MPLS: Router Roles

Customer Edge or CE router – this device has no knowledge of MPLS protocol and does not send any labeld packets but is directly connected to an LSR (PE) on the MPLS VPN.

Provider Edge or PE router – A LSP that shares a link with at least one CE router, providing MPLS functions at the edge of the MPLS VPN, includes functions such as iBGP and VRF tables.  PE routers also learn all customer routes and keep different customer routing tables separate. PEs must also impose MPLS headers on non MPLs packets they receive.

Provider or P router – a LSR that does not have a direct link to a CE router, this device simply forwards label switched packets only, and allows the LSR to ignore customer VPN routes.

P and PE routers run LDP and an IGP to support unicast routing, the IGP though only advertises routes for subnets on the MPLS network, NOT routes from the customer network. instead customer routes are put into separate customer routing tables called VRFs, then PEs use iBGP to exchange these customer routes with other PEs, never advertising routes to the P routers.

MPLS headers added to ingress packets include:

  • Outer MPLS header (Sbit = 0) with label value that causes the packet to be label switched to the egress PE
  • Inner MPLS header (Sbit = 1) label identifies the egress VRF on which to base the forwarding decision.MPLSHeaderPath.PNG
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s