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CSCO11598534
Beginner

BGP Best Path Selection - Aggregate VS Redistributed

Hello all,

I would like your help, explaining why BGP picks the best path for the following situation. Please see the diagram. R1-R2-R3 are EIGRP neighbors. R2 and R3 are eBGP neighbors with R4.

Both R2 and R3 learn about the 1.1.1.1/32 prefix from R1

Both R2 and R3 do mutual redistribution between EIGRP and BGP

Both R2 and R3 locally create a BGP aggregate 1.1.0.0/16

basic.jpg

I would expect that R4 would learn both aggregates created by R2 and R3. But here what really happens:

R2#sh ip bgp 1.1.0.0
BGP routing table entry for 1.1.0.0/16, version 10
Paths: (1 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 1
Local, (aggregated by 2 2.2.2.2)
0.0.0.0 from 0.0.0.0 (2.2.2.2)
Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate, best

 

R3#sh ip bgp 1.1.0.0
BGP routing table entry for 1.1.0.0/16, version 9
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1
Refresh Epoch 1
Local, (aggregated by 2 3.3.3.3)
0.0.0.0 from 0.0.0.0 (3.3.3.3)
Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate
Refresh Epoch 1
Local
23.23.23.2 from 0.0.0.0 (3.3.3.3)
Origin incomplete, metric 2560002816, localpref 100, weight 32768, valid, sourced, best

 

R4#sh ip bgp 1.1.0.0
BGP routing table entry for 1.1.0.0/16, version 12
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1
Refresh Epoch 6
2
34.34.34.3 from 34.34.34.3 (3.3.3.3)
Origin incomplete, metric 2560002816, localpref 100, valid, external
Refresh Epoch 6
2, (aggregated by 2 2.2.2.2)
24.24.24.2 from 24.24.24.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best

 

Either R2 or R3 (depends on who created the aggregate first), picks as best path the locally redistributed from EIGRP route that learned from the other router! (Please see R3 output)

Can someone please explain to me why this is the case?

Thank you!

 

1 ACCEPTED SOLUTION

Accepted Solutions
Martin L
VIP Advocate

 

R4 did learn both routes; one from R2 and one from R3 but R4 BGP picks 1 Best by default based on Origin code I think. 

in case of R3, I think following step #3 on the best patch selection made decision here. right? 

Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.

Local paths that are sourced by the network or redistribute commands are preferred over local aggregates that are sourced by the aggregate-address command.

Local, (aggregated by 2 3.3.3.3)
0.0.0.0 from 0.0.0.0 (3.3.3.3)
Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate

Local
23.23.23.2 from 0.0.0.0 (3.3.3.3)
Origin incomplete, metric 2560002816, localpref 100, weight 32768, valid, sourced, best

 

 

Regards, ML
**Please Rate All Helpful Responses **

View solution in original post

3 REPLIES 3
Martin L
VIP Advocate

 

R4 did learn both routes; one from R2 and one from R3 but R4 BGP picks 1 Best by default based on Origin code I think. 

in case of R3, I think following step #3 on the best patch selection made decision here. right? 

Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.

Local paths that are sourced by the network or redistribute commands are preferred over local aggregates that are sourced by the aggregate-address command.

Local, (aggregated by 2 3.3.3.3)
0.0.0.0 from 0.0.0.0 (3.3.3.3)
Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate

Local
23.23.23.2 from 0.0.0.0 (3.3.3.3)
Origin incomplete, metric 2560002816, localpref 100, weight 32768, valid, sourced, best

 

 

Regards, ML
**Please Rate All Helpful Responses **

View solution in original post

Hey Martin,

Can you please provide a legit link where it is clearly stated that local network/redistribute commands are preferred over local aggregate?