01-05-2013 02:16 PM - edited 03-07-2019 10:55 AM
Hi everybody
"* " means Valid in BGP . What does BGP consider before declaring a route as valid?
For example, below, R1 is receiving an update about 6.0.0.0/8 from its ibgp peer. The next hop is 196.196.196.1 is not reachable by R1 but yet R1 still considers a valid route ( * is attached next to 6.0.0.0/8).
R1#show ip bgp
BGP table version is 8, local router ID is 196.196.196.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*i6.0.0.0 195.195.195.5 0 100 0 6 i
*> 11.0.0.0 0.0.0.0 0 32768 i
R1#show ip route
C 196.196.196.0/24 is directly connected, Serial0/1
R 6.0.0.0/8 [120/1] via 188.188.188.2, 00:00:24, Serial0/0
R 189.189.0.0/16 [120/1] via 188.188.188.2, 00:00:24, Serial0/0
188.188.0.0/24 is subnetted, 1 subnets
C 188.188.188.0 is directly connected, Serial0/0
S 11.0.0.0/8 is directly connected, Serial0/0
Thanks and have a great weekend.
Solved! Go to Solution.
01-05-2013 05:55 PM
Hi Sarah,
Next hop reach ability is the first criteria for BGP path selection. A path the has an unreachable next-hop is not valid even though the route is shown in the routing table.
after next-hop the next one is highest weight (which is Cisco proprietary) then local preference then locally originated route and so on....
Have a great weekend!
Time to go watch wild card games
HTH
Reza
01-05-2013 02:53 PM
Hi Sarah,
The next hop is 196.196.196.1 is not reachable
You mean 195.195.195.5 right?
because 196.196.196.0/24 is assigned to serial0/1
What is the topology looks like?
Thanks,
Reza
01-05-2013 03:50 PM
Hi Reza
sorry for the typo
s0---------------188.188.188.0.0--s0 R1
|
R2
|s1 ------189.189.189.0 -----------s1 R3-----195.195.195.0-------(ebgp)-------R5 (AS 6)
R2,R1 and R3 are in AS 5. R3 is learning about update 6.0.0.0 via ebgp peer R6
R3 is advertising 6.0.0.0 with next hop 195.195.195.6 ( i.e the ip address of R5 connected to R3) to its ibgp peers R2 and R1.
we are running Rip in side As 5 but we are not advertising 195.195.195.0 under rip as a result R1 can not reach 195.195.195.0 network.
The goal is to see will R1 consider the update via ibgp peer R3 about 6.0.0.0/8 as valid or not if R1 can not find the next hop i.e 195.195.195.6 in its routing table?
So my question is what BGP take into account to consider a route as valid? ( apparently reachability of next hop is not checked to declare a route as valid by bgp)
thanks and have a great weekend
01-05-2013 05:55 PM
Hi Sarah,
Next hop reach ability is the first criteria for BGP path selection. A path the has an unreachable next-hop is not valid even though the route is shown in the routing table.
after next-hop the next one is highest weight (which is Cisco proprietary) then local preference then locally originated route and so on....
Have a great weekend!
Time to go watch wild card games
HTH
Reza
01-05-2013 08:00 PM
Hi Reza
How are you doing ?
In my example, I prevent rip from advertising195.195.195.0 network . I want to see if a router does not know the path to reach next hop, will it still consider the route as valid? Route might not be valid but I noticed * still attached to the prefix in bgp table>
R1#show ip bgp
BGP table version is 8, local router ID is 196.196.196.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*i6.0.0.0 195.195.195.5 0 100 0 6 i
=========================================================
So back to same question, what does bgp consider first before attaching * to the prefix in its bgp table ?
I hope you are enjoying " WILD CARD GAME".
have a great evening
11-10-2021 03:47 AM
Hi
It is unlikely that a route with an unavailable next-hop will be added to the routing table, but the same route can be marked as valid in the bgp table:
In addition, the literature distinguishes between these two criteria. For example: Step 1. Verify that the NRLI is valid and that the next-hop address can be resolved in the global RIB.
From here, the question arises of what a valid route is.
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