 >> Last write before reset 00:00:00, attempted 0, written 0 >> Second last write before reset 00:00:00, attempted 0, written 0 >> Last write pulse rcvd Jun 22 14:05:41.332 last full not set pulse count >> 223 >> Last write pulse rcvd before reset 00:00:00 >> Socket not armed for io, armed for read, armed for write >> Last write thread event before reset 00:00:00, second last 00:00:00 >> Last KA expiry before reset 00:00:00, second last 00:00:00 >> Last KA error before reset 00:00:00, KA not sent 00:00:00 >> Last KA start before reset 00:00:00, second last 00:00:00 >> Precedence: internet >> Enforcing first AS is enabled >> Multi-protocol capability received >> Neighbor capabilities: >> Route refresh: advertised (old + new) and received (old + new) >> 4-byte AS: advertised >> Address family IPv4 Unicast: advertised and received >> Received 118 messages, 0 notifications, 0 in queue >> Sent 110 messages, 0 notifications, 0 in queue >> Minimum time between advertisement runs is 0 secs >> >> For Address Family: IPv4 Unicast >> BGP neighbor version 105 >> Update group: 0.1 Filter-group: 0.4 No Refresh request being processed >> Route refresh request: received 4, sent 4 >> >> * Policy for incoming advertisements is CUST1 Policy for outgoing >> advertisements is CUST1* >> 1 accepted prefixes, 1 are bestpaths >> *Cumulative no. of prefixes denied: 0.* >> Prefix advertised 8, suppressed 11, withdrawn 4 >> Maximum prefixes allowed 1048576 >> Threshold for warning message 75%, restart interval 0 min >> My AS number is allowed 3 times in received updates >> An EoR was not received during read-only mode >> Last ack version 105, Last synced ack version 0 >> Outstanding version objects: current 0, max 2 >> Additional-paths operation: None >> >> >> >> >> Is IOS-XR behaving right and needs special treatment here? >> >> >> >> >> Thanks and appreciate your response >> >> >> Is IOS-XR behaving right or does it need any special treatment here? >> >> Thanks and appreciate your response >> >> >> Blogs and organic groups at http://www.ccie.net >> >> _______________________________________________________________________ >> Subscription information may be found at: >> http://www.groupstudy.com/list/CCIELab.html Blogs and organic groups at http://www.ccie.netReceived on Sun Jun 22 2014 - 22:59:13 ART
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