eiBGP multipath

[Sam-CCNP]

Hi,

I've been doing BGP multipath labs and it seem that step 8, lowest IGP metric to the next hop, is skipped when you are using multipath eibgp.

For example, R8 (below) has two neighbours - 9.9.9.9 which is iBGP, and 11.11.11.11 is eBGP:

R8#sh run | s bgp
router bgp 8
 bgp log-neighbor-changes
 neighbor 9.9.9.9 remote-as 8
 neighbor 9.9.9.9 update-source Loopback8
 neighbor 11.11.11.11 remote-as 1
 neighbor 11.11.11.11 disable-connected-check
 maximum-paths eibgp 2

R8#sh ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 9
Paths: (2 available, best #1, table default)
Multipath: eiBGP
Flag: 0x100
Advertised to update-groups:
10
Refresh Epoch 3
1
11.11.11.11 (metric 3) from 11.11.11.11 (11.11.11.11)
Origin IGP, metric 0, localpref 100, valid, external, multipath, best
rx pathid: 0, tx pathid: 0x0
Refresh Epoch 2
1
9.9.9.9 (metric 2) from 9.9.9.9 (192.168.89.9)
Origin IGP, metric 0, localpref 100, valid, internal, multipath(oldest)
rx pathid: 0, tx pathid: 0

I cant find anything in Cisco documentation that confirms this behaviour. There is the command bgp bestpath igp-metric ignore but I'm not using it, and it's not active in the show run all.

Anyone have any idea what is going on?

Thanks for any help.

Sam

[paul driver]

Hello
By the looks of it, Med isnt being selected here,best path seems ebgp over Ibgp , the reason I would say is the meds are from different ASNs ( alway compare med needs to be enabled) for med comparison

[Sam-CCNP]

Hi Paul,

I was referring to the #8 IGP metric (not the MED) and it not being equal, and as to why multipath still continues. But after closer inspection I found the answer in Cisco's best path algorithm documentation:

7. Prefer eBGP over iBGP paths.

If bestpath is selected, go to Step 9 (multipath).

8. Prefer the path with the lowest IGP metric to the BGP next hop.

Continue, even if bestpath is already selected.

9. Determine if multiple paths require installation in the routing table for BGP Multipath .

Continue, if bestpath is not yet selected.

So best path does skip #8 if eBGP is preferred over iBGP. Let me know if you agree.

Thanks,

Sam

[M02@rt37]

Hello @Sam-CCNP 

If you refer here:

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html#toc-hId-1165513981

So yes! correct!

[paul driver]

Hello

---

@Sam-CCNP wrote:

Hi Paul,

I was referring to the #8 IGP metric (not the MED) and it not being equal, and as to why multipath still continues. But after closer inspection I found the answer in Cisco's best path algorithm documentation:

---

Yes - i believe you was-- my mistake then in that case you are correct - that's the reason step 8 was ignored