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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:

("Router A" displayed meaning [AB] in my scenario)

In scenario 2, the [AB] cluster is split into undependant A and B units, having an eBGP session with SP-A and SP-B respectively. They are also running iBGP between themselves, as it can be seen here:

I have considered 2 wariants to that scenario:

2a. Router A and B run VRRP between them for both intranet (MLS) and DMZ.

2b. Router A and B run iBGP with MLS.

My plans and preferences are:

a) to request at least partial BGP updates from both ISPs,

b) to be able to load-balance both outbound and inbound internet traffic on both internet links.

c) to keep MLS within "campus block" (out of company's edge block following Cisco's ECNM guidelines, so to my understanding avoid iBGP on it).

d) to keep the setup preferably simple for me and my substitute admin.

Following these preferences, I believe that scenario 1 is best suited. Owing to the HA nature of Router [AB], I cannot see any redundancy caveats, as usually associated with the single-router setup.

My questions are:

- am I missing any other attractive option with the hardware available?

- am I missing any pros and cons of the scenarios mentioned?

Will appreciate your opinions.



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