January 16, 2006

Sham Links vs. Virtual Links?(SL)

Let's continue our talking about sham-link and virtual-link, today is sham-link's turn.

Think about a scenario as this.
U'r Company has chosen MPLS/VPN service for connecting the abroad branches from the service provider, and use OSPF as the routing protocol. While at the same time U suggest that the company should also buy a E1/T1 link, act as a backup path for the MPLS/VPN. This ensure the high available for U'r network. So, the gateway router will be connected to the SP that offers U MPLS/VPN service. And this gateway router, we call it Customer Edge (CE) router, and the link is a primary link for data transmission. Then the router which U'r SP used to connect U'r site we call it Provider Edge (PE) router. Don't forget u'r backup link, the CE router may connected to another or the same SP, but a different WAN service -- E1/T1. U design the CE router will retain the communication with the remote site through this backup E1/T1 even if the MPLS/VPN service is down somehow. This backup link is known as backdoor path.

Although OSPF PE-CE connections assume that the only path between two client sites is across the MPLS VPN backbone, backdoor paths between VPN sites may exist. If these sites belong to the same OSPF area, the path over a backdoor link will always be selected because OSPF prefers intraarea paths to interarea paths. (PE routers advertise OSPF routes learned over the VPN backbone as interarea paths.) For this reason, OSPF backdoor links between VPN sites must be taken into account so that routing is performed based on policy. Now this is why we talking about sham-link here.

At this moment, U'r CE router may received two kinds of routes to reach the remote site,
1. Received from the MPLS/VPN that redistributed from BGP to OSPF, this route create U'r primary path, and flood as LSA3 (inter-area) route.
2. Received directly form the remote site router by the E1/T1 backdoor path, this route create U'r backup path, and flood as LSA1(intra-area) route.

Unexpected situation happens, the primary path and the backup path changes their role because OSPF prefers intraarea paths to interarea paths as we explain above.

If the backdoor links between sites are used only for backup purposes and do not participate in the VPN service, then the default route selection shown in the preceding example is not acceptable. To reestablish the desired path selection over the MPLS VPN backbone, U must create an additional OSPF intra-area (logical) link between ingress and egress VRFs on the relevant PE routers. This link is called a sham-link.

Note: A sham-link is required between any two VPN sites that belong to the same OSPF area and share an OSPF backdoor link. If no backdoor link exists between the sites, no sham-link is required.

Because the sham-link is seen as an intra-area link between PE routers, an OSPF adjacency is created and database exchange (for the particular OSPF process) occurs across the link. The PE router can then flood LSAs between sites from across the MPLS VPN backbone. As a result, the desired intra-area connectivity is created by specify a lower metric for the sham-link, than which is specified for the backdoor path.

In summary, U should configure an OSPF sham link under the following circumstances:

* Two CE routers are linked together by a Layer 3 VPN.
* These CE routers are in the same OSPF area.
* An intraarea link is configured between the two CE routers.

If there is no intraarea link between the CE routers, U do not need to configure an OSPF sham link.OSPF treats the link through the Layer 3 VPN as an interarea link. By default, OSPF prefers intraarea links to interarea links, so OSPF selects the backup intraarea link as the active path. This is not acceptable in configurations where the intraarea link is not the expected primary path for traffic between the CE routers.An OSPF sham link is also an intraarea link, except that it is configured between the PE routers. U can configure the metric for the sham link to ensure that the path over the Layer 3 VPN is preferred to a backup path over an intraarea link connecting the CE routers.

16 Comments:

At 1/17/2006 12:10:00 AM, Anonymous chruch said...

Give some command line of the sham link?

 
At 1/17/2006 12:12:00 AM, Anonymous grrc said...

nice blog

 
At 2/02/2007 12:50:00 AM, Blogger Proactiv Solution said...

Beer Glasses

Logo Glassware and Beer Drinking Supplies!

 
At 2/03/2007 11:23:00 AM, Anonymous Anonymous said...

Hello! ;)
oh... what distracted comments!
what do U consider about it?

 
At 2/05/2007 08:13:00 AM, Anonymous Anonymous said...

Hey, you have a great blog here! I'm definitely going to bookmark you!

I have a asians site. It pretty much covers asians related stuff.

Come and check it out if you get time :-)

 
At 2/06/2007 02:25:00 AM, Anonymous Anonymous said...

Hi, everyone you have a great blog here! Check out this site beaver site. a lot of cool stuff here about beaver .

Come and check it out if you get time :-)

 
At 2/06/2007 09:20:00 AM, Anonymous Anonymous said...

Hey has anyone been to this site before breast site. it seems very much about breast .

Cum and check it out if you get time ;-)

 
At 2/07/2007 04:12:00 PM, Anonymous Anonymous said...

WOW! Have you seen this site about nudity . It has great nudity pictures.

 
At 2/09/2007 03:17:00 PM, Anonymous Anonymous said...

WOW! Have you seen this site about video . It has great video pictures.

