Network Virtualization using Extreme Fabric Connect

Network Virtualization Using Extreme Fabric Connect
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Caution
Extreme Networks Fabric L3 VSN BEBs are capable of IP routing traffic received from the
fabric back into the fabric in a single IP routed hop.
ISIS IP Route Types and Protocol Preference
The Extreme Fabric Connect L3 service types (L3 VSN and IP Shortcuts) are able to announce two different
types of IP routes in IS-IS using the both the I-SID-IPv4 and I-SID-IPv6 TLVs as well as with the regular TLV
135 (Extended IP Reachability) for IPv4 and TLV 236 for IPv6 routes. These are referred to as IS-IS Internal
IP routes and IS-IS External IP routes and in many ways, offer a similar distinction as what OSPF offers with
LSA-5 external Type1 and Type2 routes.
Note
Technically, unlike OSPF, all IP routes are “external” to IS-IS, since IS-IS does not operate
on top of IP. Yet we shall refer to “internal” IS-IS routes as those routes which derive their
metric from the calculated SPB Fabric path cost as opposed to the route metric carried in
the IS-IS TLV.
Every IP route advertised in SPB IS-IS TLVs has a metric associated with the route and carried in the TLV
itself. This metric is typically obtained from the original route metric on the IP routing table of the BEB that
redistributed the route into IS-IS. We will refer to this metric as the “external” metric of the route.
Tip
The external metric of an IS-IS IP route can be manipulated via use of redistribution route
policies.
When a different BEB, in the same VSN routing domain (L3 I-SID) considers that IS-IS IP route for inclusion
in its IP routing table, a separate “internal” metric for that IP route is computed. The internal metric is no
other than the SPB’s shortest path cost to reach the BEB that originated the TLV containing the IP route.
That is, the internal metric is in fact the SPB path cost metric.
In the original Extreme Fabric Connect implementation, all IS-IS IP routes were considered “internal” type
route and were considered for inclusion in the IP routing tables based on their internal metric alone, with
the external metric only being used as a tie breaker.
Since then, the Fabric Connect implementation has been enhanced to support a new “external” type route
that can now be considered for inclusion in the IP routing table based on its external metric alone
Note
Encoding of the IS-IS external route type is included within the I-SID-IPv4 and I-SID-IPv6
TLVs used with L3 VSNs and is included in a sub-TLV of TLVs 135 (IPv4) and 236 (IPv6)
used with GRT IP Shortcuts.
The following table provides a breakdown of the IS-IS IP route selection criteria when the same IP route is
seen advertised by more than one distant BEB in the IS-IS LSDB.