BGP - adj-RIB-in - loc-RIB - adj-RIB-out

[kfarrington]

Guys,

* Adj-RIBs-In. The unedited routing information sent by neighboring routers.

* Loc-RIB. The actual routing information the router uses, developed from Adj-RIBs-In.

* Adj-RIBs-Out. The information the router chooses to send to neighboring routers.

So if I have an entry in the loc-rib which is not in the routing table, and there is no other entry in the routing table via another routing process, what could be the cause.

Also, can someone confirm the following.

1. prefix recv via eBGP/iBGP into to local router adj-RIB-In

2. loc-rib takes both prefixes into loc-rib and then compares which is best (or does loc-rib only have the best-path in this table from multiple entries in the adj-RIB-ins?)

3. BGP attempts to transfer "best" prefix from loc-rib to routers forwarding table

4. BGP then moves "best" prefix to adj-rib-out and attempts to send to peers

I do have an entry in the loc-rib (or the bgp table is showing it as the best path, with the ">" next to it, but it does not make it into the routing table.

router1#sh ip bgp 69.69.69.0 255.255.255.128

BGP routing table entry for 69.69.69.0/25, version 814

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Not advertised to any peer

65000, (aggregated by 65000 71.71.71.71), (received & used)

70.70.70.70 from 70.70.70.70 (71.71.71.71)

Origin IGP, metric 2000, localpref 100, valid, external, atomic-aggregate, best

Community: 65000:1

router1#sh ip route 69.69.69.0 255.255.255.128

% Subnet not in table

router1#

router1>sh ip bgp | incl 69.69.69

*> 69.69.69.0/25 70.70.70.70 2000 0 65000 i

router1>

Can anyone please help?

Many thx,

Ken

[kfarrington]

Sorry to bother, but reading the rfc i have the following :-

Still cant work out at what stage BGP attempts to put "its" best selected route into the routers routing table, (ie, if another route existss, it would be ribbed out in bgp)

I hope someone can awnser this point.

Thx

Ken

The Decision Process takes place in three distinct phases, each triggered by a different event:

a) Phase 1 is responsible for calculating the degree of preference

for each route received from a BGP speaker located in a

neighboring autonomous system, and for advertising to the other

BGP speakers in the local autonomous system the routes that have

the highest degree of preference for each distinct destination.

b) Phase 2 is invoked on completion of phase 1. It is responsible

for choosing the best route out of all those available for each

distinct destination, and for installing each chosen route into

the appropriate Loc-RIB.

c) Phase 3 is invoked after the Loc-RIB has been modified. It is

responsible for disseminating routes in the Loc-RIB to each peer

located in a neighboring autonomous system, according to the

policies contained in the PIB. Route aggregation and information

reduction can optionally be performed within this phase.

[Harold Ritter]

Ken,

Loc-rib should have all the paths that made it through the inbound policy. The best path selection is then ran and the resulting prefix is installed in the RIB (if no other route from a source with a better AD is already present).

It looks like the BGP path should be installed in the RIB. Could you try to clear the route (clear ip ro 69.69.69.0 255.255.255.128) and see if it installs it.

Cheers mate,

[kfarrington]

sorry to appear silly here,

when you say "The best path selection is then ran and the resulting prefix is installed in the RIB"

do you mean the routers Routing table.

I get confused with RIB/FIB/RTs etc etc ;-)

I did clear the route and it has worked.

So one last point if I may, Can you confirm that this is the correct order or is 5 done before 4, ie RT populated and then passed to adj-rib-out.

1. prefix received by multiple neighbors and stored in adj-rib-in

2. policy is consulted, and if policy allows, multiple entries for the same prefix could then stored in loc-rib

3. descision process runs, and picks only one path

4. one "best" path is passed to adj-rib-out

5. Best path is passed to routers routing table if no other entry is in the RT with a lower AD.

The reason I asked, is that this route was added to an inbound policy, and a "clear ip bgp * soft in" was done, and this situation happened. I am just wondering if I had done a "clear ip bgp * soft out" also, this situation would not have happened.

I hope this makes sense, as I think my explanation is not very good :(

Many thx Mr H

Ken

[Harold Ritter · Accepted solution]

Ken,

Yes, by RIB I mean routing table.

Step 5. is probably happening before step 4. but is not a requirement for step 4 to happen. What I mean is that the route doesn't need to be installed in the RIB in order to be propagated to be passed to the adj-rib-out.

I'm not sure what caused the original issue but doing the "clear ip soft in" should have been enough.

You are welcome mate,

[kfarrington]

Top fella as always :)