* means valid route in bgp.what is valid?

[sarahr202]

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.

[Reza Sharifi]

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

[sarahr202]

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

[Reza Sharifi · Accepted solution]

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

[sarahr202]

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

[gornication]

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.