iBGP route selection

[Steph1963]

Hi to All,

I am presently studying BGP route selection and I am having a hard time with iBGP route selection.
Next questions are based on the following topology.

BGP Route Selection between iBGP and eBGP routes.

Does any criteria using weight or local preference is used to choose between a iBGP and eBGP path.
Or only criteria like shortest AS path, prefer eBGP routes over iBGP, lowest IGP metric or BGP next hop is used to determined the best path.

In the following example,

   Network          Next Hop            Metric LocPrf Weight Path
*>i131.1.0.0/24     131.2.1.4                0    100      0 100 i
*                   192.1.2.6                              0 400 100

q) Doest the best path selection is base on the shortest AS?

In the following example,

   Network          Next Hop            Metric LocPrf Weight Path
* i192.1.3.0        131.2.1.4                0    100      0 100 i
*>                  192.1.2.6                0             0 400 i

q) Does the best path selection is base on eBGP over iBGP preference?

Router R3

R3#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     131.1.0.0/24 is subnetted, 1 subnets
B       131.1.0.0 [200/0] via 131.2.1.4, 00:03:55
     131.2.0.0/24 is subnetted, 2 subnets
B       131.2.1.0 [200/0] via 131.2.2.2, 00:04:03
C       131.2.2.0 is directly connected, FastEthernet0/1
     131.4.0.0/24 is subnetted, 1 subnets
B       131.4.0.0 [20/0] via 192.1.2.6, 00:04:03
B    192.1.1.0/24 [200/0] via 131.2.1.4, 00:03:55
C    192.1.2.0/24 is directly connected, FastEthernet0/0
B    192.1.3.0/24 [20/0] via 192.1.2.6, 00:04:03

BGP Route Selection between two iBGP routes.

In the followig example,

   Network          Next Hop            Metric LocPrf Weight Path
* i131.2.1.0/24     131.2.1.4                0    100      0 i
*>i                 131.2.2.2                0    100      0 i

q) What is the criteria used to determine the best path here since both local pref are the same and both path are iBGP.

Does smallest metric to BGP next-hop is used in a case where only iBGP is used as IGP (all equal to 0 according to show ip route above) or does the lowest BGP router id is used.

Thanks for your help

Stephane

[Giuseppe Larosa]

Hello Stephane,

the BGP best path selection looks for an ordered list of parameters to be checked, and yes a path learned on an iBGP session can be preferred over a path learned over an eBGP session for example for the shortest AS path or other criteria as in first example

1) dhortest AS path

2) eBGP over iBGP

3)

>>Does smallest metric to BGP next-hop is used in a case where only iBGP is used as IGP (all equal to 0 according to show ip route above) or does the lowest BGP router id is used.

lowest IGP metric to next-hop first then lowest BGP router-id

see

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

Hope to help

Giuseppe