Subject: [Idr] Loc-Rib and Adj-Rib-In

From: John Smith <[email-masked]>
Date: Fri, 18 Jun 2004 10:29:59 +0100 (BST)

Hi,

Consider an instance where BGP is redistributing OSPF routes in it. BGP runs its decision
process and finds that these are the best routes and that it needs to announce them to
its peers. From a purist's language, will these routes now be placed in the "Loc-Rib" ?

The routes that we receive from our peers are placed in the "Adj-Rib-In". What about the
routes that are redistributed (static/RIP/ISIS/OSPF/etc) in BGP?

Thanks,
Smith

___________________________________________________________ALL-NEW Yahoo! Messenger - sooooo many all-new ways to express yourself http://uk.messenger.yahoo.com

_______________________________________________
Idr mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/idr

---

From: Jeffrey Haas <[email-masked]>
Date: Mon, 21 Jun 2004 12:32:17 -0400

> routes that are redistributed (static/RIP/ISIS/OSPF/etc) in BGP?

It is intentionally hand-waved in the specification.  The closest
reference you will find to such a thing (from draft -23):

   All routes in the Loc-RIB are processed into Adj-RIBs-Out according
   to configured policy. This policy MAY exclude a route in the Loc-RIB
   from being installed in a particular Adj-RIB-Out. A route SHALL NOT
   be installed in the Adj-Rib-Out unless the destination and NEXT_HOP
   described by this route may be forwarded appropriately by the Routing
   Table. If a route in Loc-RIB is excluded from a particular Adj-RIB-
   Out the previously advertised route in that Adj-RIB-Out MUST be with-
   drawn from service by means of an UPDATE message (see 9.2).

IMO, this would imply that a redistributed route enters directly into
the Adj-Rib-Out of a peer potentially in preference to the best selected
bgp route.

>From the BGP MIB standpoint, just because a route is the best bgp route
doesn't mean we expect to see it in the adj-rib-out.

-- 
Jeff Haas 
NextHop Technologies

---

From: "Andras Csaszar \(ETH/RL\)" <[email-masked]>
Date: Tue, 22 Jun 2004 09:36:17 +0200

Hi!

I would like to ask a related question. When do BGP routes get advertised
into OSPF?

- When received in an UPDATE message? That is, when it gets into Adj-RIB-In?
Or,
- when it gets selected as best-route? That is, when it is put into the
Loc-RIB?

In the first case, I think it's hard to ensure that OSPF uses the same
routes as BGP thinks.  Although it might be possible by setting the OSPF
metrics appropriately. (?)

In the second case there is an scenario where a multi-homed stub network
prefers to use one outgoing link with BGP, therefore does not advertise the
other route via OSPF's AS_External_LSA. Now consider that an interor node
looses connectivity to the preferred egress router but otherwise would have
connectivity to the other egress node, but it won't forward traffic to this
egress node since it doesn't know that it accepts outgoing traffic. Anyway,
in this case how will this interior node ever learn about this route? (I
mean, the two egress nodes running BGP still prefer the first egress link,
even though some interior nodes can't access it any longer.)

Am I missing something, or which one is right?

Thanks,
Andras

----Original Message----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Jeffrey Haas Sent: 2004. junius 21. 18:32
To: John Smith
Cc: [email-masked]
Subject: Re: [Idr] Loc-Rib and Adj-Rib-In

>> Consider an instance where BGP is redistributing OSPF routes in it.
>> BGP runs its decision process and finds that these are the best
>> routes and that it needs to announce them to its peers. From a
>> purist's language, will these routes now be placed in the "Loc-Rib"
>> ?  
>> 
>> The routes that we receive from our peers are placed in the
>> "Adj-Rib-In". What about the routes that are redistributed
>> (static/RIP/ISIS/OSPF/etc) in BGP? 
> 
>    to configured policy. This policy MAY exclude a route in the
>    Loc-RIB from being installed in a particular Adj-RIB-Out. A route
>    SHALL NOT be installed in the Adj-Rib-Out unless the destination
>    and NEXT_HOP described by this route may be forwarded
>    appropriately by the Routing Table. If a route in Loc-RIB is
>    excluded from a particular Adj-RIB- Out the previously advertised
>    route in that Adj-RIB-Out MUST be with- drawn from service by
> means of an UPDATE message (see 9.2). 
>

---

From: Curtis Villamizar <[email-masked]>
Date: Tue, 22 Jun 2004 15:57:45 -0400

In message <[email-masked]>
> I would like to ask a related question. When do BGP routes get advertised

When you want to bring down the network since that's what advertising
anywhere close to full BGP routes into OSPF will do for you.