 
At 3/03/2007 06:02:00 AM, Anonymous Anonymous said...

voip service


Hi, just wondering if anyone has ever tried SKYPE for telephone calls over the internet?? A pal of mine says it's brilliant and works out much cheaper than conventional calls, just wondered if anyone else has signed up for it?

 
At 3/09/2007 09:58:00 AM, Anonymous Anonymous said...

Hi all!
It's really nice site. Keep so!
Look it:
free+hardcore+sex+video
free+hardcore+sex+video
http://12351.freegoodies.nl/forum_an.php?forum=12351
pinkworld
pinkworld
http://pinkworld1.21.forumer.com
couple+erotic+fetish+guide+sex
couple+erotic+fetish+guide+sex
http://coupleerotic.jconserv.net
free+sex+trailer
free+sex+trailer
http://12350.freegoodies.nl/forum_an.php?forum=12350
gay+porn+movie
gay+porn+movie
http://gaymovie.21.forumer.com
free+sex+chat+rooms
free+sex+chat+rooms
http://sexchat.jconserv.net

 
At 3/20/2007 09:59:00 PM, Anonymous Anonymous said...

single source

 
At 3/28/2016 08:20:00 AM, Blogger oakleyses said...

burberry handbags, ugg boots, nike outlet, tiffany and co, christian louboutin, oakley sunglasses, ray ban sunglasses, replica watches, uggs outlet, uggs on sale, cheap oakley sunglasses, michael kors outlet, kate spade outlet, prada handbags, louis vuitton outlet, polo outlet, jordan shoes, tory burch outlet, oakley sunglasses wholesale, gucci handbags, replica watches, louis vuitton outlet, nike air max, louis vuitton, michael kors outlet online, christian louboutin shoes, prada outlet, chanel handbags, michael kors outlet, oakley sunglasses, ugg boots, oakley sunglasses, tiffany jewelry, uggs outlet, ray ban sunglasses, nike free, michael kors outlet online, nike air max, christian louboutin outlet, louis vuitton, longchamp outlet, christian louboutin uk, michael kors outlet online, ray ban sunglasses, polo ralph lauren outlet online, louis vuitton outlet, michael kors outlet online, longchamp outlet

 
At 3/28/2016 08:23:00 AM, Blogger oakleyses said...

mulberry uk, nike tn, abercrombie and fitch uk, nike air max, nike free run, lululemon canada, true religion outlet, oakley pas cher, longchamp pas cher, nike free uk, sac hermes, nike blazer pas cher, hollister uk, true religion jeans, new balance, air max, coach outlet store online, north face uk, coach outlet, north face, michael kors pas cher, true religion outlet, nike roshe run uk, nike air max uk, polo lacoste, michael kors, hollister pas cher, vans pas cher, nike air max uk, polo ralph lauren, jordan pas cher, timberland pas cher, nike air force, nike roshe, burberry pas cher, converse pas cher, michael kors outlet, sac vanessa bruno, ray ban pas cher, ralph lauren uk, kate spade, replica handbags, ray ban uk, michael kors, louboutin pas cher, true religion outlet, coach purses, guess pas cher, hogan outlet, sac longchamp pas cher

 
At 3/28/2016 08:25:00 AM, Blogger oakleyses said...

mont blanc pens, reebok outlet, mcm handbags, bottega veneta, abercrombie and fitch, hollister, nike roshe run, nike trainers uk, longchamp uk, celine handbags, insanity workout, iphone 6s plus cases, instyler, iphone cases, s6 case, mac cosmetics, herve leger, soccer jerseys, wedding dresses, north face outlet, new balance shoes, ferragamo shoes, iphone 5s cases, lululemon, ralph lauren, timberland boots, nike huaraches, soccer shoes, chi flat iron, asics running shoes, iphone 6s cases, jimmy choo outlet, valentino shoes, hollister clothing, iphone 6 cases, ipad cases, babyliss, vans outlet, nfl jerseys, north face outlet, giuseppe zanotti outlet, beats by dre, p90x workout, louboutin, oakley, ghd hair, baseball bats, hermes belt, iphone 6 plus cases, nike air max

 
At 3/28/2016 08:28:00 AM, Blogger oakleyses said...

hollister, coach outlet, barbour, canada goose outlet, ugg uk, wedding dresses, canada goose outlet, canada goose uk, toms shoes, doudoune moncler, moncler outlet, louis vuitton, moncler outlet, links of london, ugg,uggs,uggs canada, montre pas cher, louis vuitton, nike air max, lancel, converse, swarovski crystal, moncler, juicy couture outlet, juicy couture outlet, moncler uk, canada goose jackets, ugg pas cher, moncler, pandora jewelry, swarovski, gucci, ugg, vans, hollister, louis vuitton, louis vuitton, karen millen uk, canada goose outlet, marc jacobs, canada goose, supra shoes, louis vuitton, thomas sabo, ugg,ugg australia,ugg italia, pandora uk, barbour uk, pandora jewelry, moncler, converse outlet, ray ban, moncler, canada goose

 

Post a Comment

Links to this post:

Create a Link

<< Home