From: Volkov, Dmitry (Toronto - BCE) (dmitry_volkov@xxxxxxxxx)
Date: Wed Aug 28 2002 - 22:53:36 GMT-3
Guys,
I just tested it. It's NOT TRUE. MEDs are compared BY DEFAULT !
I have 3 routes on common ethernet segment
r1 160.100.2.1; r10 160.100.2.250 and R8 160.100.2.254
r10 R8
| |
r1-----------------
R10 & R8 in AS 2001, R1 in AS 2010
R8 advert networks via network statements
R1 sees routes:
Network Next Hop Metric LocPrf Weight Path
*> 10.10.10.0/28 133.7.4.4 0 100 0 (65302) 2020 i
* 160.100.100.0/24 160.100.2.254 0 0 2001 i
*> 160.100.2.254 0 2001 i
* 160.100.128.0/24 160.100.2.254 0 0 2001 i
*> 160.100.2.254 0 2001 i
* 160.100.129.0/24 160.100.2.254 0 0 2001 i
*> 160.100.2.254 0 2001 i
* 160.100.130.0/24 160.100.2.254 0 0 2001 i
*> 160.100.2.254 0 2001 i
No any metrics in place no any bgp determenistic meds in place.
Tiebreiker in BGP selection process is Oldest route.
Whenever I did on R1: neighbor R8 shut no shut or neigbor R10 shut no shu
best route on R1 was changing.
After I put route-map on R10 advertising network 160.100.100.0 with metric
121 towards to R1,
this network also has best route via R8 !!! - So MED plays his role in BGP
best path desision process ! without any bgp determ med on any routers.
r1#sh ip bgp 160.100.100.0
BGP routing table entry for 160.100.100.0/24, version 40
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
133.7.2.2 160.100.2.250
2001
160.100.2.254 from 160.100.2.254 (192.190.102.1)
Origin IGP, metric 0, localpref 100, valid, external, best
2001
160.100.2.254 from 160.100.2.250 (101.101.101.101)
Origin IGP, metric 121, localpref 100, valid, external
Dmitry
> -----Original Message-----
> From: Mingzhou Nie [mailto:mnie@yahoo.com]
> Sent: Wednesday, August 28, 2002 9:40 PM
> To: Nick Shah; Volkov, Dmitry (Toronto - BCE); Jon O'Nan
> Cc: ccielab@groupstudy.com
> Subject: Re: deterministic med
>
>
> Nick,
>
> Don't believe that's true. MED is always compared whether "bgp
> deterministic med " enabled to not. the command simply changes the
> comparison behavior a little bit by grouping routes from same neiborgh
> and compare.
>
> In the given example, if you change the order as below.
>
> entry1: AS(PATH) 500, med 150, external, rid 172.16.13.1
> entry2: AS(PATH) 500, med 100, internal, rid 172.16.8.4
> entry3: AS(PATH) 100, med 200, external, rid 1.1.1.1
>
> even w/o the command, entry2 will be chosen over entry1.
>
> ming
>
> --- Nick Shah <nshah@connect.com.au> wrote:
> > > Can we say that by default (bgp deterministic-med is not enabled)
> > > Metric(MED) is not compared between ANY (doesn't matter from the
> > same AS
> > or
> > > not) routes with the same path lenghts ?
> >
> > Think it this way, whats the BGP path decision process, Weight,
> > localpref,
> > origin code, ASPATH, MED (stop here). Now unless bgp
> > deterministic-med is
> > enabled, MED comparison is skipped.
> >
> > So if all else be the same till here (till MED) and you really want
> > MED to
> > influence path, then turn on 'BGP DETERMINISTIC MED'
> >
> > > I.e. MED - is not taken into consideration in the
> > > BGP best path selection alghorithm ? (by default) So forget about
> > MED
> > > completely until enable "bgp-deterministic-med" or "bgp
> > > always-compare-med" ?
> >
> > Correct
> >
> > > Also this sentence:
> > > (MED is used in this comparison by default because the winners of
> > each
> > group
> > > are now from different ASs - enabling bgp always-compare-med
> > changes this
> > > default behavior).
> > > from http://www.cisco.com/warp/public/459/37.html
> > >
> > > Is it mistake ?? Should I read (MED is NOT used in this
> > comparision....)
> > > because it doesn't compare MEDs from different ASs until "bgp
> > > always-compare-med" is enabled ?
> >
> > That looks like a'convoluted statement' and a typo.
> >
> > The correct document is : (meaning, one which confirms the actual
> > behaviour
> > is )
> > http://www.cisco.com/warp/public/459/bgp-med.html
> >
> > //snip//Enabling the bgp deterministic-med command ensures the
> > comparison of
> > the MED variable when choosing routes advertised by different peers
> > in the
> > same autonomous system (AS). Enabling the bgp always-compare-med
> > command
> > ensures the comparison of the MED for paths from neighbors in
> > different
> > autonomous systems. The bgp always-compare-med command is
> useful when
> > multiple service providers or enterprises agree on a uniform policy
> > for
> > setting MED. Thus, for network X, if Internet Service Provider (ISP)
> > A sets
> > the MED to 10, and ISP B sets the MED to 20, then both ISPs agree
> > that ISP A
> > has the better performing path to X. //snip//
> >
> > rgds
> >
> > Nick
> >
> > >
> > > > -----Original Message-----
> > > > From: Nick Shah [mailto:nshah@connect.com.au]
> > > > Sent: Wednesday, August 28, 2002 7:36 PM
> > > > To: Jon O'Nan; ccielab@groupstudy.com
> > > > Subject: Re: deterministic med
> > > >
> > > >
> > > > Jon
> > > >
> > > > I hope you have already read this paper..
> > > > http://www.cisco.com/warp/customer/459/bgp-med.html
> > > >
> > > > regardless of that. I will list down the effect of these 2
> > commands.
> > > >
> > > > * By default (with none of these commands entered) MED is NOT
> > > > compared when
> > > > selecting the best path.
> > > > * Turning on bgp deterministic-med, turns on the feature to
> > > > compare MED,
> > > > however it only compares MED, if prefix is received from same
> > > > neighbor and
> > > > the best man wins
> > > > * Turning on bgp always-compare-med, starts comparing 'same
> > > > prefix' recd.
> > > > from 'different neighbors' and the best man wins
> > > > * Turning on both of them may really not be required, but it
> > basically
> > > > compares if prefixes are recd. in groups of AS's (say same
> > > > prefix from AS
> > > > 200, via 2 neighbors, and same prefix from AS 100 via 1
> > neighbor).
> > > >
> > > > rgds
> > > > Nick
> > > > ----- Original Message -----
> > > > From: "Jon O'Nan" <jononan@columbus.rr.com>
> > > > To: <ccielab@groupstudy.com>
> > > > Sent: Thursday, August 29, 2002 8:58 AM
> > > > Subject: deterministic med
> > > >
> > > >
> > > > > Am I correct that:
> > > > >
> > > > > bgp deterministic med - compares med within AS
> > > > >
> > > > > VS.
> > > > >
> > > > > bgp always-compare-med - compares med from another AS
> > > > >
> > > > >
> > > > > Thanks
> > > > > Jon
> > > > >
This archive was generated by hypermail 2.1.4 : Sat Sep 07 2002 - 19:48:41 GMT-3