Quick BGP Question - Path Selection

[ksbolton1]

Hello all. I'm doing a BGP lab and I'm a bit curious about the best path selection I see on one of the routers. Basically, I'm used to the best path algorithm being Next-hop reachable? - Weight - Local preference - Locally injected? - AS path length - Origin - MED - Network type - IGP metric to next hop.

In the pic below, the path chosen for prefix 17.1.1.0/24 is via 8.1.2.1. The next path is via 8.1.1.1. It makes sense when you look at the actual network diagram as this router (ISP4 in the diagram) has a direct link to 8.1.2.1, however based on the tie breaker used between the paths (local preference since weight is the same), why wouldn't the 8.1.1.1 next-hop path be chosen? It has a higher local preference and everything else matches.

[Francesco Molino · Accepted solution]

Hi

The local-pref for the 8.1.1.1 has a value of 100 which is the default value.
If you do a sh ip bgp 17.1.1.1 you'll see the detail of all paths and confirm local-pref is the same.

[ksbolton1]

Ah, thank you for clearing that up. I assumed that the default value local preference value was only 100 for iBGP learned routes, but I see they're the same when I drill down as you've suggested. I imagine the tie breaker then becomes eBGP vs iBGP with the former winning that tie.

[Joseph W. Doherty]

"I imagine the tie breaker then becomes eBGP vs iBGP with the former winning that tie."

Normally, yes.

By default, Cisco assigns a "better: AD value or eBGP routes over iBGP routes.

[Harold Ritter]

Can you attach the output of a "show bgp ipv4 uni 17.1.1.0 255.255.255.0".

Regards,

[ksbolton1]

Thank you for your quick reply. I had made a couple changes in the time since I made this post but I think I see what's caused the specific path to be selected. As Francesco outlined, the tie breaker wasn't local pref as I thought, it was the neighbor type.

BGP routing table entry for 17.1.1.0/24, version 28
Paths: (2 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 1
65003 65000
8.1.10.1 (metric 3) from 8.8.8.3 (8.8.8.3)
Origin IGP, metric 0, localpref 100, weight 150, valid, internal, best
Originator: 8.8.8.4, Cluster list: 8.8.8.3
rx pathid: 0, tx pathid: 0x0
Refresh Epoch 1
65000
8.1.1.1 from 8.1.1.1 (17.17.17.2)
Origin IGP, metric 0, localpref 100, valid, external
rx pathid: 0, tx pathid: 0