Curtis

---

From: Jeffrey Haas <[email-masked]>
Date: Tue, 22 Jun 2004 19:38:22 -0400

> - when it gets selected as best-route? That is, when it is put into the

Answer number 3: When it becomes the active route in the Routing Table,
or some implementation equivalent of this abstraction.

> egress node since it doesn't know that it accepts outgoing traffic.

In a given BGP Autonomous System, each BGP speaker may have a distinct
BGP route as the best BGP route.  When this is the case, if each of
these BGP speakers is an OSPF speaker and these routes are injected
into OSPF, you will have multiple candidate AS External routes.

There are multiple reasons why a given BGP route may be selected
as the best BGP route on multiple BGP speakers.  The BGP tie
breaking process will show many of them.

-- 
Jeff Haas 
NextHop Technologies

---

From: "András Császár \(ETH/RL\)" <[email-masked]>
Date: Wed, 23 Jun 2004 09:41:11 +0200

Sure, yes, I maybe was not precise enough when asking my question. I did not
mean full BGP routes, but only the available destination prefixes (maybe
after route aggregation) reachable through a given BGP egress router. (I
mean interior routers (running IGP only) have to be able to forward traffic
to external prefixes as well, and I'm considering here either a mult-homed
stub or any AS with multiple providers or peering relations.)

I think reachable prefixes are given to the IGP after they are installed
into Loc-RIB (or even to Adj-RIB-Out?), and only if the given BGP speaker
received the reachable destination prefix UPDATE message through eBGP.

Am I right?

András

----Original Message----
From: Curtis Villamizar [mailto:[email-masked]]
Sent: 2004. június 22. 21:58
To: Andras Csaszar (ETH/RL)
Cc: [email-masked]
Subject: Re: [Idr] Loc-Rib and Adj-Rib-In

>> I would like to ask a related question. When do BGP routes get
>> advertised into OSPF?
> 
>

---

From: Curtis Villamizar <[email-masked]>
Date: Thu, 24 Jun 2004 18:51:59 -0400

In message <[email-masked]>
> received the reachable destination prefix UPDATE message through eBGP.

BGP routes are not given to the IGP at all.  IBGP is used to pass the
routing information to thw interior of an AS and acrosss the AS.

The next hop for all BGP routes are looked up in the IGP and the
forwarding entry is created based on those two peices of information.

Implementation details vary with some optimizations done to reduce the
work when the IGP changes (trading off space for a time gain) but the
above description is otherwise accurate.

Curtis

---

From: "András Császár \(ETH/RL\)" <[email-masked]>
Date: Fri, 25 Jun 2004 10:24:17 +0200

> routing information to thw interior of an AS and acrosss the AS.

Well, that's interesting. iBGP across AS, sure; that is, between edge nodes
(which nodes probably also use eBGP to communicate with neighbour AS
router(s)). But iBGP to the interior... To every single router... Does this
mean that typically every router is a BGP speaker inside an AS? In practice
there are really no interior routers which only run IGP?

iBGP requires full mesh interconnection between iBGP speakers, and if every
single router inside an AS runs BGP, then this full mesh would be pretty
hard to maintain. So, route reflectors are then really used in practice by
operators in order to avoid this full mesh?

Thanks,
András

---

From: Curtis Villamizar <[email-masked]>
Date: Fri, 25 Jun 2004 11:06:28 -0400

In message <[email-masked]>
"Andr s Cs sz r \(ETH/RL\)" writes:
> Well, that's interesting. iBGP across AS, sure; that is, between edge
> nodes (which nodes probably also use eBGP to communicate with
> neighbour AS router(s)). But iBGP to the interior... To every single
> router... Does this mean that typically every router is a BGP speaker
> inside an AS? In practice there are really no interior routers which
> only run IGP?

Yes that is exactly how it is used.  Although route reflectors are
typically used to avoid a full mesh of BGP speakers.

> iBGP requires full mesh interconnection between iBGP speakers, and if
> every single router inside an AS runs BGP, then this full mesh would
> be pretty hard to maintain. So, route reflectors are then really used
> in practice by operators in order to avoid this full mesh?

Exactly.

Quite a bit of efficiency consideration in the BGP implementation is
required for this to work.  This is why naive BGP implementations in
the mid to late 90s failed so miserably in the ISP lab before even
getting to the field (and occasionally new ones continue to do so if
they even get that far).

Curtis