03-26-2012 07:36 AM
Hi Everyone;
I hope everybody is keeping well.
I have a situation here.
I have a client hwo is unning a bgp session with us using a 1G ethenet connection (asr9000).
The same client now wishes to have another bgp session with us on a 10G interface (asr 9000).
I had assigned them a temporary IP prefix from a /29 range. The original bgp peering (on 1 g connection) uses a /30 range.
Current configs on my side are as follows:
neighbor 86.107.28.242 (10 G)
remote-as xxxx
use neighbor-group abc
description xxx
address-family ipv4 unicast
route-policy bbb in
route-policy ccc out
neighbor 86.107.23.210 (1G)
remote-as xxxx
use neighbor-group abc
description xxx
address-family ipv4 unicast
route-policy bbb in
route-policy ccc out
There is no intention to implement bgp laod balancing feature (using mltipath or otherwise). Do you think this config would work fine?
regards
03-26-2012 07:57 AM
Hi,
Yes, that would work. You’ll get 2 sessions with absolutely identical parameters for all routes except the peer’s IP address. Hence, the best path would be the path with the lowest peer address, routes from 86.107.23.210 would be taken as the best in this case.
BGP Best Path Selection Algorithm
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml
Regards,
/A
03-26-2012 08:09 AM
Thanks Alexei.
So I do not need to set multipath value to 2? if not, please advise how the peering would get established?
regards
03-26-2012 08:14 AM
Hi Jalal,
Your config from the first post would work just fine. Source and destination IP addresses are different, so there are no any issues making both sessions UP even if physically that is the same router.
Regards,
/A
03-26-2012 11:55 AM
Thanks alot Alexei again.
Just one thing, i wanted check with you. I read somewhere that 2 bgp neighbors connected through physical interfaces(not loop backs), can only have one active bgp peerig session between them. If two same cost links are used to i interconnect them only one would work.
Since in my scenario, there two pysical connections between the two neighbors and these connections are not equal cost links. One is 1G and the other is a 10G. I wonder what might happen here, which link would be chosen by bgp to select the routes to be installed in the routing tbale of the client?
regards
03-26-2012 12:15 PM
BGP would choose a path based on BGP Best Path Selection Algorithm
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml
in this scenario, the best path would be the path with the lowest peer address, hence routes learned from 86.107.23.210 would be taken as the best (compare to routes from 86.107.28.242 ) in this case and installed into the routing table.
Regard,
/A
03-26-2012 12:35 PM
Thanks again boss.
Would they still be able to see the routes coming in through the 10 G link though?
regards
03-27-2012 01:06 AM
What you get is something like this. BGP will learn same network over 2 sessions. Here the net 3.3.3.0/24 was learnt from 100.100.46.6 and 100.100.64.6 (that is the same router here)
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 3.3.3.0/24 100.100.46.6 0 0 10 ?
* 100.100.64.6 0 0 10 ?
100.100.46.6 is taken as the best (marked with > ) as the IP address is lower compare to 100.100.64.6
RP/0/0/CPU0:R4#sh bgp 3.3.3.0/24 bestpath-compare
Tue Mar 27 10:46:12.441 EEST
BGP routing table entry for 3.3.3.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 16 16
Last Modified: Mar 26 17:52:21.000 for 16:53:52
Paths: (2 available, best #1)
Advertised to update-groups (with more than one peer):
0.2
Path #1: Received by speaker 0
Advertised to update-groups (with more than one peer):
0.2
10
100.100.46.6 from 100.100.46.6 (192.168.0.6)
Origin incomplete, metric 0, localpref 100, valid, external, best, group-best
Received Path ID 0, Local Path ID 1, version 16
best of AS 10, Overall best
Path #2: Received by speaker 0
Not advertised to any peer
10
100.100.64.6 from 100.100.64.6 (192.168.0.6)
Origin incomplete, metric 0, localpref 100, valid, external
Received Path ID 0, Local Path ID 0, version 0
Higher neighbor address than best path (path #1)
The routing table has only the best path installed
RP/0/0/CPU0:R4#show route 3.3.3.0/24
Tue Mar 27 10:46:47.425 EEST
Routing entry for 3.3.3.0/24
Known via "bgp 65000", distance 20, metric 0
Tag 10, type external
Installed Mar 26 17:52:20.911 for 16:54:26
Routing Descriptor Blocks
100.100.46.6, from 100.100.46.6
Route metric is 0
No advertising protos.
If the 1G link fail, the routing table would install routes pointing to 100.100.64.6.
But you can modify the behavior and distribute the routes among the links as you wish or put a ebgp multipath to load balance between them.
for example here is the routing table when ebgp multipath is enabled:
RP/0/0/CPU0:R4#show route 3.3.3.0/24
Tue Mar 27 11:05:08.512 EEST
Routing entry for 3.3.3.0/24
Known via "bgp 65000", distance 20, metric 0
Tag 10, type external
Installed Mar 27 10:59:56.936 for 00:05:12
Routing Descriptor Blocks
100.100.46.6, from 100.100.46.6, BGP multi path
Route metric is 0
100.100.64.6, from 100.100.64.6, BGP multi path
Route metric is 0
No advertising protos.
Regards,
/A
01-16-2018 04:22 AM
Hi,
just One query,
in this Case, BGP Prefer routes from the peer with the lowest peer IP address.
here the selection will work ( between 2 IPv4 peer IP's)
in case, I have one BGP neighbor is IPv4 and One more neighbor ship with IPv6.
and getting the same prefix x.x.x.x/24 via both the neighbors , How selection will work !!!
BR//
Dipankar.
01-16-2018 05:29 AM
Hello,
I guess what you mean is that you carry one address family - e.g. IPv4 using IPv4 and IPv6 transport. Situation is that IPv4 NLRI supports only IPv4 NH and IPv6 NRLI supports only IPv6 nexthops. Thus using this approach you will still need to use route-map to modify transport NH to the one understood by your address-family. Meaning for IPv4 AF you will keep IPv4 transport NH but modify IPv6 transport to IPv4.
In this case you will again end up comparing IPv4 against IPv4 and IPv6 against IPv6 as in your first example.
HTH
Niko
01-28-2018 10:09 PM
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