25.02.2013 Views

TCP/IP Tutorial and Technical Overview - IBM Redbooks

TCP/IP Tutorial and Technical Overview - IBM Redbooks

TCP/IP Tutorial and Technical Overview - IBM Redbooks

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

7. R22 reviews the level-2 label appended by R21 <strong>and</strong> realizes it is the<br />

penultimate hop in the R2-R3 tunnel. R22 pops the level-2 label (Lc) <strong>and</strong><br />

forwards the packet to R3. The label stack contains one entry.<br />

A.2.4 MPLS stacks in a BGP environment<br />

The network shown in Figure A-4 shows three autonomous systems. The<br />

environment contains two classes of <strong>IP</strong> routing:<br />

► Each autonomous system runs an IGP to maintain connectivity within the AS.<br />

For example, R2, R21, R22, <strong>and</strong> R3 use OSPF to maintain routes within<br />

AS 2.<br />

► Each autonomous systems runs BGP to maintain connectivity between<br />

autonomous systems. For example, border routers R1, R2, R3, <strong>and</strong> R4 use<br />

BGP to exchange inter-AS routing information.<br />

EBGP<br />

Figure A-4 Connecting autonomous systems in an MPLS environment<br />

In this sample network, it is desirable to avoid distributing BGP-learned routes to<br />

devices which are not BGP border routers (for example, R21, R22). This<br />

minimizes the CPU processing required to maintain the <strong>IP</strong> routing table on these<br />

devices. It also eliminates the need to run a BGP routing algorithm on these<br />

devices.<br />

An MPLS LSP stack can be used to implement this environment. In this<br />

configuration, BGP routes are distributed only to BGP peers, <strong>and</strong> not to interior<br />

routers that lie along the hop-by-hop path between peers. LSP tunnels are<br />

configured so that:<br />

► Each peer distributes a label for each address prefix that it distributes through<br />

BGP. These labels are distributed to peers within the same AS.<br />

936 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

L3 L1 <strong>IP</strong> hdr Payload<br />

IBGP<br />

EBGP<br />

IGP R1 R2 R3 R4 IGP<br />

R21<br />

R22<br />

L1 <strong>IP</strong> hdr Payload<br />

L2 L1 <strong>IP</strong> hdr Payload<br />

AS 1 AS 2 AS 3

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!