
Configuring IP Services
5-28 114065 Rev. A
Using the External Route Tag in an ASE
An OSPF AS external route advertisement includes an external route tag field.
This field allows boundary routers in an AS to exchange information about
external routes. (The specific nature of this information is outside the scope of
OSPF.)
By default, Bay Networks boundary routers that generate ASEs set the external
route tag field to 0. For a boundary router running OSPF and BGP, the network
administrator can configure OSPF to set the external route tag field with a value in
accordance with RFC 1403, “OSPF/BGP Interaction.”
In
Figure 5-5, for example, boundary router A running OSPF and BGP learns
external routes via BGP:
1. Router A generates an ASE describing the route. OSPF fills in the external
route tag with BGP-specific information according to RFC 1403.
2. Router A injects the ASE into the AS, and OSPF routers flood the ASE
throughout the AS.
3. Router B, which runs OSPF and BGP, receives the ASE. Router B generates a
BGP update, using the contents of the external route tag to set the Origin and
AS Path attributes in the update.
Note: The route weight value will appear to be greater than the route’s original
metric. For this reason, all routers advertising a particular network must use
the same metric type -- Type 1 or Type 2. If not, the router that receives the
advertisements may choose the wrong route.
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