Dual-homing: single clustered router vs. two routers
While preparing a multihome BGP-based setup for my customer, I ran into a dilemma: use a single clustered router or two routers?
Router [AB] stands for my customer's logical edge router, currently statically interconnecting company's LAN, DMZ and internet.
Router [AB] is build up of two physical units (A and B) assuring HA in Active-Active mode (it isn't from Cisco but still IGPs- and BGP-capable and Cisco VSS might be the analogy here). It has ca. 750 MB RAM available.
Apart from [AB], there are two older and out-of-service units with 512 MB total RAM, otherwise identical to [AB] - let's call them [CD]. They can be used or not at my discretion.
The AS300 stands for the company's DMZ (with services available publicly). The MLS is a LAN core switch doing both switching and routing (collapsed two-tier LAN topology). This MLS is both IGPs- and BGP-capable (Cisco Catalyst 4500-series).
In scenario 1, Router [AB] is an eBGP peer for both ISP1 and ISP2 as displayed here: