Subject: Re: egress LSR

From: Jim Sullivan <[email-masked]>
Date: Thu, 14 Oct 1999 08:13:15 -0400

Santosh, 
I have added some comments below. 

At 10:32 AM 10/12/1999 -0400, Eric Gray wrote:
>Santosh,
>
>You wrote:
>> 
>> Hello All,
>>     I am implementing ietf-draft-05.txt for LDP. My doubt is :
>> How does an LSR decide that it is the egress router for the particular LSP?
>
>An LSR could be implemented, or configured, to egress packets 
>for any of a number of reasons, but it would 'decide' it is
>the egress for a particular LSP when the FEC (or perhaps other
>criteria used) in setting up the LSP is associated with a next
>hop that is not a labeling peer or is not associated with any
>next hop (other than 'self').

In order to answer this question I would first look at section 
3.1.4 of the architecture spec "LSP egress and LSP proxy egress"
Here is the text from the architecture spec. I have inserted 
some comments in []
  
 An LSR R is considered to be an "LSP Egress" LSR for address prefix X
   if and only if one of the following conditions holds:

      1. R has an address Y, such that X is the address prefix in R's
         routing table which is the longest match for Y, or

[I think this is the case where X corresponds to one of this LSRs 
 interface addresses(including mask)]    ]

      2. R contains in its routing tables one or more address prefixes Y
         such that X is a proper initial substring of Y, but R's "LSP
         previous hops" for X do not contain any such address prefixes
         Y; that is, R is a "deaggregation point" for address prefix X.

[ An example. If in your routing tables you had the following
  address prefixes { 128.16.0.0/16, 128.16.1.0/24, and 128.16.2.0/24 
  and your upstream had only 128.16.0.0/16] then you are the deaggregation
  point for 128.16.0.0/16. The interesting question is how to determine 
  what is in your upstream(for this FEC) LSR's forwarding DB. One way
  to figure this out is to look at routing configuration to determine if
  128.16.0.0/16 is being advertised as an aggregated route, in which case
  the upstream will not have the more specific routes.  Another case 
  where this could occur is if a static route is configured and routing
  is configured not to advertise such a route. ( I don't know if this 
  is a configurable option, and if so why one would do this ?). If this 
  were allowed then determining if you are the deaggregation point for 
  a FEC is a little more difficult because when a static route is added
  you would have to determine the list of less specific routes and 
  terminate the associated LSPs.]  

   An LSR R1 is considered to be an "LSP Proxy Egress" LSR for address
   prefix X if and only if:

      1. R1's next hop for X is R2, and R1 and R2 are not label
         distribution peers with respect to X (perhaps because R2 does
         not support MPLS), or

[ For me this case is a little unclear.  Consider the following cases.
  - My downstream for this FEC does not support MPLS. 
  - My downstream for this FEC is an LSR but my LDP session to that peer is
not up.
  - I have a session to my downstream peer, however when I requested 
  a label for the FEC he responded with either "no route" or "no resource" 
  notification. 
  It seems pretty certain that in first case we would consider ourselves
the egress
  but how about the second two cases ? 
  My inclination is that we would not want to be the egress in the last two
cases. 
  
 ]  

      2. R1 has been configured to act as an LSP Proxy Egress for X

>
>> Thanx in advance.
>> Santosh Gupta
>
>-- 
>Eric Gray		Email: mailto:[email-masked]
>Voice: (978)960-6162
>